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From "Michael Busch (JIRA)" <>
Subject [jira] Commented: (LUCENE-2324) Per thread DocumentsWriters that write their own private segments
Date Thu, 22 Jul 2010 17:53:52 GMT


Michael Busch commented on LUCENE-2324:

Perhaps a single byte[] with length prefixes (like the field cache has).  A single int could
then represent a term (it would just be an offset into the byte[], which is field-specific,
so no need to store the field each time).

Yeah that's pretty much how TermsHashPerField works.  I agree with Mike, 
let's reuse that code.

Hmm, but... this opto only helps in that we don't have to merge the
doc stores if we merge segments that already share their doc stores.
But if (say) I have 2 threads indexing, and I'm indexing lots of docs
and each DWPT has written 5 segments, we will then merge these 10
segments, and must merge the doc stores at that point. So the sharing
isn't really buying us much (just not closing old files & opening new
ones, which is presumably negligible)?

Yeah that's true.  I agree it won't help much. I think we should just 
remove the doc stores, great simplification (which should also make 
parallel indexing a bit easier :) ).  

Another thing: it looks like finishFlushedSegment is sync'd on the IW
instance, but, it need not be sync'd for all of that? EG
readerPool.get(), applyDeletes, building the CFS, may not need to be
inside the sync block?

Thanks for the hint.  I need to carefully go over all the synchronization, 
there are likely more problems.  

> Per thread DocumentsWriters that write their own private segments
> -----------------------------------------------------------------
>                 Key: LUCENE-2324
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael Busch
>            Assignee: Michael Busch
>            Priority: Minor
>             Fix For: Realtime Branch
>         Attachments: lucene-2324.patch, lucene-2324.patch, LUCENE-2324.patch
> 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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