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From "Earwin Burrfoot (JIRA)" <>
Subject [jira] Commented: (LUCENE-2755) Some improvements to CMS
Date Wed, 17 Nov 2010 08:53:13 GMT


Earwin Burrfoot commented on LUCENE-2755:

If we proceed w/ your proposal, that is basically the MS/ME polling MP, and not IW doing so,
how would IW know about the running merges and pending ones? Today IW tracks those two lists
so that if you need to abort merges, it knows which ones to abort.

We can workaround aborting the running merges by introducing a MS.abort()-like method. But
what about MP? Now the lists are divided between too entities (MP and MS), and aborting a
MP does not make sense (doable, but I don't think it belongs there). 
There are no lists at all with my approach. At least no "pending" list, that one gets recalculated
each time we poll MP and it never gets out, neither gets stored inside.
There's a kind of implicit "in flight" list - MS has the knowledge of its threads that are
currently doing things. And if you want to go around aborting things, MS is probably the right
place to do this.

bq. Maybe we can have MS.abort() poll MP for next merges until it returns null, and throwing
all the returned ones away - that can be done.
So, just I said - that's not needed. MP is empty, it has no state.

bq. Should we, in the scope of this issue, make IW a required settable parameter on MS, like
we do w/ MP?
For the love of God, no. I'd like to see it removed from MP too.
It's only natural to pass the same instance of Policy or Scheduler to different Writers, so
they have the same behaviour and share Scheduler resources (insanely important if you have
fifteen indexes like I do and don't want them to rape hardware with fifteen simultaneous merges).
It is against the nature to pass Writer to Policy. Does the Policy need to write anything
on its own, when it decides to? No. It should advice, not act.

> 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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