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From "Mike Malone (JIRA)" <>
Subject [jira] Created: (CASSANDRA-1358) Message deserializer pool will never grow beyond a single thread.
Date Wed, 04 Aug 2010 18:14:15 GMT
Message deserializer pool will never grow beyond a single thread.

                 Key: CASSANDRA-1358
             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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