[jira] [Created] (CASSANDRA-8670) Large columns + NIO memory pooling causes excessive direct memory usage

["Ariel Weisberg (JIRA)" <ji...@apache.org>]

Ariel Weisberg created CASSANDRA-8670:
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             Summary: Large columns + NIO memory pooling causes excessive direct memory usage
                 Key: CASSANDRA-8670
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8670
             Project: Cassandra
          Issue Type: Bug
          Components: Core
            Reporter: Ariel Weisberg
            Assignee: Ariel Weisberg


If you provide a large byte array to NIO and ask it to populate the byte array from a socket it will allocate a thread local byte buffer that is the size of the requested read no matter how large it is. Old IO wraps new IO for sockets (but not files) so old IO is effected as well.

Even If you are using Buffered{Input | Output}Stream you can end up passing a large byte array to NIO. The byte array read method will pass the array to NIO directly if it is larger then the internal buffer.  

Passing large cells between nodes as part of intra-cluster messaging can cause the NIO pooled buffers to quickly reach a high watermark and stay there. This ends up costing 2x the largest cell size because there is a buffer for input and output since they are different threads. This is further multiplied by the number of nodes in the cluster - 1 since each has a dedicated thread pair with separate thread locals.

Anecdotally it appears that the cost is doubled beyond that although it isn't clear why. Possibly the control connections or possibly there is some way in which multiple 

Need a workload in CI that tests the advertised limits of cells on a cluster. It would be reasonable to ratchet down the max direct memory for the test to trigger failures if a memory pooling issue is introduced. I don't think we need to test concurrently pulling in a lot of them, but it should at least work serially.



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