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https://issues.apache.org/jira/browse/CASSANDRA-6271?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13870700#comment-13870700
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Sylvain Lebresne commented on CASSANDRA-6271:
---------------------------------------------
As a side note, would have been nice to preserve the comment on top of:
{noformat}
if (reconciled == update)
indexer.update(replaced, reconciled);
else
indexer.update(update, reconciled);
{noformat}
It's pretty hard to understand what this code is about otherwise (especially
since the code made no sense in trunk, it's a reliqua of old times that needs
to be removed; will open a separate ticket to remove in 2.0 while we're at it).
> 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
> Labels: performance
> Fix For: 2.1
>
> Attachments: 0001-Always-call-ReplaceFunction.txt, oprate.svg,
> tmp.patch
>
>
> 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
> (excluding the actual ByteBuffer).
> I suggest using a sorted array; changes are supplied at-once, as opposed to
> one at a time, and if < 10% of the keys in the array change (and data equal
> to < 10% of the size of the key array) 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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