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From "Yonik Seeley (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2755) Some improvements to CMS
Date Thu, 11 Nov 2010 14:06:13 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2755?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12931015#action_12931015
] 

Yonik Seeley commented on LUCENE-2755:
--------------------------------------

bq. CMS sorts merges by segments size, doc-based and not bytes-based. Since the default MP
is LogByteSizeMP, and I hardly believe people care about doc-based size segments anymore,
I think we should switch the default impl.

That's probably a good idea.
We should also scale this by the number of deleted docs... so if a segment has 10,000 documents,
is 10MB in size, and has 2000 deleted documents, then we should consider it as 8MB for the
purposes of selecting to merge?

> Some improvements to CMS
> ------------------------
>
>                 Key: LUCENE-2755
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2755
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Shai Erera
>            Assignee: Shai Erera
>            Priority: Minor
>             Fix For: 3.1, 4.0
>
>
> While running optimize on a large index, I've noticed several things that got me to read
CMS code more carefully, and find these issues:
> * CMS may hold onto a merge if maxMergeCount is hit. That results in the MergeThreads
taking merges from the IndexWriter until they are exhausted, and only then that blocked merge
will run. I think it's unnecessary that that merge will be blocked.
> * CMS sorts merges by segments size, doc-based and not bytes-based. Since the default
MP is LogByteSizeMP, and I hardly believe people care about doc-based size segments anymore,
I think we should switch the default impl. There are two ways to make it extensible, if we
want:
> ** Have an overridable member/method in CMS that you can extend and override - easy.
> ** Have OneMerge be comparable and let the MP determine the order (e.g. by bytes, docs,
calibrate deletes etc.). Better, but will need to tap into several places in the code, so
more risky and complicated.
> On the go, I'd like to add some documentation to CMS - it's not very easy to read and
follow.
> I'll work on a patch.

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