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Posted to users@tomcat.apache.org by Christopher Schultz <ch...@christopherschultz.net> on 2023/09/05 19:38:24 UTC
Virtual Threads
All,
I have some questions about Virtual Threads and their use within Tomcat.
Note that only Tomcat 11 currently has support for Virtual Threads when
running on a version 19 or later JVM.
My (admittedly limited) understanding is that the use of Virtual
Threads, specifically within Tomcat, will allow Tomcat to use fewer
platform threads (e.g. native OS threads) to support a larger number of
JVM/application threads for request-processing. The justification for
this improvement is that request-processing threads spend a lot of time
in an I/O-blocked state which, when using Virtual Threads, would no
longer consume a platform thread.
There appears to only be one setting to enable Virtual Threads in Tomcat:
<Connector ... useVirtualThreads="true|false" />
or
<Executor ... useVirtualThreads="true|false" />
In both cases, there is only one setting to affect the number of
"threads" (by any description) which the Executor will ultimately use
(Connectors without an explicit Executor will create a non-shared
Executor to be used internally). That setting is "maxThreads" which
limits the total number of "threads" that the Executor will create.
The implementation of the VirtualThreadExecutor does not seem to have
any upper-bound on the number of Virtual Threads that will be created at
all. I believe this means that a large number of incoming requests (i.e.
a burst) will result in the creation of a large number of Virtual
Threads. Without an upper-bound, we are expecting that the JVM's
(virtual) thread scheduler will schedule each application thread to be
mounted to a platform thread in the order it was added to the queue
(essentially in request-order).
Before Virtual Threads were introduced, the Executor would use a queue
of requests to dispatch to available (non-Virtual) threads which would
use that thread until the request was complete, then return the thread
to the pool. With Virtual Threads, the same thing is essentially still
happening except that (a) Tomcat no longer manages the thread pool (a
JVM-defined one is being used instead) and (b) requests are immediately
handed a (Virtual) thread to carry their execution.
I believe there are some subtle differences in how Tomcat will behave,
now. As an example, if I have two applications running, say, the Tomcat
Manager application and the Tomcat Examples application, without using
Virtual Threads, each application's thread pools should be "fair" within
the context of each application: requests are processed in the order
they are received in Manager and Examples, separately. If all requests
are equally "expensive" and everything is "fair", then requests to the
Examples application are scheduled alongside those to the Manager
application, and they can both execute simultaneously as well as
separately-manage the order in which the requests are processed.
Once Virtual Threads are introduced, the requests are filed into a
single JVM-wide thread-scheduling system where activity in one
application can affect the other.
Let's replace Examples with RealWorldApp, an application that is
expected to be used by users. Maybe a LOT of users. Without Virtual
Threads, a high number of requests to RealWorldApp will not cause
starvation of requests to (maybe the more important, at least for
admins) the Manager. Once Virtual Threads are introduced, a limitless
number of requests can be queued in front of a request to Manager, which
can experience starvation.
While Tomcat did not previously implement any specific priority-queuing
of requests, the use of separate Executors for each application
effectively provided that kind of thing. Yes, each Executor can be
configured separately to either use Virtual Threads or not, and so
Manager can be configured to NOT use Virtual Threads while RealWorldApp
can be configured to use Virtual Threads and the balance is "restored".
But it is no longer possible to have RealWorldApp and RealWorldApp2 and
Manager all with equally probable request-scheduling when using Virtual
Threads. You can pick some subset of applications to get (essentially)
priority by NOT using Virtual Threads, but the whole set of applications
running in a single JVM will share a single Executor with FIFO behavior.
If an application creates Virtual Threads (hey, why not?! they are
cheaper!) then they will be scheduled alongside the request-processing
threads as well.
Do I understand things correctly? Is my scenario of request-starvation
for a little-used but potentially critical application (e.g. Manager) a
real concern, or am I missing something fundamental about the way
Virtual Threads are scheduled relative to other Virtual Threads, and
relative to non-virtual threads?
-chris
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Re: Virtual Threads
Posted by Mark Thomas <ma...@apache.org>.
On 07/09/2023 15:41, Christopher Schultz wrote:
> On 9/6/23 16:29, Mark Thomas wrote:
<snip/>
>> There isn't
>> much point using an executor with virtual threads.
>
> Okay then.... perche
> https://tomcat.apache.org/tomcat-11.0-doc/config/executor.html#Virtual_Thread_Implementation ?
That is the internal executor we use to provide virtual threads to the
connector. The same code exists in all current versions.
Not sure why it was documented. It may have been while we were exploring
what was possible. I'd lean towards removing that section from the
Tomcat 11 docs.
Mark
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Re: Virtual Threads
Posted by Christopher Schultz <ch...@christopherschultz.net>.
Mark,
On 9/6/23 16:29, Mark Thomas wrote:
> On 06/09/2023 21:24, Christopher Schultz wrote:
>> On 9/6/23 03:29, Mark Thomas wrote:
>>> On 05/09/2023 22:02, Christopher Schultz wrote:
>
> <snip/>
>
>>>> Thanks for the correction. I just did a quick docs[1] search for
>>>> "virtual" in Tomcat 10.x for example and I didn't see
>>>> useVirtualThreads, so I assumed it wasn't in there. Maybe we need
>>>> some documentation back-ports?
>>>
>>> Odd. It is there as an attribute on the Connector. And in 9.0.x and
>>> 8.5.x too.
>>
>> https://tomcat.apache.org/tomcat-10.0-doc/config/executor.html
>>
>> I don't find the word "virtual" anywhere on that page.
>
> It is a Connector attribute, not an Executor attribute. There isn't much
> point using an executor with virtual threads.
Okay then.... perche
https://tomcat.apache.org/tomcat-11.0-doc/config/executor.html#Virtual_Thread_Implementation
?
-chris
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Re: Virtual Threads
Posted by Mark Thomas <ma...@apache.org>.
On 06/09/2023 21:24, Christopher Schultz wrote:
> On 9/6/23 03:29, Mark Thomas wrote:
>> On 05/09/2023 22:02, Christopher Schultz wrote:
<snip/>
>>> Thanks for the correction. I just did a quick docs[1] search for
>>> "virtual" in Tomcat 10.x for example and I didn't see
>>> useVirtualThreads, so I assumed it wasn't in there. Maybe we need
>>> some documentation back-ports?
>>
>> Odd. It is there as an attribute on the Connector. And in 9.0.x and
>> 8.5.x too.
>
> https://tomcat.apache.org/tomcat-10.0-doc/config/executor.html
>
> I don't find the word "virtual" anywhere on that page.
It is a Connector attribute, not an Executor attribute. There isn't much
point using an executor with virtual threads.
>> For those of you that are finding this topic interesting, I'll have a
>> talk on this at the ASF conference, Community Over Code in Halifax.
>>
>> https://communityovercode.org/schedule-list/#TH001
>>
>> More broadly, there are usually a reasonable number of Tomcat
>> committers present at ASF conferences so this is a great opportunity
>> to speak face to face with the committers.
>
> +1
Mark
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Re: Virtual Threads
Posted by Christopher Schultz <ch...@christopherschultz.net>.
Mark,
On 9/6/23 03:29, Mark Thomas wrote:
> On 05/09/2023 22:02, Christopher Schultz wrote:
>> Mark,
>>
>> On 9/5/23 15:55, Mark Thomas wrote:
>>> On 05/09/2023 20:38, Christopher Schultz wrote:
>>>> All,
>>>>
>>>> I have some questions about Virtual Threads and their use within
>>>> Tomcat. Note that only Tomcat 11 currently has support for Virtual
>>>> Threads when running on a version 19 or later JVM.
>>>
>>> Not quite. All current versions support virtual threads when running
>>> on Java 21 or later.
>>
>> Thanks for the correction. I just did a quick docs[1] search for
>> "virtual" in Tomcat 10.x for example and I didn't see
>> useVirtualThreads, so I assumed it wasn't in there. Maybe we need some
>> documentation back-ports?
>
> Odd. It is there as an attribute on the Connector. And in 9.0.x and
> 8.5.x too.
https://tomcat.apache.org/tomcat-10.0-doc/config/executor.html
I don't find the word "virtual" anywhere on that page.
>>> The virtual thread scheduler uses a work stealing queue so it isn't
>>> quite FIFO.
>>
>> That sounds like nit-picking to me.
>
> It is, but it is nit-picking for a reason. Violetta did some testing
> which showed switching to virtual threads showed a small (about 1%)
> throughput improvement it also showed a much broader spread of
> latencies. That increase in spread is due to the work-stealing queue.
> For those users watch latency carefully, the change was viewed as a
> significant drawback of switching to virtual threads.
That's a significant point, thanks for reiterating it.
> <snip/>
>
>>> I think you have summed things up pretty well. I don't see a way with
>>> the current API to specify multiple virtual thread schedulers (which
>>> is what I think you would need to address this).
>>
>> Yeah, in all of the coverage I've seen online, there are no examples
>> where Virtual Threads are used with an "executor" other than one that
>> (a) accepts Runnable tasks and (b) just generates a Virtual Thread
>> from that task which appears to be "bound" to the build-in default
>> JVM-wide Virtual Thread executor.
>>
>> It would be potentially interesting to see an API which would allow
>> different Executors to be used with Virtual Threads, even though the
>> whole point of the entire thing is to avoid the (application-level)
>> complexity of a thread pool/executor/etc. I think it does make sense,
>> however, to be able to prioritize your threads a little bit. I haven't
>> read anything about Virtual Threads and their priorities, so I assume
>> it's just not part of anything user-facing at this point.
>
> I haven't seen anything that suggests that there will be any such API
> but to be fair, I haven't been following the Loom project that closely.
>
> For those of you that are finding this topic interesting, I'll have a
> talk on this at the ASF conference, Community Over Code in Halifax.
>
> https://communityovercode.org/schedule-list/#TH001
>
> More broadly, there are usually a reasonable number of Tomcat committers
> present at ASF conferences so this is a great opportunity to speak face
> to face with the committers.
+1
-chris
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Re: Virtual Threads
Posted by Mark Thomas <ma...@apache.org>.
On 05/09/2023 22:02, Christopher Schultz wrote:
> Mark,
>
> On 9/5/23 15:55, Mark Thomas wrote:
>> On 05/09/2023 20:38, Christopher Schultz wrote:
>>> All,
>>>
>>> I have some questions about Virtual Threads and their use within
>>> Tomcat. Note that only Tomcat 11 currently has support for Virtual
>>> Threads when running on a version 19 or later JVM.
>>
>> Not quite. All current versions support virtual threads when running
>> on Java 21 or later.
>
> Thanks for the correction. I just did a quick docs[1] search for
> "virtual" in Tomcat 10.x for example and I didn't see useVirtualThreads,
> so I assumed it wasn't in there. Maybe we need some documentation
> back-ports?
Odd. It is there as an attribute on the Connector. And in 9.0.x and
8.5.x too.
>> The virtual thread scheduler uses a work stealing queue so it isn't
>> quite FIFO.
>
> That sounds like nit-picking to me.
It is, but it is nit-picking for a reason. Violetta did some testing
which showed switching to virtual threads showed a small (about 1%)
throughput improvement it also showed a much broader spread of
latencies. That increase in spread is due to the work-stealing queue.
For those users watch latency carefully, the change was viewed as a
significant drawback of switching to virtual threads.
<snip/>
>> I think you have summed things up pretty well. I don't see a way with
>> the current API to specify multiple virtual thread schedulers (which
>> is what I think you would need to address this).
>
> Yeah, in all of the coverage I've seen online, there are no examples
> where Virtual Threads are used with an "executor" other than one that
> (a) accepts Runnable tasks and (b) just generates a Virtual Thread from
> that task which appears to be "bound" to the build-in default JVM-wide
> Virtual Thread executor.
>
> It would be potentially interesting to see an API which would allow
> different Executors to be used with Virtual Threads, even though the
> whole point of the entire thing is to avoid the (application-level)
> complexity of a thread pool/executor/etc. I think it does make sense,
> however, to be able to prioritize your threads a little bit. I haven't
> read anything about Virtual Threads and their priorities, so I assume
> it's just not part of anything user-facing at this point.
I haven't seen anything that suggests that there will be any such API
but to be fair, I haven't been following the Loom project that closely.
For those of you that are finding this topic interesting, I'll have a
talk on this at the ASF conference, Community Over Code in Halifax.
https://communityovercode.org/schedule-list/#TH001
More broadly, there are usually a reasonable number of Tomcat committers
present at ASF conferences so this is a great opportunity to speak face
to face with the committers.
Mark
---------------------------------------------------------------------
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For additional commands, e-mail: users-help@tomcat.apache.org
Re: Virtual Threads
Posted by Christopher Schultz <ch...@christopherschultz.net>.
Mark,
On 9/5/23 15:55, Mark Thomas wrote:
> On 05/09/2023 20:38, Christopher Schultz wrote:
>> All,
>>
>> I have some questions about Virtual Threads and their use within
>> Tomcat. Note that only Tomcat 11 currently has support for Virtual
>> Threads when running on a version 19 or later JVM.
>
> Not quite. All current versions support virtual threads when running on
> Java 21 or later.
Thanks for the correction. I just did a quick docs[1] search for
"virtual" in Tomcat 10.x for example and I didn't see useVirtualThreads,
so I assumed it wasn't in there. Maybe we need some documentation
back-ports?
>> My (admittedly limited) understanding is that the use of Virtual
>> Threads, specifically within Tomcat, will allow Tomcat to use fewer
>> platform threads (e.g. native OS threads) to support a larger number
>> of JVM/application threads for request-processing. The justification
>> for this improvement is that request-processing threads spend a lot of
>> time in an I/O-blocked state which, when using Virtual Threads, would
>> no longer consume a platform thread.
>>
>> There appears to only be one setting to enable Virtual Threads in Tomcat:
>>
>> <Connector ... useVirtualThreads="true|false" />
>>
>> or
>>
>> <Executor ... useVirtualThreads="true|false" />
>>
>> In both cases, there is only one setting to affect the number of
>> "threads" (by any description) which the Executor will ultimately use
>> (Connectors without an explicit Executor will create a non-shared
>> Executor to be used internally). That setting is "maxThreads" which
>> limits the total number of "threads" that the Executor will create.
>>
>> The implementation of the VirtualThreadExecutor does not seem to have
>> any upper-bound on the number of Virtual Threads that will be created
>> at all. I believe this means that a large number of incoming requests
>> (i.e. a burst) will result in the creation of a large number of
>> Virtual Threads. Without an upper-bound, we are expecting that the
>> JVM's (virtual) thread scheduler will schedule each application thread
>> to be mounted to a platform thread in the order it was added to the
>> queue (essentially in request-order).
>
> The virtual thread scheduler uses a work stealing queue so it isn't
> quite FIFO.
That sounds like nit-picking to me. I suppose if each new Virtual Thread
is assigned to a platform thread when it's initially queued, things can
be executed in an not-strictly-FIFO-manner, but one has to assume work
loads are equal (which they likely are not) and work is spread evenly
across all the platform threads (which is likely) to have a general
conversation about all this. I think the work-stealing ForkJoinPool is
about as close to FIFO as you can get without introducing more expensive
contention in order to enforce strict FIFO while not gaining much in the
way of meaningful "fairness".
But it's probably worth mentioning that the queue might not be "strictly
fair". What *is* fair, though -- and maybe more fair than in the past?
-- is that pipelined requests have to get back in line instead of
jumping the queue. I think the old blocking JIO connector would allow
pipelined requests to skip the queue, but all NIO-based connectors are
"more fair" than that.
>> Before Virtual Threads were introduced, the Executor would use a queue
>> of requests to dispatch to available (non-Virtual) threads which would
>> use that thread until the request was complete, then return the thread
>> to the pool. With Virtual Threads, the same thing is essentially still
>> happening except that (a) Tomcat no longer manages the thread pool (a
>> JVM-defined one is being used instead) and (b) requests are
>> immediately handed a (Virtual) thread to carry their execution.
>>
>> I believe there are some subtle differences in how Tomcat will behave,
>> now. As an example, if I have two applications running, say, the
>> Tomcat Manager application and the Tomcat Examples application,
>> without using Virtual Threads, each application's thread pools should
>> be "fair" within the context of each application: requests are
>> processed in the order they are received in Manager and Examples,
>> separately. If all requests are equally "expensive" and everything is
>> "fair", then requests to the Examples application are scheduled
>> alongside those to the Manager application, and they can both execute
>> simultaneously as well as separately-manage the order in which the
>> requests are processed.
>
> The above assumes each application has a separate thread pool (which
> implies a separate Connector).
Yes, I'm sorry, I completely skipped over the "fact" that a separate
<Connector> would be used for such a "priority" application such as the
Manager. My example was assuming that each application had the
possibility to use a separate <Executor>.
>> Once Virtual Threads are introduced, the requests are filed into a
>> single JVM-wide thread-scheduling system where activity in one
>> application can affect the other.
>
> Correct.
>
>> Let's replace Examples with RealWorldApp, an application that is
>> expected to be used by users. Maybe a LOT of users. Without Virtual
>> Threads, a high number of requests to RealWorldApp will not cause
>> starvation of requests to (maybe the more important, at least for
>> admins) the Manager. Once Virtual Threads are introduced, a limitless
>> number of requests can be queued in front of a request to Manager,
>> which can experience starvation.
>>
>> While Tomcat did not previously implement any specific
>> priority-queuing of requests, the use of separate Executors for each
>> application effectively provided that kind of thing. Yes, each
>> Executor can be configured separately to either use Virtual Threads or
>> not, and so Manager can be configured to NOT use Virtual Threads while
>> RealWorldApp can be configured to use Virtual Threads and the balance
>> is "restored". But it is no longer possible to have RealWorldApp and
>> RealWorldApp2 and Manager all with equally probable request-scheduling
>> when using Virtual Threads. You can pick some subset of applications
>> to get (essentially) priority by NOT using Virtual Threads, but the
>> whole set of applications running in a single JVM will share a single
>> Executor with FIFO behavior. If an application creates Virtual Threads
>> (hey, why not?! they are cheaper!) then they will be scheduled
>> alongside the request-processing threads as well.
>>
>> Do I understand things correctly? Is my scenario of request-starvation
>> for a little-used but potentially critical application (e.g. Manager)
>> a real concern, or am I missing something fundamental about the way
>> Virtual Threads are scheduled relative to other Virtual Threads, and
>> relative to non-virtual threads?
>
> I think you have summed things up pretty well. I don't see a way with
> the current API to specify multiple virtual thread schedulers (which is
> what I think you would need to address this).
Yeah, in all of the coverage I've seen online, there are no examples
where Virtual Threads are used with an "executor" other than one that
(a) accepts Runnable tasks and (b) just generates a Virtual Thread from
that task which appears to be "bound" to the build-in default JVM-wide
Virtual Thread executor.
It would be potentially interesting to see an API which would allow
different Executors to be used with Virtual Threads, even though the
whole point of the entire thing is to avoid the (application-level)
complexity of a thread pool/executor/etc. I think it does make sense,
however, to be able to prioritize your threads a little bit. I haven't
read anything about Virtual Threads and their priorities, so I assume
it's just not part of anything user-facing at this point.
Thanks,
-chris
[1] https://tomcat.apache.org/tomcat-10.1-doc/config/executor.html
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Re: Virtual Threads
Posted by Mark Thomas <ma...@apache.org>.
On 05/09/2023 20:38, Christopher Schultz wrote:
> All,
>
> I have some questions about Virtual Threads and their use within Tomcat.
> Note that only Tomcat 11 currently has support for Virtual Threads when
> running on a version 19 or later JVM.
Not quite. All current versions support virtual threads when running on
Java 21 or later.
> My (admittedly limited) understanding is that the use of Virtual
> Threads, specifically within Tomcat, will allow Tomcat to use fewer
> platform threads (e.g. native OS threads) to support a larger number of
> JVM/application threads for request-processing. The justification for
> this improvement is that request-processing threads spend a lot of time
> in an I/O-blocked state which, when using Virtual Threads, would no
> longer consume a platform thread.
>
> There appears to only be one setting to enable Virtual Threads in Tomcat:
>
> <Connector ... useVirtualThreads="true|false" />
>
> or
>
> <Executor ... useVirtualThreads="true|false" />
>
> In both cases, there is only one setting to affect the number of
> "threads" (by any description) which the Executor will ultimately use
> (Connectors without an explicit Executor will create a non-shared
> Executor to be used internally). That setting is "maxThreads" which
> limits the total number of "threads" that the Executor will create.
>
> The implementation of the VirtualThreadExecutor does not seem to have
> any upper-bound on the number of Virtual Threads that will be created at
> all. I believe this means that a large number of incoming requests (i.e.
> a burst) will result in the creation of a large number of Virtual
> Threads. Without an upper-bound, we are expecting that the JVM's
> (virtual) thread scheduler will schedule each application thread to be
> mounted to a platform thread in the order it was added to the queue
> (essentially in request-order).
The virtual thread scheduler uses a work stealing queue so it isn't
quite FIFO.
> Before Virtual Threads were introduced, the Executor would use a queue
> of requests to dispatch to available (non-Virtual) threads which would
> use that thread until the request was complete, then return the thread
> to the pool. With Virtual Threads, the same thing is essentially still
> happening except that (a) Tomcat no longer manages the thread pool (a
> JVM-defined one is being used instead) and (b) requests are immediately
> handed a (Virtual) thread to carry their execution.
>
> I believe there are some subtle differences in how Tomcat will behave,
> now. As an example, if I have two applications running, say, the Tomcat
> Manager application and the Tomcat Examples application, without using
> Virtual Threads, each application's thread pools should be "fair" within
> the context of each application: requests are processed in the order
> they are received in Manager and Examples, separately. If all requests
> are equally "expensive" and everything is "fair", then requests to the
> Examples application are scheduled alongside those to the Manager
> application, and they can both execute simultaneously as well as
> separately-manage the order in which the requests are processed.
The above assumes each application has a separate thread pool (which
implies a separate Connector).
> Once Virtual Threads are introduced, the requests are filed into a
> single JVM-wide thread-scheduling system where activity in one
> application can affect the other.
Correct.
> Let's replace Examples with RealWorldApp, an application that is
> expected to be used by users. Maybe a LOT of users. Without Virtual
> Threads, a high number of requests to RealWorldApp will not cause
> starvation of requests to (maybe the more important, at least for
> admins) the Manager. Once Virtual Threads are introduced, a limitless
> number of requests can be queued in front of a request to Manager, which
> can experience starvation.
>
> While Tomcat did not previously implement any specific priority-queuing
> of requests, the use of separate Executors for each application
> effectively provided that kind of thing. Yes, each Executor can be
> configured separately to either use Virtual Threads or not, and so
> Manager can be configured to NOT use Virtual Threads while RealWorldApp
> can be configured to use Virtual Threads and the balance is "restored".
> But it is no longer possible to have RealWorldApp and RealWorldApp2 and
> Manager all with equally probable request-scheduling when using Virtual
> Threads. You can pick some subset of applications to get (essentially)
> priority by NOT using Virtual Threads, but the whole set of applications
> running in a single JVM will share a single Executor with FIFO behavior.
> If an application creates Virtual Threads (hey, why not?! they are
> cheaper!) then they will be scheduled alongside the request-processing
> threads as well.
>
> Do I understand things correctly? Is my scenario of request-starvation
> for a little-used but potentially critical application (e.g. Manager) a
> real concern, or am I missing something fundamental about the way
> Virtual Threads are scheduled relative to other Virtual Threads, and
> relative to non-virtual threads?
I think you have summed things up pretty well. I don't see a way with
the current API to specify multiple virtual thread schedulers (which is
what I think you would need to address this).
Mark
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