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From "Jonathan Ellis (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-1358) Message deserializer pool will never grow beyond a single thread.
Date Wed, 04 Aug 2010 21:00:18 GMT


Jonathan Ellis commented on CASSANDRA-1358:

bq. So are the message deserializer threads blocking on a response from (or pushing a task
onto a queue for) a different thread pool then?


bq. If the intent is to have a single threaded executor, the second argument should be 1

true, but possibly moot, because:

bq. it's unable to process gossip requests / responses in a timely manner

good point.  we have a separate connection for gossip and acks so they won't be blocked by
a large request on the outbound path, but combining them both through the same MDP on the
inbound side defeats that to a large degree.

probably we should just get rid of MDP, and do the deserialize on the IncomingTcpConnection
thread.  (This would require moving the flow control from CASSANDRA-685 / CASSANDRA-1284 into

> Message deserializer pool will never grow beyond a single thread.
> -----------------------------------------------------------------
>                 Key: CASSANDRA-1358
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 0.6.3
>         Environment: All.
>            Reporter: Mike Malone
>            Priority: Minor
> The message deserialization process can become a bottleneck that prevents efficient resource
utilization because the executor that manages the deserialization process will never grow
beyond a single thread. The message deserializer executor is instantiated in the MessagingService
constructor as a JMXEnableThreadPoolExecutor, which extends java.util.concurrent.ThreadPoolExecutor.
The thread pool is instantiated with a corePoolSize of 1 and a maximumPoolSize of Runtime.getRuntime().availableProcessors().
But, according to the ThreadPoolExecutor documentation "using an unbounded queue (for example
a LinkedBlockingQueue without a predefined capacity) will cause new tasks to be queued in
cases where all corePoolSize threads are busy. Thus, no more than corePoolSize threads will
ever be created. (And the value of the maximumPoolSize therefore doesn't have any effect.)"
> The message deserializer pool uses a LinkedBlockingQueue, so there will never be more
than one deserialization thread. This issue became a problem in our production cluster when
the MESSAGE-DESERIALIZER-POOL began to back up on a node that was only lightly loaded. We
increased the core pool size to 4 and the situation improved, but the deserializer pool was
still backing up while the machine was not fully utilized (less than 100% CPU utilization).
This leads me to think that the deserializer thread is blocking on some sort of I/O, which
seems like it shouldn't happen.

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