Re: Discussion: Scheduling across runner and SDKHarness in Portability framework

[Robert Bradshaw <ro...@google.com>]

Currently the best solution we've come up with is that we must process an
unbounded number of bundles concurrently to avoid deadlock. Especially in
the batch case, this may be wasteful as we bring up workers for many stages
that are not actually executable until upstream stages finish. Since it may
be invasive to require runners to only schedule stages that can be actively
worked on, I've been thinking about what we could do in the common runner
libraries themselves. One idea is to postpone the actual sending of a
process bundle request until there is data on the channel to consume. With
Reads as Impulses, and triggers as data, all bundles are driven by some
input.

This would mean we never ask the SDK to process bundles it cannot
immediately start working on. There is still the open question of whether
being able to *start* a bundle implies that one is able to *finish* a
bundle (i.e. do any runners start bundles and then block, pending other
bundle completion, before closing the data channel (though clearly a runner
can chop a bundle off at any point if it wants)).

Does this approach sound feasible?


On Thu, Aug 30, 2018 at 2:54 AM Ankur Goenka <go...@google.com> wrote:

> I managed to write a small document based on the discussion.
> Please take a look at
> https://docs.google.com/document/d/1oAXVPbJ0dzj2_8LXEWFAgqCP5Tpld3q5B3QU254PQ6A/edit?usp=sharing
>
>
> On Tue, Aug 21, 2018 at 11:01 PM Henning Rohde <he...@google.com> wrote:
>
>> Sending bundles that cannot be executed, i.e., the situation described to
>> cause deadlock in Flink in the beginning of the thread with mapB. The
>> discussion of exposing (or assuming an infinitely large) concurrency level
>> -- while a useful concept in its own right -- came around as a way to
>> unblock mapB.
>>
>> On Tue, Aug 21, 2018 at 2:16 PM Lukasz Cwik <lc...@google.com> wrote:
>>
>>> Henning, can you clarify by what you mean with send non-executable
>>> bundles to the SDK harness and how it is useful for Flink?
>>>
>>> On Tue, Aug 21, 2018 at 2:01 PM Henning Rohde <he...@google.com>
>>> wrote:
>>>
>>>> I think it will be useful to the runner to know upfront what the
>>>> fundamental threading capabilities are for the SDK harness (say, "fixed",
>>>> "linear", "dynamic", ..) so that the runner can upfront make a good static
>>>> decision on #harnesses and how many resources they should each have. It's
>>>> wasteful to give the Foo SDK a whole many-core machine with TBs of memory,
>>>> if it can only support a single bundle at a time. I think this is also in
>>>> line with what Thomas and Luke are suggesting.
>>>>
>>>> However, it still seems to me to be a semantically problematic idea to
>>>> send non-executable bundles to the SDK harness. I understand it's useful
>>>> for Flink, but is that really the best path forward?
>>>>
>>>>
>>>>
>>>> On Mon, Aug 20, 2018 at 5:44 PM Ankur Goenka <go...@google.com> wrote:
>>>>
>>>>> That's right.
>>>>> To add to it. We added multi threading to python streaming as a single
>>>>> thread is sub optimal for streaming use case.
>>>>> Shall we move towards a conclusion on the SDK bundle processing upper
>>>>> bound?
>>>>>
>>>>> On Mon, Aug 20, 2018 at 1:54 PM Lukasz Cwik <lc...@google.com> wrote:
>>>>>
>>>>>> Ankur, I can see where you are going with your argument. I believe
>>>>>> there is certain information which is static and won't change at pipeline
>>>>>> creation time (such as Python SDK is most efficient doing one bundle at a
>>>>>> time) and some stuff which is best at runtime, like memory and CPU limits,
>>>>>> worker count.
>>>>>>
>>>>>> On Mon, Aug 20, 2018 at 1:47 PM Ankur Goenka <go...@google.com>
>>>>>> wrote:
>>>>>>
>>>>>>> I would prefer to to keep it dynamic as it can be changed by the
>>>>>>> infrastructure or the pipeline author.
>>>>>>> Like in case of Python, number of concurrent bundle can be changed
>>>>>>> by setting pipeline option worker_count. And for Java it can be computed
>>>>>>> based on the cpus on the machine.
>>>>>>>
>>>>>>> For Flink runner, we can use the worker_count parameter for now to
>>>>>>> increase the parallelism. And we can have 1 container for each mapPartition
>>>>>>> task on Flink while reusing containers as container creation is expensive
>>>>>>> especially for Python where it installs a bunch of dependencies. There is 1
>>>>>>> caveat though. I have seen machine crashes because of too many simultaneous
>>>>>>> container creation. We can rate limit container creation in the code to
>>>>>>> avoid this.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Ankur
>>>>>>>
>>>>>>> On Mon, Aug 20, 2018 at 9:20 AM Lukasz Cwik <lc...@google.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> +1 on making the resources part of a proto. Based upon what Henning
>>>>>>>> linked to, the provisioning API seems like an appropriate place to provide
>>>>>>>> this information.
>>>>>>>>
>>>>>>>> Thomas, I believe the environment proto is the best place to add
>>>>>>>> information that a runner may want to know about upfront during pipeline
>>>>>>>> pipeline creation. I wouldn't stick this into PipelineOptions for the long
>>>>>>>> term.
>>>>>>>> If you have time to capture these thoughts and update the
>>>>>>>> environment proto, I would suggest going down that path. Otherwise anything
>>>>>>>> short term like PipelineOptions will do.
>>>>>>>>
>>>>>>>> On Sun, Aug 19, 2018 at 5:41 PM Thomas Weise <th...@apache.org>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> For SDKs where the upper limit is constant and known upfront, why
>>>>>>>>> not communicate this along with the other harness resource info as part of
>>>>>>>>> the job submission?
>>>>>>>>>
>>>>>>>>> Regarding use of GRPC headers: Why not make this explicit in the
>>>>>>>>> proto instead?
>>>>>>>>>
>>>>>>>>> WRT runner dictating resource constraints: The runner actually may
>>>>>>>>> also not have that information. It would need to be supplied as part of the
>>>>>>>>> pipeline options? The cluster resource manager needs to allocate resources
>>>>>>>>> for both, the runner and the SDK harness(es).
>>>>>>>>>
>>>>>>>>> Finally, what can be done to unblock the Flink runner / Python
>>>>>>>>> until solution discussed here is in place? An extra runner option for SDK
>>>>>>>>> singleton on/off?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Sat, Aug 18, 2018 at 1:34 AM Ankur Goenka <go...@google.com>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> Sounds good to me.
>>>>>>>>>> GRPC Header of the control channel seems to be a good place to
>>>>>>>>>> add upper bound information.
>>>>>>>>>> Added jiras:
>>>>>>>>>> https://issues.apache.org/jira/browse/BEAM-5166
>>>>>>>>>> https://issues.apache.org/jira/browse/BEAM-5167
>>>>>>>>>>
>>>>>>>>>> On Fri, Aug 17, 2018 at 10:51 PM Henning Rohde <
>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> Regarding resources: the runner can currently dictate the
>>>>>>>>>>> mem/cpu/disk resources that the harness is allowed to use via the
>>>>>>>>>>> provisioning api. The SDK harness need not -- and should not -- speculate
>>>>>>>>>>> on what else might be running on the machine:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> https://github.com/apache/beam/blob/0e14965707b5d48a3de7fa69f09d88ef0aa48c09/model/fn-execution/src/main/proto/beam_provision_api.proto#L69
>>>>>>>>>>>
>>>>>>>>>>> A realistic startup-time computation in the SDK harness would be
>>>>>>>>>>> something simple like: max(1, min(cpu*100, mem_mb/10)) say, and use that at
>>>>>>>>>>> most number of threads. Or just hardcode to 300. Or a user-provided value.
>>>>>>>>>>> Whatever the value is the maximum number of bundles in flight allowed at
>>>>>>>>>>> any given time and needs to be communicated to the runner via some message.
>>>>>>>>>>> Anything beyond would be rejected (but this shouldn't happen, because the
>>>>>>>>>>> runner should respect that number).
>>>>>>>>>>>
>>>>>>>>>>> A dynamic computation would use the same limits from the SDK,
>>>>>>>>>>> but take into account its own resource usage (incl. the usage by running
>>>>>>>>>>> bundles).
>>>>>>>>>>>
>>>>>>>>>>> On Fri, Aug 17, 2018 at 6:20 PM Ankur Goenka <go...@google.com>
>>>>>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> I am thinking upper bound to be more on the lines of
>>>>>>>>>>>> theocratical upper limit or any other static high value beyond which the
>>>>>>>>>>>> SDK will reject bundle verbosely. The idea is that SDK will not keep
>>>>>>>>>>>> bundles in queue while waiting on current bundles to finish. It will simply
>>>>>>>>>>>> reject any additional bundle.
>>>>>>>>>>>> Beyond this I don't have a good answer to dynamic upper bound.
>>>>>>>>>>>> As SDK does not have the complete picture of processes on the machine with
>>>>>>>>>>>> which it share resources, resources might not be a good proxy for upper
>>>>>>>>>>>> bound from the SDK point of view.
>>>>>>>>>>>>
>>>>>>>>>>>> On Fri, Aug 17, 2018 at 6:01 PM Lukasz Cwik <lc...@google.com>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Ankur, how would you expect an SDK to compute a realistic
>>>>>>>>>>>>> upper bound (upfront or during pipeline computation)?
>>>>>>>>>>>>>
>>>>>>>>>>>>> First thought that came to my mind was that the SDK would
>>>>>>>>>>>>> provide CPU/memory/... resourcing information and the runner making a
>>>>>>>>>>>>> judgement call as to whether it should ask the SDK to do more work or less
>>>>>>>>>>>>> but its not an explicit don't do more then X bundles in parallel.
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 5:55 PM Ankur Goenka <
>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> Makes sense. Having exposed upper bound on concurrency with
>>>>>>>>>>>>>> optimum concurrency can give a good balance. This is good information to
>>>>>>>>>>>>>> expose while keeping the requirements from the SDK simple. SDK can publish
>>>>>>>>>>>>>> 1 as the optimum concurrency and upper bound to keep things simple.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Runner introspection of upper bound on concurrency is
>>>>>>>>>>>>>> important for correctness while introspection of optimum concurrency is
>>>>>>>>>>>>>> important for efficiency. This separates efficiency and correctness
>>>>>>>>>>>>>> requirements.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 5:05 PM Henning Rohde <
>>>>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I agree with Luke's observation, with the caveat that "infinite
>>>>>>>>>>>>>>> amount of bundles in parallel" is limited by the available resources. For
>>>>>>>>>>>>>>> example, the Go SDK harness will accept an arbitrary amount of parallel
>>>>>>>>>>>>>>> work, but too much work will cause either excessive GC pressure with
>>>>>>>>>>>>>>> crippling slowness or an outright OOM. Unless it's always 1, a reasonable
>>>>>>>>>>>>>>> upper bound will either have to be provided by the user or computed from
>>>>>>>>>>>>>>> the mem/cpu resources given. Of course, as some bundles takes more
>>>>>>>>>>>>>>> resources than others, so any static value will be an estimate or ignore
>>>>>>>>>>>>>>> resource limits.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> That said, I do not like that an "efficiency" aspect becomes
>>>>>>>>>>>>>>> a subtle requirement for correctness due to Flink internals. I fear that
>>>>>>>>>>>>>>> road leads to trouble.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 4:26 PM Ankur Goenka <
>>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> The later case of having a of supporting single bundle
>>>>>>>>>>>>>>>> execution at a time on SDK and runner not using this flag is exactly the
>>>>>>>>>>>>>>>> reason we got into the Dead Lock here.
>>>>>>>>>>>>>>>> I agree with exposing SDK optimum concurrency level ( 1 in
>>>>>>>>>>>>>>>> later case ) and let runner decide to use it or not. But at the same time
>>>>>>>>>>>>>>>> expect SDK to handle infinite amount of bundles even if its not efficient.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>> Ankur
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 4:11 PM Lukasz Cwik <
>>>>>>>>>>>>>>>> lcwik@google.com> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I believe in practice SDK harnesses will fall into one of
>>>>>>>>>>>>>>>>> two capabilities, can process effectively an infinite amount of bundles in
>>>>>>>>>>>>>>>>> parallel or can only process a single bundle at a time.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I believe it is more difficult for a runner to handle the
>>>>>>>>>>>>>>>>> latter case well and to perform all the environment management that would
>>>>>>>>>>>>>>>>> make that efficient. It may be inefficient for an SDK but I do believe it
>>>>>>>>>>>>>>>>> should be able to say that I'm not great at anything more then a single
>>>>>>>>>>>>>>>>> bundle at a time but utilizing this information by a runner should be
>>>>>>>>>>>>>>>>> optional.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 1:53 PM Ankur Goenka <
>>>>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> To recap the discussion it seems that we have come-up
>>>>>>>>>>>>>>>>>> with following point.
>>>>>>>>>>>>>>>>>> SDKHarness Management and initialization.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>    1. Runner completely own the work assignment to
>>>>>>>>>>>>>>>>>>    SDKHarness.
>>>>>>>>>>>>>>>>>>    2. Runner should know the capabilities and capacity
>>>>>>>>>>>>>>>>>>    of SDKHarness and should assign work accordingly.
>>>>>>>>>>>>>>>>>>    3. Spinning up of SDKHarness is runner's
>>>>>>>>>>>>>>>>>>    responsibility and it can be done statically (a fixed pre configured number
>>>>>>>>>>>>>>>>>>    of SDKHarness) or dynamically or based on certain other configuration/logic
>>>>>>>>>>>>>>>>>>    which runner choose.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> SDKHarness Expectation. This is more in question and we
>>>>>>>>>>>>>>>>>> should outline the responsibility of SDKHarness.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>    1. SDKHarness should publish how many concurrent
>>>>>>>>>>>>>>>>>>    tasks it can execute.
>>>>>>>>>>>>>>>>>>    2. SDKHarness should start executing all the tasks
>>>>>>>>>>>>>>>>>>    items assigned in parallel in a timely manner or fail task.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Also to add to simplification side. I think for better
>>>>>>>>>>>>>>>>>> adoption, we should have simple SDKHarness as well as simple Runner
>>>>>>>>>>>>>>>>>> integration to encourage integration with more runner. Also many runners
>>>>>>>>>>>>>>>>>> might not expose some of the internal scheduling characteristics so we
>>>>>>>>>>>>>>>>>> should not expect scheduling characteristics for runner integration.
>>>>>>>>>>>>>>>>>> Moreover scheduling characteristics can change based on pipeline type,
>>>>>>>>>>>>>>>>>> infrastructure, available resource etc. So I am a bit hesitant to add
>>>>>>>>>>>>>>>>>> runner scheduling specifics for runner integration.
>>>>>>>>>>>>>>>>>> A good balance between SDKHarness complexity and Runner
>>>>>>>>>>>>>>>>>> integration can be helpful in easier adoption.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>> Ankur
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 12:22 PM Henning Rohde <
>>>>>>>>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Finding a good balance is indeed the art of portability,
>>>>>>>>>>>>>>>>>>> because the range of capability (and assumptions) on both sides is wide.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> It was originally the idea to allow the SDK harness to
>>>>>>>>>>>>>>>>>>> be an extremely simple bundle executer (specifically, single-threaded
>>>>>>>>>>>>>>>>>>> execution one instruction at a time) however inefficient -- a more
>>>>>>>>>>>>>>>>>>> sophisticated SDK harness would support more features and be more
>>>>>>>>>>>>>>>>>>> efficient. For the issue described here, it seems problematic to me to send
>>>>>>>>>>>>>>>>>>> non-executable bundles to the SDK harness under the expectation that the
>>>>>>>>>>>>>>>>>>> SDK harness will concurrently work its way deeply enough down the
>>>>>>>>>>>>>>>>>>> instruction queue to unblock itself. That would be an extremely subtle
>>>>>>>>>>>>>>>>>>> requirement for SDK authors and one practical question
>>>>>>>>>>>>>>>>>>> becomes: what should an SDK do with a bundle instruction that it doesn't
>>>>>>>>>>>>>>>>>>> have capacity to execute? If a runner needs to make
>>>>>>>>>>>>>>>>>>> such assumptions, I think that information should probably rather be
>>>>>>>>>>>>>>>>>>> explicit along the lines of proposal 1 -- i.e., some kind of negotiation
>>>>>>>>>>>>>>>>>>> between resources allotted to the SDK harness (a preliminary variant are in
>>>>>>>>>>>>>>>>>>> the provisioning api) and what the SDK harness in return can do (and a
>>>>>>>>>>>>>>>>>>> valid answer might be: 1 bundle at a time irrespectively of resources
>>>>>>>>>>>>>>>>>>> given) or a per-bundle special "overloaded" error response. For other
>>>>>>>>>>>>>>>>>>> aspects, such as side input readiness, the runner handles that complexity
>>>>>>>>>>>>>>>>>>> and the overall bias has generally been to move complexity to the runner
>>>>>>>>>>>>>>>>>>> side.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> The SDK harness and initialization overhead is entirely
>>>>>>>>>>>>>>>>>>> SDK, job type and even pipeline specific. A docker container is also just a
>>>>>>>>>>>>>>>>>>> process, btw, and doesn't inherently carry much overhead. That said, on a
>>>>>>>>>>>>>>>>>>> single host, a static docker configuration is generally a lot simpler to
>>>>>>>>>>>>>>>>>>> work with.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Henning
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 10:18 AM Thomas Weise <
>>>>>>>>>>>>>>>>>>> thw@apache.org> wrote:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> It is good to see this discussed!
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> I think there needs to be a good balance between the
>>>>>>>>>>>>>>>>>>>> SDK harness capabilities/complexity and responsibilities. Additionally the
>>>>>>>>>>>>>>>>>>>> user will need to be able to adjust the runner behavior, since the type of
>>>>>>>>>>>>>>>>>>>> workload executed in the harness also is a factor. Elsewhere we already
>>>>>>>>>>>>>>>>>>>> discussed that the current assumption of a single SDK harness instance per
>>>>>>>>>>>>>>>>>>>> Flink task manager brings problems with it and that there needs to be more
>>>>>>>>>>>>>>>>>>>> than one way how the runner can spin up SDK harnesses.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> There was the concern that instantiation if multiple
>>>>>>>>>>>>>>>>>>>> SDK harnesses per TM host is expensive (resource usage, initialization time
>>>>>>>>>>>>>>>>>>>> etc.). That may hold true for a specific scenario, such as batch workloads
>>>>>>>>>>>>>>>>>>>> and the use of Docker containers. But it may look totally different for a
>>>>>>>>>>>>>>>>>>>> streaming topology or when SDK harness is just a process on the same host.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>>>> Thomas
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 8:36 AM Lukasz Cwik <
>>>>>>>>>>>>>>>>>>>> lcwik@google.com> wrote:
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> SDK harnesses were always responsible for executing
>>>>>>>>>>>>>>>>>>>>> all work given to it concurrently. Runners have been responsible for
>>>>>>>>>>>>>>>>>>>>> choosing how much work to give to the SDK harness in such a way that best
>>>>>>>>>>>>>>>>>>>>> utilizes the SDK harness.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> I understand that multithreading in python is
>>>>>>>>>>>>>>>>>>>>> inefficient due to the global interpreter lock, it would be upto the runner
>>>>>>>>>>>>>>>>>>>>> in this case to make sure that the amount of work it gives to each SDK
>>>>>>>>>>>>>>>>>>>>> harness best utilizes it while spinning up an appropriate number of SDK
>>>>>>>>>>>>>>>>>>>>> harnesses.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 7:32 AM Maximilian Michels <
>>>>>>>>>>>>>>>>>>>>> mxm@apache.org> wrote:
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> Hi Ankur,
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> Thanks for looking into this problem. The cause seems
>>>>>>>>>>>>>>>>>>>>>> to be Flink's
>>>>>>>>>>>>>>>>>>>>>> pipelined execution mode. It runs multiple tasks in
>>>>>>>>>>>>>>>>>>>>>> one task slot and
>>>>>>>>>>>>>>>>>>>>>> produces a deadlock when the pipelined operators
>>>>>>>>>>>>>>>>>>>>>> schedule the SDK
>>>>>>>>>>>>>>>>>>>>>> harness DoFns in non-topological order.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> The problem would be resolved if we scheduled the
>>>>>>>>>>>>>>>>>>>>>> tasks in topological
>>>>>>>>>>>>>>>>>>>>>> order. Doing that is not easy because they run in
>>>>>>>>>>>>>>>>>>>>>> separate Flink
>>>>>>>>>>>>>>>>>>>>>> operators and the SDK Harness would have to have
>>>>>>>>>>>>>>>>>>>>>> insight into the
>>>>>>>>>>>>>>>>>>>>>> execution graph (which is not desirable).
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> The easiest method, which you proposed in 1) is to
>>>>>>>>>>>>>>>>>>>>>> ensure that the
>>>>>>>>>>>>>>>>>>>>>> number of threads in the SDK harness matches the
>>>>>>>>>>>>>>>>>>>>>> number of
>>>>>>>>>>>>>>>>>>>>>> ExecutableStage DoFn operators.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> The approach in 2) is what Flink does as well. It
>>>>>>>>>>>>>>>>>>>>>> glues together
>>>>>>>>>>>>>>>>>>>>>> horizontal parts of the execution graph, also in
>>>>>>>>>>>>>>>>>>>>>> multiple threads. So I
>>>>>>>>>>>>>>>>>>>>>> agree with your proposed solution.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> Best,
>>>>>>>>>>>>>>>>>>>>>> Max
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> On 17.08.18 03:10, Ankur Goenka wrote:
>>>>>>>>>>>>>>>>>>>>>> > Hi,
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > tl;dr Dead Lock in task execution caused by limited
>>>>>>>>>>>>>>>>>>>>>> task parallelism on
>>>>>>>>>>>>>>>>>>>>>> > SDKHarness.
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > *Setup:*
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> >   * Job type: /*Beam Portable Python Batch*/ Job on
>>>>>>>>>>>>>>>>>>>>>> Flink standalone
>>>>>>>>>>>>>>>>>>>>>> >     cluster.
>>>>>>>>>>>>>>>>>>>>>> >   * Only a single job is scheduled on the cluster.
>>>>>>>>>>>>>>>>>>>>>> >   * Everything is running on a single machine with
>>>>>>>>>>>>>>>>>>>>>> single Flink task
>>>>>>>>>>>>>>>>>>>>>> >     manager.
>>>>>>>>>>>>>>>>>>>>>> >   * Flink Task Manager Slots is 1.
>>>>>>>>>>>>>>>>>>>>>> >   * Flink Parallelism is 1.
>>>>>>>>>>>>>>>>>>>>>> >   * Python SDKHarness has 1 thread.
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > *Example pipeline:*
>>>>>>>>>>>>>>>>>>>>>> > Read -> MapA -> GroupBy -> MapB -> WriteToSink
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > *Issue:*
>>>>>>>>>>>>>>>>>>>>>> > With multi stage job, Flink schedule different
>>>>>>>>>>>>>>>>>>>>>> dependent sub tasks
>>>>>>>>>>>>>>>>>>>>>> > concurrently on Flink worker as long as it can get
>>>>>>>>>>>>>>>>>>>>>> slots. Each map tasks
>>>>>>>>>>>>>>>>>>>>>> > are then executed on SDKHarness.
>>>>>>>>>>>>>>>>>>>>>> > Its possible that MapB gets to SDKHarness before
>>>>>>>>>>>>>>>>>>>>>> MapA and hence gets
>>>>>>>>>>>>>>>>>>>>>> > into the execution queue before MapA. Because we
>>>>>>>>>>>>>>>>>>>>>> only have 1 execution
>>>>>>>>>>>>>>>>>>>>>> > thread on SDKHarness, MapA will never get a chance
>>>>>>>>>>>>>>>>>>>>>> to execute as MapB
>>>>>>>>>>>>>>>>>>>>>> > will never release the execution thread. MapB will
>>>>>>>>>>>>>>>>>>>>>> wait for input from
>>>>>>>>>>>>>>>>>>>>>> > MapA. This gets us to a dead lock in a simple
>>>>>>>>>>>>>>>>>>>>>> pipeline.
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > *Mitigation:*
>>>>>>>>>>>>>>>>>>>>>> > Set worker_count in pipeline options more than the
>>>>>>>>>>>>>>>>>>>>>> expected sub tasks
>>>>>>>>>>>>>>>>>>>>>> > in pipeline.
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > *Proposal:*
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> >  1. We can get the maximum concurrency from the
>>>>>>>>>>>>>>>>>>>>>> runner and make sure
>>>>>>>>>>>>>>>>>>>>>> >     that we have more threads than max concurrency.
>>>>>>>>>>>>>>>>>>>>>> This approach
>>>>>>>>>>>>>>>>>>>>>> >     assumes that Beam has insight into runner
>>>>>>>>>>>>>>>>>>>>>> execution plan and can
>>>>>>>>>>>>>>>>>>>>>> >     make decision based on it.
>>>>>>>>>>>>>>>>>>>>>> >  2. We dynamically create thread and cache them
>>>>>>>>>>>>>>>>>>>>>> with a high upper bound
>>>>>>>>>>>>>>>>>>>>>> >     in SDKHarness. We can warn if we are hitting
>>>>>>>>>>>>>>>>>>>>>> the upper bound of
>>>>>>>>>>>>>>>>>>>>>> >     threads. This approach assumes that runner does
>>>>>>>>>>>>>>>>>>>>>> a good job of
>>>>>>>>>>>>>>>>>>>>>> >     scheduling and will distribute tasks more or
>>>>>>>>>>>>>>>>>>>>>> less evenly.
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > We expect good scheduling from runners so I prefer
>>>>>>>>>>>>>>>>>>>>>> approach 2. It is
>>>>>>>>>>>>>>>>>>>>>> > simpler to implement and the implementation is not
>>>>>>>>>>>>>>>>>>>>>> runner specific. This
>>>>>>>>>>>>>>>>>>>>>> > approach better utilize resource as it creates only
>>>>>>>>>>>>>>>>>>>>>> as many threads as
>>>>>>>>>>>>>>>>>>>>>> > needed instead of the peak thread requirement.
>>>>>>>>>>>>>>>>>>>>>> > And last but not the least, it gives runner control
>>>>>>>>>>>>>>>>>>>>>> over managing truly
>>>>>>>>>>>>>>>>>>>>>> > active tasks.
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > Please let me know if I am missing something and
>>>>>>>>>>>>>>>>>>>>>> your thoughts on the
>>>>>>>>>>>>>>>>>>>>>> > approach.
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>> > Thanks,
>>>>>>>>>>>>>>>>>>>>>> > Ankur
>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>

***UNCHECKED*** Re: Discussion: Scheduling across runner and SDKHarness in Portability framework

[Robert Bradshaw <ro...@google.com>]

On Fri, Sep 14, 2018 at 3:01 PM Thomas Weise <th...@apache.org> wrote:

> That's actually how the Flink runner already works - bundle processing
> starts when elements are available (see FlinkExecutableStageFunction for
> batch mode).
>
> But we still have the possibility of the SDK getting concurrent requests
> due to parallelism (and pipelined execution).
>

Concurrent requests should be fine, but are there cases (in batch) where a
bundle that was started cannot finish without other bundles finishing
first? What pipelining would we see if the graph is of the form
ExecutableStage - GBK - ExecutableStage - GBK - ... (or is it not always a
digraph of this form, possibly with branching)?


>
> Thanks,
> Thomas
>
>
> On Fri, Sep 14, 2018 at 2:56 AM Robert Bradshaw <ro...@google.com>
> wrote:
>
>> Currently the best solution we've come up with is that we must process an
>> unbounded number of bundles concurrently to avoid deadlock. Especially in
>> the batch case, this may be wasteful as we bring up workers for many stages
>> that are not actually executable until upstream stages finish. Since it may
>> be invasive to require runners to only schedule stages that can be actively
>> worked on, I've been thinking about what we could do in the common runner
>> libraries themselves. One idea is to postpone the actual sending of a
>> process bundle request until there is data on the channel to consume. With
>> Reads as Impulses, and triggers as data, all bundles are driven by some
>> input.
>>
>> This would mean we never ask the SDK to process bundles it cannot
>> immediately start working on. There is still the open question of whether
>> being able to *start* a bundle implies that one is able to *finish* a
>> bundle (i.e. do any runners start bundles and then block, pending other
>> bundle completion, before closing the data channel (though clearly a runner
>> can chop a bundle off at any point if it wants)).
>>
>> Does this approach sound feasible?
>>
>>
>> On Thu, Aug 30, 2018 at 2:54 AM Ankur Goenka <go...@google.com> wrote:
>>
>>> I managed to write a small document based on the discussion.
>>> Please take a look at
>>> https://docs.google.com/document/d/1oAXVPbJ0dzj2_8LXEWFAgqCP5Tpld3q5B3QU254PQ6A/edit?usp=sharing
>>>
>>>
>>> On Tue, Aug 21, 2018 at 11:01 PM Henning Rohde <he...@google.com>
>>> wrote:
>>>
>>>> Sending bundles that cannot be executed, i.e., the situation described
>>>> to cause deadlock in Flink in the beginning of the thread with mapB. The
>>>> discussion of exposing (or assuming an infinitely large) concurrency level
>>>> -- while a useful concept in its own right -- came around as a way to
>>>> unblock mapB.
>>>>
>>>> On Tue, Aug 21, 2018 at 2:16 PM Lukasz Cwik <lc...@google.com> wrote:
>>>>
>>>>> Henning, can you clarify by what you mean with send non-executable
>>>>> bundles to the SDK harness and how it is useful for Flink?
>>>>>
>>>>> On Tue, Aug 21, 2018 at 2:01 PM Henning Rohde <he...@google.com>
>>>>> wrote:
>>>>>
>>>>>> I think it will be useful to the runner to know upfront what the
>>>>>> fundamental threading capabilities are for the SDK harness (say, "fixed",
>>>>>> "linear", "dynamic", ..) so that the runner can upfront make a good static
>>>>>> decision on #harnesses and how many resources they should each have. It's
>>>>>> wasteful to give the Foo SDK a whole many-core machine with TBs of memory,
>>>>>> if it can only support a single bundle at a time. I think this is also in
>>>>>> line with what Thomas and Luke are suggesting.
>>>>>>
>>>>>> However, it still seems to me to be a semantically problematic idea
>>>>>> to send non-executable bundles to the SDK harness. I understand it's useful
>>>>>> for Flink, but is that really the best path forward?
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Mon, Aug 20, 2018 at 5:44 PM Ankur Goenka <go...@google.com>
>>>>>> wrote:
>>>>>>
>>>>>>> That's right.
>>>>>>> To add to it. We added multi threading to python streaming as a
>>>>>>> single thread is sub optimal for streaming use case.
>>>>>>> Shall we move towards a conclusion on the SDK bundle processing
>>>>>>> upper bound?
>>>>>>>
>>>>>>> On Mon, Aug 20, 2018 at 1:54 PM Lukasz Cwik <lc...@google.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> Ankur, I can see where you are going with your argument. I believe
>>>>>>>> there is certain information which is static and won't change at pipeline
>>>>>>>> creation time (such as Python SDK is most efficient doing one bundle at a
>>>>>>>> time) and some stuff which is best at runtime, like memory and CPU limits,
>>>>>>>> worker count.
>>>>>>>>
>>>>>>>> On Mon, Aug 20, 2018 at 1:47 PM Ankur Goenka <go...@google.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> I would prefer to to keep it dynamic as it can be changed by the
>>>>>>>>> infrastructure or the pipeline author.
>>>>>>>>> Like in case of Python, number of concurrent bundle can be changed
>>>>>>>>> by setting pipeline option worker_count. And for Java it can be computed
>>>>>>>>> based on the cpus on the machine.
>>>>>>>>>
>>>>>>>>> For Flink runner, we can use the worker_count parameter for now to
>>>>>>>>> increase the parallelism. And we can have 1 container for each mapPartition
>>>>>>>>> task on Flink while reusing containers as container creation is expensive
>>>>>>>>> especially for Python where it installs a bunch of dependencies. There is 1
>>>>>>>>> caveat though. I have seen machine crashes because of too many simultaneous
>>>>>>>>> container creation. We can rate limit container creation in the code to
>>>>>>>>> avoid this.
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Ankur
>>>>>>>>>
>>>>>>>>> On Mon, Aug 20, 2018 at 9:20 AM Lukasz Cwik <lc...@google.com>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> +1 on making the resources part of a proto. Based upon what
>>>>>>>>>> Henning linked to, the provisioning API seems like an appropriate place to
>>>>>>>>>> provide this information.
>>>>>>>>>>
>>>>>>>>>> Thomas, I believe the environment proto is the best place to add
>>>>>>>>>> information that a runner may want to know about upfront during pipeline
>>>>>>>>>> pipeline creation. I wouldn't stick this into PipelineOptions for the long
>>>>>>>>>> term.
>>>>>>>>>> If you have time to capture these thoughts and update the
>>>>>>>>>> environment proto, I would suggest going down that path. Otherwise anything
>>>>>>>>>> short term like PipelineOptions will do.
>>>>>>>>>>
>>>>>>>>>> On Sun, Aug 19, 2018 at 5:41 PM Thomas Weise <th...@apache.org>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> For SDKs where the upper limit is constant and known upfront,
>>>>>>>>>>> why not communicate this along with the other harness resource info as part
>>>>>>>>>>> of the job submission?
>>>>>>>>>>>
>>>>>>>>>>> Regarding use of GRPC headers: Why not make this explicit in the
>>>>>>>>>>> proto instead?
>>>>>>>>>>>
>>>>>>>>>>> WRT runner dictating resource constraints: The runner actually
>>>>>>>>>>> may also not have that information. It would need to be supplied as part of
>>>>>>>>>>> the pipeline options? The cluster resource manager needs to allocate
>>>>>>>>>>> resources for both, the runner and the SDK harness(es).
>>>>>>>>>>>
>>>>>>>>>>> Finally, what can be done to unblock the Flink runner / Python
>>>>>>>>>>> until solution discussed here is in place? An extra runner option for SDK
>>>>>>>>>>> singleton on/off?
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On Sat, Aug 18, 2018 at 1:34 AM Ankur Goenka <go...@google.com>
>>>>>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> Sounds good to me.
>>>>>>>>>>>> GRPC Header of the control channel seems to be a good place to
>>>>>>>>>>>> add upper bound information.
>>>>>>>>>>>> Added jiras:
>>>>>>>>>>>> https://issues.apache.org/jira/browse/BEAM-5166
>>>>>>>>>>>> https://issues.apache.org/jira/browse/BEAM-5167
>>>>>>>>>>>>
>>>>>>>>>>>> On Fri, Aug 17, 2018 at 10:51 PM Henning Rohde <
>>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Regarding resources: the runner can currently dictate the
>>>>>>>>>>>>> mem/cpu/disk resources that the harness is allowed to use via the
>>>>>>>>>>>>> provisioning api. The SDK harness need not -- and should not -- speculate
>>>>>>>>>>>>> on what else might be running on the machine:
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> https://github.com/apache/beam/blob/0e14965707b5d48a3de7fa69f09d88ef0aa48c09/model/fn-execution/src/main/proto/beam_provision_api.proto#L69
>>>>>>>>>>>>>
>>>>>>>>>>>>> A realistic startup-time computation in the SDK harness would
>>>>>>>>>>>>> be something simple like: max(1, min(cpu*100, mem_mb/10)) say, and use that
>>>>>>>>>>>>> at most number of threads. Or just hardcode to 300. Or a user-provided
>>>>>>>>>>>>> value. Whatever the value is the maximum number of bundles in flight
>>>>>>>>>>>>> allowed at any given time and needs to be communicated to the runner via
>>>>>>>>>>>>> some message. Anything beyond would be rejected (but this shouldn't happen,
>>>>>>>>>>>>> because the runner should respect that number).
>>>>>>>>>>>>>
>>>>>>>>>>>>> A dynamic computation would use the same limits from the SDK,
>>>>>>>>>>>>> but take into account its own resource usage (incl. the usage by running
>>>>>>>>>>>>> bundles).
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 6:20 PM Ankur Goenka <
>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> I am thinking upper bound to be more on the lines of
>>>>>>>>>>>>>> theocratical upper limit or any other static high value beyond which the
>>>>>>>>>>>>>> SDK will reject bundle verbosely. The idea is that SDK will not keep
>>>>>>>>>>>>>> bundles in queue while waiting on current bundles to finish. It will simply
>>>>>>>>>>>>>> reject any additional bundle.
>>>>>>>>>>>>>> Beyond this I don't have a good answer to dynamic upper
>>>>>>>>>>>>>> bound. As SDK does not have the complete picture of processes on the
>>>>>>>>>>>>>> machine with which it share resources, resources might not be a good proxy
>>>>>>>>>>>>>> for upper bound from the SDK point of view.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 6:01 PM Lukasz Cwik <lc...@google.com>
>>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Ankur, how would you expect an SDK to compute a realistic
>>>>>>>>>>>>>>> upper bound (upfront or during pipeline computation)?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> First thought that came to my mind was that the SDK would
>>>>>>>>>>>>>>> provide CPU/memory/... resourcing information and the runner making a
>>>>>>>>>>>>>>> judgement call as to whether it should ask the SDK to do more work or less
>>>>>>>>>>>>>>> but its not an explicit don't do more then X bundles in parallel.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 5:55 PM Ankur Goenka <
>>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Makes sense. Having exposed upper bound on concurrency with
>>>>>>>>>>>>>>>> optimum concurrency can give a good balance. This is good information to
>>>>>>>>>>>>>>>> expose while keeping the requirements from the SDK simple. SDK can publish
>>>>>>>>>>>>>>>> 1 as the optimum concurrency and upper bound to keep things simple.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Runner introspection of upper bound on concurrency is
>>>>>>>>>>>>>>>> important for correctness while introspection of optimum concurrency is
>>>>>>>>>>>>>>>> important for efficiency. This separates efficiency and correctness
>>>>>>>>>>>>>>>> requirements.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 5:05 PM Henning Rohde <
>>>>>>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I agree with Luke's observation, with the caveat that "infinite
>>>>>>>>>>>>>>>>> amount of bundles in parallel" is limited by the available resources. For
>>>>>>>>>>>>>>>>> example, the Go SDK harness will accept an arbitrary amount of parallel
>>>>>>>>>>>>>>>>> work, but too much work will cause either excessive GC pressure with
>>>>>>>>>>>>>>>>> crippling slowness or an outright OOM. Unless it's always 1, a reasonable
>>>>>>>>>>>>>>>>> upper bound will either have to be provided by the user or computed from
>>>>>>>>>>>>>>>>> the mem/cpu resources given. Of course, as some bundles takes more
>>>>>>>>>>>>>>>>> resources than others, so any static value will be an estimate or ignore
>>>>>>>>>>>>>>>>> resource limits.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> That said, I do not like that an "efficiency" aspect
>>>>>>>>>>>>>>>>> becomes a subtle requirement for correctness due to Flink internals. I fear
>>>>>>>>>>>>>>>>> that road leads to trouble.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 4:26 PM Ankur Goenka <
>>>>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> The later case of having a of supporting single bundle
>>>>>>>>>>>>>>>>>> execution at a time on SDK and runner not using this flag is exactly the
>>>>>>>>>>>>>>>>>> reason we got into the Dead Lock here.
>>>>>>>>>>>>>>>>>> I agree with exposing SDK optimum concurrency level ( 1
>>>>>>>>>>>>>>>>>> in later case ) and let runner decide to use it or not. But at the same
>>>>>>>>>>>>>>>>>> time expect SDK to handle infinite amount of bundles even if its not
>>>>>>>>>>>>>>>>>> efficient.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>> Ankur
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 4:11 PM Lukasz Cwik <
>>>>>>>>>>>>>>>>>> lcwik@google.com> wrote:
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> I believe in practice SDK harnesses will fall into one
>>>>>>>>>>>>>>>>>>> of two capabilities, can process effectively an infinite amount of bundles
>>>>>>>>>>>>>>>>>>> in parallel or can only process a single bundle at a time.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> I believe it is more difficult for a runner to handle
>>>>>>>>>>>>>>>>>>> the latter case well and to perform all the environment management that
>>>>>>>>>>>>>>>>>>> would make that efficient. It may be inefficient for an SDK but I do
>>>>>>>>>>>>>>>>>>> believe it should be able to say that I'm not great at anything more then a
>>>>>>>>>>>>>>>>>>> single bundle at a time but utilizing this information by a runner should
>>>>>>>>>>>>>>>>>>> be optional.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 1:53 PM Ankur Goenka <
>>>>>>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> To recap the discussion it seems that we have come-up
>>>>>>>>>>>>>>>>>>>> with following point.
>>>>>>>>>>>>>>>>>>>> SDKHarness Management and initialization.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>    1. Runner completely own the work assignment to
>>>>>>>>>>>>>>>>>>>>    SDKHarness.
>>>>>>>>>>>>>>>>>>>>    2. Runner should know the capabilities and capacity
>>>>>>>>>>>>>>>>>>>>    of SDKHarness and should assign work accordingly.
>>>>>>>>>>>>>>>>>>>>    3. Spinning up of SDKHarness is runner's
>>>>>>>>>>>>>>>>>>>>    responsibility and it can be done statically (a fixed pre configured number
>>>>>>>>>>>>>>>>>>>>    of SDKHarness) or dynamically or based on certain other configuration/logic
>>>>>>>>>>>>>>>>>>>>    which runner choose.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> SDKHarness Expectation. This is more in question and we
>>>>>>>>>>>>>>>>>>>> should outline the responsibility of SDKHarness.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>    1. SDKHarness should publish how many concurrent
>>>>>>>>>>>>>>>>>>>>    tasks it can execute.
>>>>>>>>>>>>>>>>>>>>    2. SDKHarness should start executing all the tasks
>>>>>>>>>>>>>>>>>>>>    items assigned in parallel in a timely manner or fail task.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Also to add to simplification side. I think for better
>>>>>>>>>>>>>>>>>>>> adoption, we should have simple SDKHarness as well as simple Runner
>>>>>>>>>>>>>>>>>>>> integration to encourage integration with more runner. Also many runners
>>>>>>>>>>>>>>>>>>>> might not expose some of the internal scheduling characteristics so we
>>>>>>>>>>>>>>>>>>>> should not expect scheduling characteristics for runner integration.
>>>>>>>>>>>>>>>>>>>> Moreover scheduling characteristics can change based on pipeline type,
>>>>>>>>>>>>>>>>>>>> infrastructure, available resource etc. So I am a bit hesitant to add
>>>>>>>>>>>>>>>>>>>> runner scheduling specifics for runner integration.
>>>>>>>>>>>>>>>>>>>> A good balance between SDKHarness complexity and Runner
>>>>>>>>>>>>>>>>>>>> integration can be helpful in easier adoption.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>>>> Ankur
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 12:22 PM Henning Rohde <
>>>>>>>>>>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> Finding a good balance is indeed the art of
>>>>>>>>>>>>>>>>>>>>> portability, because the range of capability (and assumptions) on both
>>>>>>>>>>>>>>>>>>>>> sides is wide.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> It was originally the idea to allow the SDK harness to
>>>>>>>>>>>>>>>>>>>>> be an extremely simple bundle executer (specifically, single-threaded
>>>>>>>>>>>>>>>>>>>>> execution one instruction at a time) however inefficient -- a more
>>>>>>>>>>>>>>>>>>>>> sophisticated SDK harness would support more features and be more
>>>>>>>>>>>>>>>>>>>>> efficient. For the issue described here, it seems problematic to me to send
>>>>>>>>>>>>>>>>>>>>> non-executable bundles to the SDK harness under the expectation that the
>>>>>>>>>>>>>>>>>>>>> SDK harness will concurrently work its way deeply enough down the
>>>>>>>>>>>>>>>>>>>>> instruction queue to unblock itself. That would be an extremely subtle
>>>>>>>>>>>>>>>>>>>>> requirement for SDK authors and one practical
>>>>>>>>>>>>>>>>>>>>> question becomes: what should an SDK do with a bundle instruction that it
>>>>>>>>>>>>>>>>>>>>> doesn't have capacity to execute? If a runner needs
>>>>>>>>>>>>>>>>>>>>> to make such assumptions, I think that information should probably rather
>>>>>>>>>>>>>>>>>>>>> be explicit along the lines of proposal 1 -- i.e., some kind of negotiation
>>>>>>>>>>>>>>>>>>>>> between resources allotted to the SDK harness (a preliminary variant are in
>>>>>>>>>>>>>>>>>>>>> the provisioning api) and what the SDK harness in return can do (and a
>>>>>>>>>>>>>>>>>>>>> valid answer might be: 1 bundle at a time irrespectively of resources
>>>>>>>>>>>>>>>>>>>>> given) or a per-bundle special "overloaded" error response. For other
>>>>>>>>>>>>>>>>>>>>> aspects, such as side input readiness, the runner handles that complexity
>>>>>>>>>>>>>>>>>>>>> and the overall bias has generally been to move complexity to the runner
>>>>>>>>>>>>>>>>>>>>> side.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> The SDK harness and initialization overhead is
>>>>>>>>>>>>>>>>>>>>> entirely SDK, job type and even pipeline specific. A docker container is
>>>>>>>>>>>>>>>>>>>>> also just a process, btw, and doesn't inherently carry much overhead. That
>>>>>>>>>>>>>>>>>>>>> said, on a single host, a static docker configuration is generally a lot
>>>>>>>>>>>>>>>>>>>>> simpler to work with.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> Henning
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 10:18 AM Thomas Weise <
>>>>>>>>>>>>>>>>>>>>> thw@apache.org> wrote:
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> It is good to see this discussed!
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> I think there needs to be a good balance between the
>>>>>>>>>>>>>>>>>>>>>> SDK harness capabilities/complexity and responsibilities. Additionally the
>>>>>>>>>>>>>>>>>>>>>> user will need to be able to adjust the runner behavior, since the type of
>>>>>>>>>>>>>>>>>>>>>> workload executed in the harness also is a factor. Elsewhere we already
>>>>>>>>>>>>>>>>>>>>>> discussed that the current assumption of a single SDK harness instance per
>>>>>>>>>>>>>>>>>>>>>> Flink task manager brings problems with it and that there needs to be more
>>>>>>>>>>>>>>>>>>>>>> than one way how the runner can spin up SDK harnesses.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> There was the concern that instantiation if multiple
>>>>>>>>>>>>>>>>>>>>>> SDK harnesses per TM host is expensive (resource usage, initialization time
>>>>>>>>>>>>>>>>>>>>>> etc.). That may hold true for a specific scenario, such as batch workloads
>>>>>>>>>>>>>>>>>>>>>> and the use of Docker containers. But it may look totally different for a
>>>>>>>>>>>>>>>>>>>>>> streaming topology or when SDK harness is just a process on the same host.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>>>>>> Thomas
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 8:36 AM Lukasz Cwik <
>>>>>>>>>>>>>>>>>>>>>> lcwik@google.com> wrote:
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> SDK harnesses were always responsible for executing
>>>>>>>>>>>>>>>>>>>>>>> all work given to it concurrently. Runners have been responsible for
>>>>>>>>>>>>>>>>>>>>>>> choosing how much work to give to the SDK harness in such a way that best
>>>>>>>>>>>>>>>>>>>>>>> utilizes the SDK harness.
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> I understand that multithreading in python is
>>>>>>>>>>>>>>>>>>>>>>> inefficient due to the global interpreter lock, it would be upto the runner
>>>>>>>>>>>>>>>>>>>>>>> in this case to make sure that the amount of work it gives to each SDK
>>>>>>>>>>>>>>>>>>>>>>> harness best utilizes it while spinning up an appropriate number of SDK
>>>>>>>>>>>>>>>>>>>>>>> harnesses.
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 7:32 AM Maximilian Michels <
>>>>>>>>>>>>>>>>>>>>>>> mxm@apache.org> wrote:
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>> Hi Ankur,
>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>> Thanks for looking into this problem. The cause
>>>>>>>>>>>>>>>>>>>>>>>> seems to be Flink's
>>>>>>>>>>>>>>>>>>>>>>>> pipelined execution mode. It runs multiple tasks in
>>>>>>>>>>>>>>>>>>>>>>>> one task slot and
>>>>>>>>>>>>>>>>>>>>>>>> produces a deadlock when the pipelined operators
>>>>>>>>>>>>>>>>>>>>>>>> schedule the SDK
>>>>>>>>>>>>>>>>>>>>>>>> harness DoFns in non-topological order.
>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>> The problem would be resolved if we scheduled the
>>>>>>>>>>>>>>>>>>>>>>>> tasks in topological
>>>>>>>>>>>>>>>>>>>>>>>> order. Doing that is not easy because they run in
>>>>>>>>>>>>>>>>>>>>>>>> separate Flink
>>>>>>>>>>>>>>>>>>>>>>>> operators and the SDK Harness would have to have
>>>>>>>>>>>>>>>>>>>>>>>> insight into the
>>>>>>>>>>>>>>>>>>>>>>>> execution graph (which is not desirable).
>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>> The easiest method, which you proposed in 1) is to
>>>>>>>>>>>>>>>>>>>>>>>> ensure that the
>>>>>>>>>>>>>>>>>>>>>>>> number of threads in the SDK harness matches the
>>>>>>>>>>>>>>>>>>>>>>>> number of
>>>>>>>>>>>>>>>>>>>>>>>> ExecutableStage DoFn operators.
>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>> The approach in 2) is what Flink does as well. It
>>>>>>>>>>>>>>>>>>>>>>>> glues together
>>>>>>>>>>>>>>>>>>>>>>>> horizontal parts of the execution graph, also in
>>>>>>>>>>>>>>>>>>>>>>>> multiple threads. So I
>>>>>>>>>>>>>>>>>>>>>>>> agree with your proposed solution.
>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>> Best,
>>>>>>>>>>>>>>>>>>>>>>>> Max
>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>> On 17.08.18 03:10, Ankur Goenka wrote:
>>>>>>>>>>>>>>>>>>>>>>>> > Hi,
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > tl;dr Dead Lock in task execution caused by
>>>>>>>>>>>>>>>>>>>>>>>> limited task parallelism on
>>>>>>>>>>>>>>>>>>>>>>>> > SDKHarness.
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > *Setup:*
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> >   * Job type: /*Beam Portable Python Batch*/ Job
>>>>>>>>>>>>>>>>>>>>>>>> on Flink standalone
>>>>>>>>>>>>>>>>>>>>>>>> >     cluster.
>>>>>>>>>>>>>>>>>>>>>>>> >   * Only a single job is scheduled on the cluster.
>>>>>>>>>>>>>>>>>>>>>>>> >   * Everything is running on a single machine
>>>>>>>>>>>>>>>>>>>>>>>> with single Flink task
>>>>>>>>>>>>>>>>>>>>>>>> >     manager.
>>>>>>>>>>>>>>>>>>>>>>>> >   * Flink Task Manager Slots is 1.
>>>>>>>>>>>>>>>>>>>>>>>> >   * Flink Parallelism is 1.
>>>>>>>>>>>>>>>>>>>>>>>> >   * Python SDKHarness has 1 thread.
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > *Example pipeline:*
>>>>>>>>>>>>>>>>>>>>>>>> > Read -> MapA -> GroupBy -> MapB -> WriteToSink
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > *Issue:*
>>>>>>>>>>>>>>>>>>>>>>>> > With multi stage job, Flink schedule different
>>>>>>>>>>>>>>>>>>>>>>>> dependent sub tasks
>>>>>>>>>>>>>>>>>>>>>>>> > concurrently on Flink worker as long as it can
>>>>>>>>>>>>>>>>>>>>>>>> get slots. Each map tasks
>>>>>>>>>>>>>>>>>>>>>>>> > are then executed on SDKHarness.
>>>>>>>>>>>>>>>>>>>>>>>> > Its possible that MapB gets to SDKHarness before
>>>>>>>>>>>>>>>>>>>>>>>> MapA and hence gets
>>>>>>>>>>>>>>>>>>>>>>>> > into the execution queue before MapA. Because we
>>>>>>>>>>>>>>>>>>>>>>>> only have 1 execution
>>>>>>>>>>>>>>>>>>>>>>>> > thread on SDKHarness, MapA will never get a
>>>>>>>>>>>>>>>>>>>>>>>> chance to execute as MapB
>>>>>>>>>>>>>>>>>>>>>>>> > will never release the execution thread. MapB
>>>>>>>>>>>>>>>>>>>>>>>> will wait for input from
>>>>>>>>>>>>>>>>>>>>>>>> > MapA. This gets us to a dead lock in a simple
>>>>>>>>>>>>>>>>>>>>>>>> pipeline.
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > *Mitigation:*
>>>>>>>>>>>>>>>>>>>>>>>> > Set worker_count in pipeline options more than
>>>>>>>>>>>>>>>>>>>>>>>> the expected sub tasks
>>>>>>>>>>>>>>>>>>>>>>>> > in pipeline.
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > *Proposal:*
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> >  1. We can get the maximum concurrency from the
>>>>>>>>>>>>>>>>>>>>>>>> runner and make sure
>>>>>>>>>>>>>>>>>>>>>>>> >     that we have more threads than max
>>>>>>>>>>>>>>>>>>>>>>>> concurrency. This approach
>>>>>>>>>>>>>>>>>>>>>>>> >     assumes that Beam has insight into runner
>>>>>>>>>>>>>>>>>>>>>>>> execution plan and can
>>>>>>>>>>>>>>>>>>>>>>>> >     make decision based on it.
>>>>>>>>>>>>>>>>>>>>>>>> >  2. We dynamically create thread and cache them
>>>>>>>>>>>>>>>>>>>>>>>> with a high upper bound
>>>>>>>>>>>>>>>>>>>>>>>> >     in SDKHarness. We can warn if we are hitting
>>>>>>>>>>>>>>>>>>>>>>>> the upper bound of
>>>>>>>>>>>>>>>>>>>>>>>> >     threads. This approach assumes that runner
>>>>>>>>>>>>>>>>>>>>>>>> does a good job of
>>>>>>>>>>>>>>>>>>>>>>>> >     scheduling and will distribute tasks more or
>>>>>>>>>>>>>>>>>>>>>>>> less evenly.
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > We expect good scheduling from runners so I
>>>>>>>>>>>>>>>>>>>>>>>> prefer approach 2. It is
>>>>>>>>>>>>>>>>>>>>>>>> > simpler to implement and the implementation is
>>>>>>>>>>>>>>>>>>>>>>>> not runner specific. This
>>>>>>>>>>>>>>>>>>>>>>>> > approach better utilize resource as it creates
>>>>>>>>>>>>>>>>>>>>>>>> only as many threads as
>>>>>>>>>>>>>>>>>>>>>>>> > needed instead of the peak thread requirement.
>>>>>>>>>>>>>>>>>>>>>>>> > And last but not the least, it gives runner
>>>>>>>>>>>>>>>>>>>>>>>> control over managing truly
>>>>>>>>>>>>>>>>>>>>>>>> > active tasks.
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > Please let me know if I am missing something and
>>>>>>>>>>>>>>>>>>>>>>>> your thoughts on the
>>>>>>>>>>>>>>>>>>>>>>>> > approach.
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>> > Thanks,
>>>>>>>>>>>>>>>>>>>>>>>> > Ankur
>>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>

Re: Discussion: Scheduling across runner and SDKHarness in Portability framework

[Thomas Weise <th...@apache.org>]

That's actually how the Flink runner already works - bundle processing
starts when elements are available (see FlinkExecutableStageFunction for
batch mode).

But we still have the possibility of the SDK getting concurrent requests
due to parallelism (and pipelined execution).

Thanks,
Thomas


On Fri, Sep 14, 2018 at 2:56 AM Robert Bradshaw <ro...@google.com> wrote:

> Currently the best solution we've come up with is that we must process an
> unbounded number of bundles concurrently to avoid deadlock. Especially in
> the batch case, this may be wasteful as we bring up workers for many stages
> that are not actually executable until upstream stages finish. Since it may
> be invasive to require runners to only schedule stages that can be actively
> worked on, I've been thinking about what we could do in the common runner
> libraries themselves. One idea is to postpone the actual sending of a
> process bundle request until there is data on the channel to consume. With
> Reads as Impulses, and triggers as data, all bundles are driven by some
> input.
>
> This would mean we never ask the SDK to process bundles it cannot
> immediately start working on. There is still the open question of whether
> being able to *start* a bundle implies that one is able to *finish* a
> bundle (i.e. do any runners start bundles and then block, pending other
> bundle completion, before closing the data channel (though clearly a runner
> can chop a bundle off at any point if it wants)).
>
> Does this approach sound feasible?
>
>
> On Thu, Aug 30, 2018 at 2:54 AM Ankur Goenka <go...@google.com> wrote:
>
>> I managed to write a small document based on the discussion.
>> Please take a look at
>> https://docs.google.com/document/d/1oAXVPbJ0dzj2_8LXEWFAgqCP5Tpld3q5B3QU254PQ6A/edit?usp=sharing
>>
>>
>> On Tue, Aug 21, 2018 at 11:01 PM Henning Rohde <he...@google.com>
>> wrote:
>>
>>> Sending bundles that cannot be executed, i.e., the situation described
>>> to cause deadlock in Flink in the beginning of the thread with mapB. The
>>> discussion of exposing (or assuming an infinitely large) concurrency level
>>> -- while a useful concept in its own right -- came around as a way to
>>> unblock mapB.
>>>
>>> On Tue, Aug 21, 2018 at 2:16 PM Lukasz Cwik <lc...@google.com> wrote:
>>>
>>>> Henning, can you clarify by what you mean with send non-executable
>>>> bundles to the SDK harness and how it is useful for Flink?
>>>>
>>>> On Tue, Aug 21, 2018 at 2:01 PM Henning Rohde <he...@google.com>
>>>> wrote:
>>>>
>>>>> I think it will be useful to the runner to know upfront what the
>>>>> fundamental threading capabilities are for the SDK harness (say, "fixed",
>>>>> "linear", "dynamic", ..) so that the runner can upfront make a good static
>>>>> decision on #harnesses and how many resources they should each have. It's
>>>>> wasteful to give the Foo SDK a whole many-core machine with TBs of memory,
>>>>> if it can only support a single bundle at a time. I think this is also in
>>>>> line with what Thomas and Luke are suggesting.
>>>>>
>>>>> However, it still seems to me to be a semantically problematic idea to
>>>>> send non-executable bundles to the SDK harness. I understand it's useful
>>>>> for Flink, but is that really the best path forward?
>>>>>
>>>>>
>>>>>
>>>>> On Mon, Aug 20, 2018 at 5:44 PM Ankur Goenka <go...@google.com>
>>>>> wrote:
>>>>>
>>>>>> That's right.
>>>>>> To add to it. We added multi threading to python streaming as a
>>>>>> single thread is sub optimal for streaming use case.
>>>>>> Shall we move towards a conclusion on the SDK bundle processing upper
>>>>>> bound?
>>>>>>
>>>>>> On Mon, Aug 20, 2018 at 1:54 PM Lukasz Cwik <lc...@google.com> wrote:
>>>>>>
>>>>>>> Ankur, I can see where you are going with your argument. I believe
>>>>>>> there is certain information which is static and won't change at pipeline
>>>>>>> creation time (such as Python SDK is most efficient doing one bundle at a
>>>>>>> time) and some stuff which is best at runtime, like memory and CPU limits,
>>>>>>> worker count.
>>>>>>>
>>>>>>> On Mon, Aug 20, 2018 at 1:47 PM Ankur Goenka <go...@google.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> I would prefer to to keep it dynamic as it can be changed by the
>>>>>>>> infrastructure or the pipeline author.
>>>>>>>> Like in case of Python, number of concurrent bundle can be changed
>>>>>>>> by setting pipeline option worker_count. And for Java it can be computed
>>>>>>>> based on the cpus on the machine.
>>>>>>>>
>>>>>>>> For Flink runner, we can use the worker_count parameter for now to
>>>>>>>> increase the parallelism. And we can have 1 container for each mapPartition
>>>>>>>> task on Flink while reusing containers as container creation is expensive
>>>>>>>> especially for Python where it installs a bunch of dependencies. There is 1
>>>>>>>> caveat though. I have seen machine crashes because of too many simultaneous
>>>>>>>> container creation. We can rate limit container creation in the code to
>>>>>>>> avoid this.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Ankur
>>>>>>>>
>>>>>>>> On Mon, Aug 20, 2018 at 9:20 AM Lukasz Cwik <lc...@google.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> +1 on making the resources part of a proto. Based upon what
>>>>>>>>> Henning linked to, the provisioning API seems like an appropriate place to
>>>>>>>>> provide this information.
>>>>>>>>>
>>>>>>>>> Thomas, I believe the environment proto is the best place to add
>>>>>>>>> information that a runner may want to know about upfront during pipeline
>>>>>>>>> pipeline creation. I wouldn't stick this into PipelineOptions for the long
>>>>>>>>> term.
>>>>>>>>> If you have time to capture these thoughts and update the
>>>>>>>>> environment proto, I would suggest going down that path. Otherwise anything
>>>>>>>>> short term like PipelineOptions will do.
>>>>>>>>>
>>>>>>>>> On Sun, Aug 19, 2018 at 5:41 PM Thomas Weise <th...@apache.org>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> For SDKs where the upper limit is constant and known upfront, why
>>>>>>>>>> not communicate this along with the other harness resource info as part of
>>>>>>>>>> the job submission?
>>>>>>>>>>
>>>>>>>>>> Regarding use of GRPC headers: Why not make this explicit in the
>>>>>>>>>> proto instead?
>>>>>>>>>>
>>>>>>>>>> WRT runner dictating resource constraints: The runner actually
>>>>>>>>>> may also not have that information. It would need to be supplied as part of
>>>>>>>>>> the pipeline options? The cluster resource manager needs to allocate
>>>>>>>>>> resources for both, the runner and the SDK harness(es).
>>>>>>>>>>
>>>>>>>>>> Finally, what can be done to unblock the Flink runner / Python
>>>>>>>>>> until solution discussed here is in place? An extra runner option for SDK
>>>>>>>>>> singleton on/off?
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On Sat, Aug 18, 2018 at 1:34 AM Ankur Goenka <go...@google.com>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> Sounds good to me.
>>>>>>>>>>> GRPC Header of the control channel seems to be a good place to
>>>>>>>>>>> add upper bound information.
>>>>>>>>>>> Added jiras:
>>>>>>>>>>> https://issues.apache.org/jira/browse/BEAM-5166
>>>>>>>>>>> https://issues.apache.org/jira/browse/BEAM-5167
>>>>>>>>>>>
>>>>>>>>>>> On Fri, Aug 17, 2018 at 10:51 PM Henning Rohde <
>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> Regarding resources: the runner can currently dictate the
>>>>>>>>>>>> mem/cpu/disk resources that the harness is allowed to use via the
>>>>>>>>>>>> provisioning api. The SDK harness need not -- and should not -- speculate
>>>>>>>>>>>> on what else might be running on the machine:
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> https://github.com/apache/beam/blob/0e14965707b5d48a3de7fa69f09d88ef0aa48c09/model/fn-execution/src/main/proto/beam_provision_api.proto#L69
>>>>>>>>>>>>
>>>>>>>>>>>> A realistic startup-time computation in the SDK harness would
>>>>>>>>>>>> be something simple like: max(1, min(cpu*100, mem_mb/10)) say, and use that
>>>>>>>>>>>> at most number of threads. Or just hardcode to 300. Or a user-provided
>>>>>>>>>>>> value. Whatever the value is the maximum number of bundles in flight
>>>>>>>>>>>> allowed at any given time and needs to be communicated to the runner via
>>>>>>>>>>>> some message. Anything beyond would be rejected (but this shouldn't happen,
>>>>>>>>>>>> because the runner should respect that number).
>>>>>>>>>>>>
>>>>>>>>>>>> A dynamic computation would use the same limits from the SDK,
>>>>>>>>>>>> but take into account its own resource usage (incl. the usage by running
>>>>>>>>>>>> bundles).
>>>>>>>>>>>>
>>>>>>>>>>>> On Fri, Aug 17, 2018 at 6:20 PM Ankur Goenka <go...@google.com>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> I am thinking upper bound to be more on the lines of
>>>>>>>>>>>>> theocratical upper limit or any other static high value beyond which the
>>>>>>>>>>>>> SDK will reject bundle verbosely. The idea is that SDK will not keep
>>>>>>>>>>>>> bundles in queue while waiting on current bundles to finish. It will simply
>>>>>>>>>>>>> reject any additional bundle.
>>>>>>>>>>>>> Beyond this I don't have a good answer to dynamic upper bound.
>>>>>>>>>>>>> As SDK does not have the complete picture of processes on the machine with
>>>>>>>>>>>>> which it share resources, resources might not be a good proxy for upper
>>>>>>>>>>>>> bound from the SDK point of view.
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 6:01 PM Lukasz Cwik <lc...@google.com>
>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> Ankur, how would you expect an SDK to compute a realistic
>>>>>>>>>>>>>> upper bound (upfront or during pipeline computation)?
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> First thought that came to my mind was that the SDK would
>>>>>>>>>>>>>> provide CPU/memory/... resourcing information and the runner making a
>>>>>>>>>>>>>> judgement call as to whether it should ask the SDK to do more work or less
>>>>>>>>>>>>>> but its not an explicit don't do more then X bundles in parallel.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 5:55 PM Ankur Goenka <
>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Makes sense. Having exposed upper bound on concurrency with
>>>>>>>>>>>>>>> optimum concurrency can give a good balance. This is good information to
>>>>>>>>>>>>>>> expose while keeping the requirements from the SDK simple. SDK can publish
>>>>>>>>>>>>>>> 1 as the optimum concurrency and upper bound to keep things simple.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Runner introspection of upper bound on concurrency is
>>>>>>>>>>>>>>> important for correctness while introspection of optimum concurrency is
>>>>>>>>>>>>>>> important for efficiency. This separates efficiency and correctness
>>>>>>>>>>>>>>> requirements.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 5:05 PM Henning Rohde <
>>>>>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> I agree with Luke's observation, with the caveat that "infinite
>>>>>>>>>>>>>>>> amount of bundles in parallel" is limited by the available resources. For
>>>>>>>>>>>>>>>> example, the Go SDK harness will accept an arbitrary amount of parallel
>>>>>>>>>>>>>>>> work, but too much work will cause either excessive GC pressure with
>>>>>>>>>>>>>>>> crippling slowness or an outright OOM. Unless it's always 1, a reasonable
>>>>>>>>>>>>>>>> upper bound will either have to be provided by the user or computed from
>>>>>>>>>>>>>>>> the mem/cpu resources given. Of course, as some bundles takes more
>>>>>>>>>>>>>>>> resources than others, so any static value will be an estimate or ignore
>>>>>>>>>>>>>>>> resource limits.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> That said, I do not like that an "efficiency" aspect
>>>>>>>>>>>>>>>> becomes a subtle requirement for correctness due to Flink internals. I fear
>>>>>>>>>>>>>>>> that road leads to trouble.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 4:26 PM Ankur Goenka <
>>>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> The later case of having a of supporting single bundle
>>>>>>>>>>>>>>>>> execution at a time on SDK and runner not using this flag is exactly the
>>>>>>>>>>>>>>>>> reason we got into the Dead Lock here.
>>>>>>>>>>>>>>>>> I agree with exposing SDK optimum concurrency level ( 1 in
>>>>>>>>>>>>>>>>> later case ) and let runner decide to use it or not. But at the same time
>>>>>>>>>>>>>>>>> expect SDK to handle infinite amount of bundles even if its not efficient.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>> Ankur
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 4:11 PM Lukasz Cwik <
>>>>>>>>>>>>>>>>> lcwik@google.com> wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I believe in practice SDK harnesses will fall into one of
>>>>>>>>>>>>>>>>>> two capabilities, can process effectively an infinite amount of bundles in
>>>>>>>>>>>>>>>>>> parallel or can only process a single bundle at a time.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I believe it is more difficult for a runner to handle the
>>>>>>>>>>>>>>>>>> latter case well and to perform all the environment management that would
>>>>>>>>>>>>>>>>>> make that efficient. It may be inefficient for an SDK but I do believe it
>>>>>>>>>>>>>>>>>> should be able to say that I'm not great at anything more then a single
>>>>>>>>>>>>>>>>>> bundle at a time but utilizing this information by a runner should be
>>>>>>>>>>>>>>>>>> optional.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 1:53 PM Ankur Goenka <
>>>>>>>>>>>>>>>>>> goenka@google.com> wrote:
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> To recap the discussion it seems that we have come-up
>>>>>>>>>>>>>>>>>>> with following point.
>>>>>>>>>>>>>>>>>>> SDKHarness Management and initialization.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>    1. Runner completely own the work assignment to
>>>>>>>>>>>>>>>>>>>    SDKHarness.
>>>>>>>>>>>>>>>>>>>    2. Runner should know the capabilities and capacity
>>>>>>>>>>>>>>>>>>>    of SDKHarness and should assign work accordingly.
>>>>>>>>>>>>>>>>>>>    3. Spinning up of SDKHarness is runner's
>>>>>>>>>>>>>>>>>>>    responsibility and it can be done statically (a fixed pre configured number
>>>>>>>>>>>>>>>>>>>    of SDKHarness) or dynamically or based on certain other configuration/logic
>>>>>>>>>>>>>>>>>>>    which runner choose.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> SDKHarness Expectation. This is more in question and we
>>>>>>>>>>>>>>>>>>> should outline the responsibility of SDKHarness.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>    1. SDKHarness should publish how many concurrent
>>>>>>>>>>>>>>>>>>>    tasks it can execute.
>>>>>>>>>>>>>>>>>>>    2. SDKHarness should start executing all the tasks
>>>>>>>>>>>>>>>>>>>    items assigned in parallel in a timely manner or fail task.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Also to add to simplification side. I think for better
>>>>>>>>>>>>>>>>>>> adoption, we should have simple SDKHarness as well as simple Runner
>>>>>>>>>>>>>>>>>>> integration to encourage integration with more runner. Also many runners
>>>>>>>>>>>>>>>>>>> might not expose some of the internal scheduling characteristics so we
>>>>>>>>>>>>>>>>>>> should not expect scheduling characteristics for runner integration.
>>>>>>>>>>>>>>>>>>> Moreover scheduling characteristics can change based on pipeline type,
>>>>>>>>>>>>>>>>>>> infrastructure, available resource etc. So I am a bit hesitant to add
>>>>>>>>>>>>>>>>>>> runner scheduling specifics for runner integration.
>>>>>>>>>>>>>>>>>>> A good balance between SDKHarness complexity and Runner
>>>>>>>>>>>>>>>>>>> integration can be helpful in easier adoption.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>>> Ankur
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 12:22 PM Henning Rohde <
>>>>>>>>>>>>>>>>>>> herohde@google.com> wrote:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Finding a good balance is indeed the art of
>>>>>>>>>>>>>>>>>>>> portability, because the range of capability (and assumptions) on both
>>>>>>>>>>>>>>>>>>>> sides is wide.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> It was originally the idea to allow the SDK harness to
>>>>>>>>>>>>>>>>>>>> be an extremely simple bundle executer (specifically, single-threaded
>>>>>>>>>>>>>>>>>>>> execution one instruction at a time) however inefficient -- a more
>>>>>>>>>>>>>>>>>>>> sophisticated SDK harness would support more features and be more
>>>>>>>>>>>>>>>>>>>> efficient. For the issue described here, it seems problematic to me to send
>>>>>>>>>>>>>>>>>>>> non-executable bundles to the SDK harness under the expectation that the
>>>>>>>>>>>>>>>>>>>> SDK harness will concurrently work its way deeply enough down the
>>>>>>>>>>>>>>>>>>>> instruction queue to unblock itself. That would be an extremely subtle
>>>>>>>>>>>>>>>>>>>> requirement for SDK authors and one practical question
>>>>>>>>>>>>>>>>>>>> becomes: what should an SDK do with a bundle instruction that it doesn't
>>>>>>>>>>>>>>>>>>>> have capacity to execute? If a runner needs to make
>>>>>>>>>>>>>>>>>>>> such assumptions, I think that information should probably rather be
>>>>>>>>>>>>>>>>>>>> explicit along the lines of proposal 1 -- i.e., some kind of negotiation
>>>>>>>>>>>>>>>>>>>> between resources allotted to the SDK harness (a preliminary variant are in
>>>>>>>>>>>>>>>>>>>> the provisioning api) and what the SDK harness in return can do (and a
>>>>>>>>>>>>>>>>>>>> valid answer might be: 1 bundle at a time irrespectively of resources
>>>>>>>>>>>>>>>>>>>> given) or a per-bundle special "overloaded" error response. For other
>>>>>>>>>>>>>>>>>>>> aspects, such as side input readiness, the runner handles that complexity
>>>>>>>>>>>>>>>>>>>> and the overall bias has generally been to move complexity to the runner
>>>>>>>>>>>>>>>>>>>> side.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> The SDK harness and initialization overhead is entirely
>>>>>>>>>>>>>>>>>>>> SDK, job type and even pipeline specific. A docker container is also just a
>>>>>>>>>>>>>>>>>>>> process, btw, and doesn't inherently carry much overhead. That said, on a
>>>>>>>>>>>>>>>>>>>> single host, a static docker configuration is generally a lot simpler to
>>>>>>>>>>>>>>>>>>>> work with.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Henning
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 10:18 AM Thomas Weise <
>>>>>>>>>>>>>>>>>>>> thw@apache.org> wrote:
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> It is good to see this discussed!
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> I think there needs to be a good balance between the
>>>>>>>>>>>>>>>>>>>>> SDK harness capabilities/complexity and responsibilities. Additionally the
>>>>>>>>>>>>>>>>>>>>> user will need to be able to adjust the runner behavior, since the type of
>>>>>>>>>>>>>>>>>>>>> workload executed in the harness also is a factor. Elsewhere we already
>>>>>>>>>>>>>>>>>>>>> discussed that the current assumption of a single SDK harness instance per
>>>>>>>>>>>>>>>>>>>>> Flink task manager brings problems with it and that there needs to be more
>>>>>>>>>>>>>>>>>>>>> than one way how the runner can spin up SDK harnesses.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> There was the concern that instantiation if multiple
>>>>>>>>>>>>>>>>>>>>> SDK harnesses per TM host is expensive (resource usage, initialization time
>>>>>>>>>>>>>>>>>>>>> etc.). That may hold true for a specific scenario, such as batch workloads
>>>>>>>>>>>>>>>>>>>>> and the use of Docker containers. But it may look totally different for a
>>>>>>>>>>>>>>>>>>>>> streaming topology or when SDK harness is just a process on the same host.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>>>>> Thomas
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 8:36 AM Lukasz Cwik <
>>>>>>>>>>>>>>>>>>>>> lcwik@google.com> wrote:
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> SDK harnesses were always responsible for executing
>>>>>>>>>>>>>>>>>>>>>> all work given to it concurrently. Runners have been responsible for
>>>>>>>>>>>>>>>>>>>>>> choosing how much work to give to the SDK harness in such a way that best
>>>>>>>>>>>>>>>>>>>>>> utilizes the SDK harness.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> I understand that multithreading in python is
>>>>>>>>>>>>>>>>>>>>>> inefficient due to the global interpreter lock, it would be upto the runner
>>>>>>>>>>>>>>>>>>>>>> in this case to make sure that the amount of work it gives to each SDK
>>>>>>>>>>>>>>>>>>>>>> harness best utilizes it while spinning up an appropriate number of SDK
>>>>>>>>>>>>>>>>>>>>>> harnesses.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> On Fri, Aug 17, 2018 at 7:32 AM Maximilian Michels <
>>>>>>>>>>>>>>>>>>>>>> mxm@apache.org> wrote:
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> Hi Ankur,
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> Thanks for looking into this problem. The cause
>>>>>>>>>>>>>>>>>>>>>>> seems to be Flink's
>>>>>>>>>>>>>>>>>>>>>>> pipelined execution mode. It runs multiple tasks in
>>>>>>>>>>>>>>>>>>>>>>> one task slot and
>>>>>>>>>>>>>>>>>>>>>>> produces a deadlock when the pipelined operators
>>>>>>>>>>>>>>>>>>>>>>> schedule the SDK
>>>>>>>>>>>>>>>>>>>>>>> harness DoFns in non-topological order.
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> The problem would be resolved if we scheduled the
>>>>>>>>>>>>>>>>>>>>>>> tasks in topological
>>>>>>>>>>>>>>>>>>>>>>> order. Doing that is not easy because they run in
>>>>>>>>>>>>>>>>>>>>>>> separate Flink
>>>>>>>>>>>>>>>>>>>>>>> operators and the SDK Harness would have to have
>>>>>>>>>>>>>>>>>>>>>>> insight into the
>>>>>>>>>>>>>>>>>>>>>>> execution graph (which is not desirable).
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> The easiest method, which you proposed in 1) is to
>>>>>>>>>>>>>>>>>>>>>>> ensure that the
>>>>>>>>>>>>>>>>>>>>>>> number of threads in the SDK harness matches the
>>>>>>>>>>>>>>>>>>>>>>> number of
>>>>>>>>>>>>>>>>>>>>>>> ExecutableStage DoFn operators.
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> The approach in 2) is what Flink does as well. It
>>>>>>>>>>>>>>>>>>>>>>> glues together
>>>>>>>>>>>>>>>>>>>>>>> horizontal parts of the execution graph, also in
>>>>>>>>>>>>>>>>>>>>>>> multiple threads. So I
>>>>>>>>>>>>>>>>>>>>>>> agree with your proposed solution.
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> Best,
>>>>>>>>>>>>>>>>>>>>>>> Max
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> On 17.08.18 03:10, Ankur Goenka wrote:
>>>>>>>>>>>>>>>>>>>>>>> > Hi,
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > tl;dr Dead Lock in task execution caused by
>>>>>>>>>>>>>>>>>>>>>>> limited task parallelism on
>>>>>>>>>>>>>>>>>>>>>>> > SDKHarness.
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > *Setup:*
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> >   * Job type: /*Beam Portable Python Batch*/ Job
>>>>>>>>>>>>>>>>>>>>>>> on Flink standalone
>>>>>>>>>>>>>>>>>>>>>>> >     cluster.
>>>>>>>>>>>>>>>>>>>>>>> >   * Only a single job is scheduled on the cluster.
>>>>>>>>>>>>>>>>>>>>>>> >   * Everything is running on a single machine with
>>>>>>>>>>>>>>>>>>>>>>> single Flink task
>>>>>>>>>>>>>>>>>>>>>>> >     manager.
>>>>>>>>>>>>>>>>>>>>>>> >   * Flink Task Manager Slots is 1.
>>>>>>>>>>>>>>>>>>>>>>> >   * Flink Parallelism is 1.
>>>>>>>>>>>>>>>>>>>>>>> >   * Python SDKHarness has 1 thread.
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > *Example pipeline:*
>>>>>>>>>>>>>>>>>>>>>>> > Read -> MapA -> GroupBy -> MapB -> WriteToSink
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > *Issue:*
>>>>>>>>>>>>>>>>>>>>>>> > With multi stage job, Flink schedule different
>>>>>>>>>>>>>>>>>>>>>>> dependent sub tasks
>>>>>>>>>>>>>>>>>>>>>>> > concurrently on Flink worker as long as it can get
>>>>>>>>>>>>>>>>>>>>>>> slots. Each map tasks
>>>>>>>>>>>>>>>>>>>>>>> > are then executed on SDKHarness.
>>>>>>>>>>>>>>>>>>>>>>> > Its possible that MapB gets to SDKHarness before
>>>>>>>>>>>>>>>>>>>>>>> MapA and hence gets
>>>>>>>>>>>>>>>>>>>>>>> > into the execution queue before MapA. Because we
>>>>>>>>>>>>>>>>>>>>>>> only have 1 execution
>>>>>>>>>>>>>>>>>>>>>>> > thread on SDKHarness, MapA will never get a chance
>>>>>>>>>>>>>>>>>>>>>>> to execute as MapB
>>>>>>>>>>>>>>>>>>>>>>> > will never release the execution thread. MapB will
>>>>>>>>>>>>>>>>>>>>>>> wait for input from
>>>>>>>>>>>>>>>>>>>>>>> > MapA. This gets us to a dead lock in a simple
>>>>>>>>>>>>>>>>>>>>>>> pipeline.
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > *Mitigation:*
>>>>>>>>>>>>>>>>>>>>>>> > Set worker_count in pipeline options more than the
>>>>>>>>>>>>>>>>>>>>>>> expected sub tasks
>>>>>>>>>>>>>>>>>>>>>>> > in pipeline.
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > *Proposal:*
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> >  1. We can get the maximum concurrency from the
>>>>>>>>>>>>>>>>>>>>>>> runner and make sure
>>>>>>>>>>>>>>>>>>>>>>> >     that we have more threads than max
>>>>>>>>>>>>>>>>>>>>>>> concurrency. This approach
>>>>>>>>>>>>>>>>>>>>>>> >     assumes that Beam has insight into runner
>>>>>>>>>>>>>>>>>>>>>>> execution plan and can
>>>>>>>>>>>>>>>>>>>>>>> >     make decision based on it.
>>>>>>>>>>>>>>>>>>>>>>> >  2. We dynamically create thread and cache them
>>>>>>>>>>>>>>>>>>>>>>> with a high upper bound
>>>>>>>>>>>>>>>>>>>>>>> >     in SDKHarness. We can warn if we are hitting
>>>>>>>>>>>>>>>>>>>>>>> the upper bound of
>>>>>>>>>>>>>>>>>>>>>>> >     threads. This approach assumes that runner
>>>>>>>>>>>>>>>>>>>>>>> does a good job of
>>>>>>>>>>>>>>>>>>>>>>> >     scheduling and will distribute tasks more or
>>>>>>>>>>>>>>>>>>>>>>> less evenly.
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > We expect good scheduling from runners so I prefer
>>>>>>>>>>>>>>>>>>>>>>> approach 2. It is
>>>>>>>>>>>>>>>>>>>>>>> > simpler to implement and the implementation is not
>>>>>>>>>>>>>>>>>>>>>>> runner specific. This
>>>>>>>>>>>>>>>>>>>>>>> > approach better utilize resource as it creates
>>>>>>>>>>>>>>>>>>>>>>> only as many threads as
>>>>>>>>>>>>>>>>>>>>>>> > needed instead of the peak thread requirement.
>>>>>>>>>>>>>>>>>>>>>>> > And last but not the least, it gives runner
>>>>>>>>>>>>>>>>>>>>>>> control over managing truly
>>>>>>>>>>>>>>>>>>>>>>> > active tasks.
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > Please let me know if I am missing something and
>>>>>>>>>>>>>>>>>>>>>>> your thoughts on the
>>>>>>>>>>>>>>>>>>>>>>> > approach.
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>> > Thanks,
>>>>>>>>>>>>>>>>>>>>>>> > Ankur
>>>>>>>>>>>>>>>>>>>>>>> >
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>