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From "Shai Erera (JIRA)" <>
Subject [jira] Commented: (LUCENE-2755) Some improvements to CMS
Date Tue, 16 Nov 2010 14:58:13 GMT


Shai Erera commented on LUCENE-2755:

bq. Ideally only IW.merge should call it (and it becomes private),

I wouldn't make it private. If I remember correctly, the Parallel Index overrode that method
to synchronize merges across all parallels.

bq. but if you're at your max merge count then CMS will stall you and the turnaround time
easily becomes seconds, which is awful.

But Mike, if you hit your maxMergeCount with large merges, then you won't run tiny merges
at all. It's only if you have room to run any merges, that this 'pausing' actually helps.
I trust you when you say you've observed that not pausing those merges hurt performance, but
I wonder in real life, how often does that happen, and whether we should incorporate that
in our code. If it's a rare case, then perhaps apps that hit it should use another MS which
pauses its threads?

> Some improvements to CMS
> ------------------------
>                 Key: LUCENE-2755
>                 URL:
>             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
> ** 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
> I'll work on a patch.

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