[jira] Commented: (LUCENE-845) If you "flush by RAM usage" then IndexWriter may over-merge

["Steven Parkes (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LUCENE-845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12520130 ] 

Steven Parkes commented on LUCENE-845:
--------------------------------------

This increases file descriptor usage in some cases, right? In the old scheme, if you set mergeFactor to 10 and maxBufferedDocs to 1000, you'd only get 10 segments with size <= 1000. But with this code, you can't bound that anymore. If I create single doc segments (perhaps by flushing based on latency), I can get 30 of them?

Of course, if what we're trying to do is manage the number of file descriptors, we should just do that, rather than using using maxBufferedDocs as a proxy (with all it's nasty overmerging behavior).

> 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
>            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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