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https://issues.apache.org/jira/browse/CASSANDRA-14983?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16809691#comment-16809691
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Benedict edited comment on CASSANDRA-14983 at 4/4/19 10:14 AM:
---------------------------------------------------------------
[~molsson] very sorry for taking so long to look at this patch, or to respond.
Thank you for producing the patch - I am looking forward to reviewing it soon.
I'm afraid I still don't quite have time - I hope to get to it in the next
couple of weeks.
The best ways I can think of to break a local request, without modifying the
underlying codebase, would be one of two possibilities:
1) Register a java-based FileSystemProvider that we can break, but this would
also require changes to the codebase, to use URIs instead of paths for the
filesystem interactions. This might not be too onerous, or might be really
awful to undertake. But it would offer us really powerful control over testing
of filesystem interactions.
2) Use a pre-existing outside-of-JVM approach. ScyllaDB seem to have written a
[fault-injecting filesystem for
Linux|https://www.scylladb.com/2016/05/02/fault-injection-filesystem-cookbook/].
This would work in CircleCI and on users developing under Linux, which is
better than nothing, and might be lower effort to introduce, though I'm not
sure if it would support a delay - and an error might not suffice for this test.
3) Byteman is also an option for a single fault injection, but in my experience
it is really unpleasant to work with, and costly to maintain. But this _might_
be the lowest effort.
4) You could mess with the local node state, copying the sstable thousands or
tens-of-thousands of times (and registering the copies with the node's Tracker
directly, since you can invoke any method you like).
We do also have prior form for modifying the real code to produce errors, but I
have a preference to avoid this.
I think 4 should be the easiest, so that might be the best bet!
was (Author: benedict):
[~molsson] very sorry for taking so long to look at this patch, or to respond.
Thank you for producing the patch - I am looking forward to reviewing it soon.
I'm afraid I still don't quite have time - I hope to get to it in the next
couple of weeks.
The best way I can think of to break a local request, without modifying the
underlying codebase, would be one of two possibilities:
1) Register a java-based FileSystemProvider that we can break, but this would
also require changes to the codebase, to use URIs instead of paths for the
filesystem interactions. This might not be too onerous, or might be really
awful to undertake. But it would offer us really powerful control over testing
of filesystem interactions.
2) Use a pre-existing outside-of-JVM approach. ScyllaDB seem to have written a
[fault-injecting filesystem for
Linux|https://www.scylladb.com/2016/05/02/fault-injection-filesystem-cookbook/].
This would work in CircleCI and on users developing under Linux, which is
better than nothing, and might be lower effort to introduce.
3) Byteman is also an option for a single fault injection, but in my experience
it is really unpleasant to work with, and costly to maintain. But this _might_
be the lowest effort.
We do also have prior form for modifying the real code to produce errors, but I
have a preference to avoid this.
FWIW, I don't think any of these approaches are _strictly_ necessary for
merging this patch, since none are trivial undertakings. But one would
certainly be preferable, and some would have wider positive impacts for the
project. They are just suggestions given your question.
> 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
> Assignee: Marcus Olsson
> Priority: Low
> 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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