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https://issues.apache.org/jira/browse/LUCENE-2324?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13005651#comment-13005651
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Jason Rutherglen commented on LUCENE-2324:
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{quote}Ie, if a given term X occurrs in 6 DWPTs (today) then we merge-sort the 
docIDs from the postings of that term, which is costly. (The "normal" merge 
that will merge these DWPTs after this issue lands just append by 
docIDs).{quote}

Right, this is the same principal motivation behind implementing DWPTs for use 
with realtime search, eg, the doc-id interleaving is too expensive to be 
performed at query time.

> Per thread DocumentsWriters that write their own private segments
> -----------------------------------------------------------------
>
>                 Key: LUCENE-2324
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2324
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael Busch
>            Assignee: Michael Busch
>            Priority: Minor
>             Fix For: Realtime Branch
>
>         Attachments: LUCENE-2324-SMALL.patch, LUCENE-2324-SMALL.patch, 
> LUCENE-2324-SMALL.patch, LUCENE-2324-SMALL.patch, LUCENE-2324-SMALL.patch, 
> LUCENE-2324.patch, LUCENE-2324.patch, LUCENE-2324.patch, LUCENE-2324.patch, 
> lucene-2324.patch, lucene-2324.patch, test.out, test.out, test.out, test.out
>
>
> See LUCENE-2293 for motivation and more details.
> I'm copying here Mike's summary he posted on 2293:
> Change the approach for how we buffer in RAM to a more isolated
> approach, whereby IW has N fully independent RAM segments
> in-process and when a doc needs to be indexed it's added to one of
> them. Each segment would also write its own doc stores and
> "normal" segment merging (not the inefficient merge we now do on
> flush) would merge them. This should be a good simplification in
> the chain (eg maybe we can remove the *PerThread classes). The
> segments can flush independently, letting us make much better
> concurrent use of IO & CPU.

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