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https://issues.apache.org/jira/browse/LUCENE-845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12483631
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Michael McCandless commented on LUCENE-845:
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This bug is actually rather serious.
If you set maxBufferedDocs to a very large number (on the expectation
that it's not used since you will manually flush by RAM usage) then
the merge policy will always merge the index down to 1 segment as soon
as it hits mergeFactor segments.
This will be an O(N^2) slowdown. EG if based on RAM you are
flushing every 100 docs, then at 1000 docs you will merge to 1
segment. Then at 1900 docs, you merge to 1 segment again. At 2800,
3700, 4600, ... (every 900 docs) you keep merging to 1 segment. Your
indexing process will get very slow because every 900 documents the
entire index is effectively being optimized.
With LUCENE-843 I'm thinking we should deprecate maxBufferedDocs
entirely and switch to flushing by RAM usage instead (you can always
manually flush every N documents in your app if for some reason you
need that). But obviously we need to resolve this bug first.
> If you "flush by RAM usage" then IndexWriter may over-merge
> -----------------------------------------------------------
>
> Key: LUCENE-845
> URL: https://issues.apache.org/jira/browse/LUCENE-845
> Project: Lucene - Java
> Issue Type: Bug
> Components: Index
> Affects Versions: 2.1
> Reporter: Michael McCandless
> Assigned To: Michael McCandless
> Priority: Minor
>
> 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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