Tomas Salfischberger created CASSANDRA-6191:
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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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