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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4937) CRAR improvements (object cache + CompressionMetadata chunk offset storage moved off-heap).
Date Thu, 15 Nov 2012 15:06:13 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13498067#comment-13498067
] 

Jonathan Ellis commented on CASSANDRA-4937:
-------------------------------------------

bq. We can't [only drop the recently-read blocks during sequential i/o] because it would require
(e.g. compaction) to know of how something is using files, it could remove cache from somebody's
fit anyway and we would use too many system calls if we do that.

You mean because RandomAccess reader doesn't know if we've been seeking back and forth?  Technically
that is true, but it looks like we only actually create RandomAccessReader with skipCache=True
when we're doing sequential reads.  So I'd be okay with just assuming it's sequential in reBuffer
if skipCache=True.

If you wanted to make it more robust you could subclass RAR to MostlySequentialRAR (still
need seek for when compaction needs two passes on wide rows) and only do skipCache in that
subclass.

This feels like a good compromise to me: it's a lot better than what we have now for your
use case: if an sstable *is* in the page cache we'd only dontneed it once gradually, instead
of repeatedly with each trySkipCache.  And it doesn't take us back to the Dark Ages of totally
stomping the page cache with cold data during compaction.

                
> CRAR improvements (object cache + CompressionMetadata chunk offset storage moved off-heap).
> -------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-4937
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4937
>             Project: Cassandra
>          Issue Type: Improvement
>    Affects Versions: 1.1.6
>            Reporter: Pavel Yaskevich
>            Assignee: Pavel Yaskevich
>            Priority: Minor
>             Fix For: 1.1.7
>
>         Attachments: CASSANDRA-4937.patch
>
>
> After good amount of testing on one of the clusters it was found that in order to improve
read latency we need to minimize allocation rate that compression involves, that minimizes
GC (as well as heap usage) and substantially decreases latency on read heavy workloads. 
> I have also discovered that RAR skip cache harms performance in situation when reads
are done in parallel with compaction working with relatively big SSTable files (few GB and
more). The attached patch removes possibility to skip cache from compressed files (I can also
add changes to RAR to remove skip cache functionality as a separate patch). 

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