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Date: Wed, 10 Jul 2013 00:39:48 +0000 (UTC)
From: "Pavel Yaskevich (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
Message-ID: <JIRA.12653702.1371648677234.22824.1373416788987@arcas>
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Subject: [jira] [Commented] (CASSANDRA-5661) Discard pooled readers for cold
 data
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    [ https://issues.apache.org/jira/browse/CASSANDRA-5661?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13704045#comment-13704045 ] 

Pavel Yaskevich commented on CASSANDRA-5661:
--------------------------------------------

I have started working on integrating multiway pool and it looks like we have two problems:

#1. As each SegmentedFile has to return unique instance using "createReader(String)" LoadingCache won't do for us, as we need get(K, Callable) per SSTableReader.
#2. as MultiWay returns a handle I changed RAR to have a setHandle method instead of passing SegmentedFile into constructor, which seems a bit hacky to me as we need to be careful in maintaining that relationship...

I did some performance testing (with attached patch) where MultiwayPool allocated per instance because we can't specify loader in borrow(...) yet, which should be a best case for it, not in terms of memory usage but contention. I loaded 5,000,000 keys with following stress command (./tools/bin/cassandra-stress -n 5000000 -S 512 -C 20 -Z LeveledCompactionStrategy) for initial data and then I made it run in a loop and was doing reads in parallel.

With writes:

Average read performance for MultiwayPool: median 6.2, 95th 11.4, 99.9th 78.8 
Average read performance for FileCacheService: median: 5.3, 95th 9.6, 99.9th 73.1

No writes, no compaction:

Average read performance for MultiwayPool: median 2.3, 95th 3.2, 99.9th 21.3 
Average read performance for FileCacheService: median: 1.7, 95th 2.9, 99.9th 19.2

I tried doing range_slice but due to timeouts I couldn't really complete test on any of the implementations, median latenties on average different by 3-4 ms.

                
> Discard pooled readers for cold data
> ------------------------------------
>
>                 Key: CASSANDRA-5661
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5661
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 1.2.1
>            Reporter: Jonathan Ellis
>            Assignee: Pavel Yaskevich
>             Fix For: 2.0
>
>         Attachments: CASSANDRA-5661-multiway-per-sstable.patch, CASSANDRA-5661.patch, DominatorTree.png, Histogram.png
>
>
> Reader pooling was introduced in CASSANDRA-4942 but pooled RandomAccessReaders are never cleaned up until the SSTableReader is closed.  So memory use is "the worst case simultaneous RAR we had open for this file, forever."
> We should introduce a global limit on how much memory to use for RAR, and evict old ones.

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