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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6271) Replace SnapTree in AtomicSortedColumns
Date Tue, 31 Dec 2013 00:04:50 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6271?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13859191#comment-13859191
] 

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

The way Stack abuses succ/prev mutation to both construct a path and navigate it confuses
the hell out of me.  

Wouldn't it be a lot simpler to add a Previous node reference (e.g. the first reference in
the Object[] node)?  Then we wouldn't need to construct the Object[][] at all in Stack or
poorly defined depth semantics; we'd just need a node and an index; almost all the complexity
goes away from succ/prev.

> 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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