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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-845) If you "flush by RAM usage" then IndexWriter may over-merge
Date Fri, 17 Aug 2007 19:34:31 GMT


Michael McCandless commented on LUCENE-845:

> Merging small segments in the reader seems like a cool idea on it's
> own.  But if it's an acceptable hit to merge in the reader, why is
> it not in the writer?

Good point.  I think it comes down to how often we expect readers to
refresh vs writers flushing.

If indeed it's 1 to 1 (as the truest "low latency" app would in fact
be, or a "single writer + reader with no separation"), then the writer
should merge them because although it's paying an O(N^2) cost to keep
the tail "short", merging on open would pay even more cost.

But if writer flushes frequently and reader re-opens less frequently
then it's better to merge on open.

Of course, if the O(N^2) cost for IndexWriter to keep a short tail is
in practice not too costly then we should just leave this in
IndexWriter.  I still need to run that test for LUCENE-845.

> Also, would this tail merging on an open be able to reduce the peak
> number of file descriptors?  It seems like to do so, the tail would
> have to be merged *before* other index files were opened, further
> complicating matters.

Right I think to keep peak descriptor usage capped we must merge the
tail, first, then open the remaining segments, which definitely
complicate things...

> If you "flush by RAM usage" then IndexWriter may over-merge
> -----------------------------------------------------------
>                 Key: LUCENE-845
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Index
>    Affects Versions: 2.1
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>            Priority: Minor
>         Attachments: LUCENE-845.patch
> I think a good way to maximize performance of Lucene's indexing for a
> given amount of RAM is to flush (writer.flush()) the added documents
> whenever the RAM usage (writer.ramSizeInBytes()) has crossed the max
> RAM you can afford.
> But, this can confuse the merge policy and cause over-merging, unless
> you set maxBufferedDocs properly.
> This is because the merge policy looks at the current maxBufferedDocs
> to figure out which segments are level 0 (first flushed) or level 1
> (merged from <mergeFactor> level 0 segments).
> I'm not sure how to fix this.  Maybe we can look at net size (bytes)
> of a segment and "infer" level from this?  Still we would have to be
> resilient to the application suddenly increasing the RAM allowed.
> The good news is to workaround this bug I think you just need to
> ensure that your maxBufferedDocs is less than mergeFactor *
> typical-number-of-docs-flushed.

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