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From "Tomas Salfischberger (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-6191) Memory exhaustion with large number of compressed SSTables
Date Sun, 13 Oct 2013 17:40:42 GMT
Tomas Salfischberger created CASSANDRA-6191:
-----------------------------------------------

             Summary: Memory exhaustion with large number of compressed SSTables
                 Key: CASSANDRA-6191
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6191
             Project: Cassandra
          Issue Type: Bug
          Components: Core
         Environment: OS: Debian 7.1
Java: Oracle 1.7.0_25
Cassandra: 1.2.10
Memory: 24GB
Heap: 8GB
            Reporter: Tomas Salfischberger


Not sure "bug" is the right description, because I can't say for sure that the large number
of SSTables is the cause of the memory issues. I'll share my research so far:

Under high read-load with a very large number of compressed SSTables (caused by the initial
default 5mb sstable_size in LCS) it seems memory is exhausted, without any room for GC to
fix this. It tries to GC but doesn't reclaim much.

The node first hits the "emergency valves" flushing all memtables, then reducing caches. And
finally logs 0.99+ heap usages and hangs with GC failure or crashes with OutOfMemoryError.

I've taken a heapdump and started analysis to find out what's wrong. The memory seems to be
used by the byte[] backing the HeapByteBuffer in the "compressed" field of org.apache.cassandra.io.compress.CompressedRandomAccessReader.
The byte[] are generally 65536 byes in size, matching the block-size of the compression.

Looking further in the heap-dump I can see that these readers are part of the pool in org.apache.cassandra.io.util.CompressedPoolingSegmentedFile.
Which is linked to the "dfile" field of org.apache.cassandra.io.sstable.SSTableReader. The
dump-file lists 45248 instances of CompressedRandomAccessReader.

Is this intended to go this way? Is there a leak somewhere? Or should there be an alternative
strategy and/or warning for cases where a node is trying to read far too many SSTables?



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