Wayne Lewis created CASSANDRA-4316:
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Summary: Compaction Throttle too bursty with large rows
Key: CASSANDRA-4316
URL: https://issues.apache.org/jira/browse/CASSANDRA-4316
Project: Cassandra
Issue Type: Bug
Components: Core
Affects Versions: 1.1.0, 1.0.1
Reporter: Wayne Lewis
In org.apache.cassandra.db.compaction.CompactionIterable the check for
compaction throttling occurs once every 1000 rows. In our workload this is much
too large as we have many large rows (16 - 100 MB).
With a 100 MB row, about 100 GB is read (and possibly written) before the
compaction throttle sleeps. This causes bursts of essentially unthrottled
compaction IO followed by a long sleep which yields inconsistence performance
and high error rates during the bursts.
We applied a workaround to check throttle every row which solved our
performance and error issues:
line 116 in org.apache.cassandra.db.compaction.CompactionIterable:
if ((row++ % 1000) == 0)
replaced with
if ((row++ % 1) == 0)
I think the better solution is to calculate how often throttle should be
checked based on the throttle rate to apply sleeps more consistently. E.g. if
16MB/sec is the limit then check for sleep after every 16MB is read so sleeps
are spaced out about every second.
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