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From "Vijay (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-4277) hsha default thread limits make no sense, and yaml comments look confused
Date Thu, 24 May 2012 04:12:41 GMT


Vijay commented on CASSANDRA-4277:

Peter, i dont understand how will latency come into picture? Selectors are woken when the
data is available, right? if for some reason your connection is taking 15 ms or what even
in the middle of a read you are better off disconnecting and reconnecting... I still dont
understand how 500 threads will help it will all hang right? how does it help?

Basically these threads (4*CPU core's) are used for selection read/write (only during that
time) and the TP executes it and the selector is woken up again when the data has to be written.
Are we having the same conversation as in CASSANDRA-3590 (but this is within the DC's where
latencies are really low)?
> hsha default thread limits make no sense, and yaml comments look confused
> -------------------------------------------------------------------------
>                 Key: CASSANDRA-4277
>                 URL:
>             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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