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https://issues.apache.org/jira/browse/CASSANDRA-4277?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13281957#comment-13281957
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Peter Schuller commented on CASSANDRA-4277:
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The point is, zoom out of all implementation details. Forget about context
switching. Just look at requests. If you have 10 000 requests with an average
latency of 15 milliseconds for example, it would take 10000*15/1000=150 seconds
to execute them with a single rpc thread. With 10 threads, 15 seconds.
So if you're trying to push 10 000 requests per second through a node, with an
average latency of 15 milliseconds, you cannot possible do so if you limit the
number of concurrent requests to less than 15*10=150.
> hsha default thread limits make no sense, and yaml comments look confused
> -------------------------------------------------------------------------
>
> Key: CASSANDRA-4277
> URL: https://issues.apache.org/jira/browse/CASSANDRA-4277
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Peter Schuller
>
> The cassandra.yaml states with respect to {{rpc_max_threads}}:
> {code}
> # For the Hsha server, the min and max both default to quadruple the number of
> # CPU cores.
> {code}
> The code seems to indeed do this. But this makes, as far as I can tell, no
> sense what-so-ever since the number of concurrent RPC threads you need is a
> function of the throughput and the average latency of requests (that includes
> synchronously waiting on network traffic).
> Defaulting to anything having to do with CPU cores seems inherently wrong. If
> a default is non-static, a closer guess might be to look at thread stack size
> and heap size and infer what "might" be reasonable.
> *NOTE*: The effect of having this too low, is "strange" (if you don't know
> what's going on) latencies observed form the client on all thrift requests
> (*any* thrift request, including e.g. {{describe_ring()}}), that isn't
> visible in any latency metric exposed by Cassandra. This is why I consider
> this "major", since unwitting users may be seeing detrimental performance for
> no good reason.
> In addition, I read this about async:
> {code}
> # async -> Nonblocking server implementation with one thread to serve
> # rpc connections. This is not recommended for high throughput use
> # cases. Async has been tested to be about 50% slower than sync
> # or hsha and is deprecated: it will be removed in the next major
> release.
> {code}
> This makes even less sense. Running with *one* rpc thread limits you to a
> single concurrent request. How was that 50% number even attained? By
> single-node testing being completely CPU bound locally on a node? The actual
> effect should be "stupidly slow" in any real situation with lots of requests
> on a cluster of many nodes and network traffic (though I didn't test that) -
> especially in the event of any kind of hiccup like a node doing GC. I agree
> that if the above is true, async should *definitely* be deprecated, but the
> reasons seem *much* stronger than implied.
> I may be missing something here, in which case I apologize,, but I
> specifically double-checked after I fixed this setting on on our our clusters
> after seeing exactly the expected side-effect of having it be too low. I
> always was under the impression that rpc_max_threads affects the number of
> RPC requests running concurrently, and code inspection (it being used for the
> worker thread limit) + the effects of client-observed latency is consistent
> with my understanding.
> I suspect the setting was set strangely by someone because the phrasing of
> the comments in {{cassandra.yaml}} strongly suggest that this should be tied
> to CPU cores, hiding the fact that this really has to do with the number of
> requests that can be serviced concurrently regardless of implementation
> details of thrift/networking being sync/async/etc.
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