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Date: Thu, 2 Jan 2014 23:27:51 +0000 (UTC)
From: "Jonathan Ellis (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
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Subject: [jira] [Commented] (CASSANDRA-6271) Replace SnapTree in
 AtomicSortedColumns
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    [ https://issues.apache.org/jira/browse/CASSANDRA-6271?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13860935#comment-13860935 ] 

Jonathan Ellis commented on CASSANDRA-6271:
-------------------------------------------

bq. Splits the parent if necessary (in the addChild method, which is called whenever we have a completed node to pass to the parent)

Hmm, I don't see an addChild, and ensureChild/addExtraChild aren't doing any obvious parent-splitting.

> 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
>         Attachments: oprate.svg
>
>
> 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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