[jira] [Comment Edited] (CASSANDRA-14983) Local reads potentially blocking remote reads

["Marcus Olsson (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-14983?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16773237#comment-16773237 ] 

Marcus Olsson edited comment on CASSANDRA-14983 at 2/20/19 6:26 PM:
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I created CASSANDRA-15022 for the SEP executor and have uploaded some patches for 3.0, 3.11 and 4.0:
|[3.0|https://github.com/emolsson/cassandra/tree/3.0-CASSANDRA-14983]|[unit tests|https://circleci.com/gh/emolsson/cassandra/13]|
|[3.11|https://github.com/emolsson/cassandra/tree/3.11-CASSANDRA-14983]|[unit tests|https://circleci.com/gh/emolsson/cassandra/16]|
|[4.0|https://github.com/emolsson/cassandra/tree/4.0-CASSANDRA-14983]|[unit tests|https://circleci.com/gh/emolsson/cassandra/19]|

I seem to be having some issues running the tests on 4.0 though. When I tried to run the tests on a clean trunk branch locally I got similar issues as the ones in the build. But the 3.0 and 3.11 branches seems to pass the unit tests.

A note for the 4.0 patch is that all requests are still performed as full data requests. My understanding is that digest request are primarily used to avoid sending large amounts of data over the network. If we always perform data requests locally we might however keep multiple copies of the data in memory if we send data requests to remote nodes as well. If we want to change that behavior I think it should be in a separate JIRA ticket.

I also looked into the transient requests and I don't think anything would change from a logic perspective by executing the local request as a transient request as it is right now. Responses seems to be considered transient by checking the replica plan rather than the response. The ReadCommand#acceptsTransient()-method only seems to be used for validation before starting to execute the request and not affecting the actual request. Basically I don't think this is causing us any bugs right now. With that said I think the transient status should be kept even for local requests, at least from a correctness perspective. WDYT, should the local request be converted to a transient request when that is the case?

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I also looked at the distributed tests but I'm not sure how to delay or lose the local requests in a good way without changing the actual code base. Do you know of a good place on the nodes read path to hook in to and delay it - I can't seem to find any?


was (Author: molsson):
I created CASSANDRA-15022 for the SEP executor and have uploaded some patches for 3.0, 3.11 and 4.0:

|[3.0|https://github.com/emolsson/cassandra/tree/3.0-CASSANDRA-14983]|[unit tests|https://circleci.com/gh/emolsson/cassandra/13]|
|[3.11|https://github.com/emolsson/cassandra/tree/3.11-CASSANDRA-14983]|[unit tests|https://circleci.com/gh/emolsson/cassandra/16]|
|[4.0|https://github.com/emolsson/cassandra/tree/4.0-CASSANDRA-14983]|[unit tests|https://circleci.com/gh/emolsson/cassandra/19]|

I seem to be having some issues running the tests on 4.0 though. When I tried to run the tests on a clean trunk branch locally I got similar issues as the ones in the build. But the 3.0 and 3.11 branches seems to pass the unit tests.

I also looked at the distributed tests but I'm not sure how to delay or lose the local requests in a good way without changing the actual code base.

> Local reads potentially blocking remote reads
> ---------------------------------------------
>
>                 Key: CASSANDRA-14983
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14983
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Consistency/Coordination
>            Reporter: Marcus Olsson
>            Priority: Minor
>         Attachments: graph_local_read.html, graph_local_read_trunk.html, local_read_trace.log
>
>
> Since CASSANDRA-4718 there is a fast path allowing local requests to continue to [work in the same thread|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/service/reads/AbstractReadExecutor.java#L157] rather than being sent over to the read stage.
> Based on the comment
> {code:java}
> // We delay the local (potentially blocking) read till the end to avoid stalling remote requests.
> {code}
> it seems like this should be performed last in the chain to avoid blocking remote requests but that does not seem to be the case when the local request is a data request. The digest request(s) are sent after the data requests are sent (and now the transient replica requests as well). When the fast path is used for local data/transient data requests this will block the next type of request from being sent away until the local read is finished and add additional latency to the request.
> In addition to this it seems like local requests are *always* data requests (might not be a problem), but the log message can say either ["digest" or "data"|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/service/reads/AbstractReadExecutor.java#L156] as the type of request.
> I have tried to run performance measurements to see the impact of this in 3.0 (by moving local requests to the end of ARE#executeAsync()) but I haven't seen any big difference yet. I'll continue to run some more tests to see if I can find a use case affected by this.
> Attaching a trace (3.0) where this happens. Reproduction:
>  # Create a three node CCM cluster
>  # Provision data with stress (rf=3)
>  # In parallel:
>  ## Start stress read run
>  ## Run multiple manual read queries in cqlsh with tracing on and local_quorum (as this does not always happen)



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