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From "Sylvain Lebresne (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-3623) use MMapedBuffer in CompressedSegmentedFile.getSegment
Date Thu, 22 Dec 2011 18:37:33 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-3623?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13174958#comment-13174958
] 

Sylvain Lebresne commented on CASSANDRA-3623:
---------------------------------------------

When I was originally checking the compression code, I did a few quick stress tests under
yourkit, and in cases where the data was small and thus entirely in page cache, the cost of
creating a RandomAccessFile on each read was taking the majority of the time. Note that I'm
not saying this to justify this ticket necessarily, since:
# we could use an object pool to avoid that cost
# my tests were really toy test, so they would need confirmation
but mmapping does avoid this cost. Just saying.

I'll also note that Snappy has a way to decompress data from direct ByteBuffer directly ([snappydoc|http://wiki.snappy-java.googlecode.com/hg/apidocs/org/xerial/snappy/Snappy.html#compress(java.nio.ByteBuffer,
java.nio.ByteBuffer)]) so this could potentially avoid 1 copy (we would go from page cache
to decompressed buffer directly). Of course we should look how well that works, but again,
just to feed the discussion.
                
> use MMapedBuffer in CompressedSegmentedFile.getSegment
> ------------------------------------------------------
>
>                 Key: CASSANDRA-3623
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3623
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 1.1
>            Reporter: Vijay
>            Assignee: Vijay
>              Labels: compression
>             Fix For: 1.1
>
>         Attachments: 0001-MMaped-Compression-segmented-file.patch
>
>
> CompressedSegmentedFile.getSegment seem to open a new file and doesnt seem to use the
MMap and hence a higher CPU on the nodes and higher latencies on reads. 
> This ticket is to implement the TODO mentioned in CompressedRandomAccessReader
> // TODO refactor this to separate concept of "buffer to avoid lots of read() syscalls"
and "compression buffer"
> but i think a separate class for the Buffer will be better.

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