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Posted to commits@cassandra.apache.org by "Benedict (JIRA)" <ji...@apache.org> on 2013/10/30 09:41:25 UTC
[jira] [Created] (CASSANDRA-6271) Replace SnapTree in
AtomicSortedColumns
Benedict created CASSANDRA-6271:
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Summary: Replace SnapTree in AtomicSortedColumns
Key: CASSANDRA-6271
URL: https://issues.apache.org/jira/browse/CASSANDRA-6271
Project: Cassandra
Issue Type: Improvement
Reporter: Benedict
Assignee: Benedict
On the write path a huge percentage of time is spent in GC (>50% in my tests, if accounting for slow down due to parallel marking). SnapTrees are both GC unfriendly due to their structure and also very expensive to keep around - each column name in AtomicSortedColumns uses > 100 bytes on average.
I suggest using a sorted array; changes are supplied at-once, as opposed to one at a time, and if < 10% of the values in the array change we simply overlay a new array of changes only over the top. Otherwise we rewrite the array. This method should ensure much less GC overhead, and also save approximately 80% of the current memory overhead.
TreeMap is similarly difficult object for the GC, and a related task might be to remove it where not strictly necessary, even though we don't keep them hanging around for long. TreeMapBackedSortedColumns, for instance, seems to be used in a lot of places where we could simply sort the columns.
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