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From "Michael Busch (JIRA)" <>
Subject [jira] Commented: (LUCENE-2324) Per thread DocumentsWriters that write their own private segments
Date Thu, 15 Apr 2010 00:53:50 GMT


Michael Busch commented on LUCENE-2324:

It's for performance. I expect there are apps where a given
thread/pool indexes certain kind of docs, ie, the app threads
themselves have "affinity" for docs with similar term distributions.
In which case, it's best (most RAM efficient) if those docs w/
presumably similar term stats are sent back to the same DW. If you
mix in different term stats into one buffer you get worse RAM

I do see your point, but I feel like we shouldn't optimize/make compromises for this use case.
 Mainly, because I think apps with such an affinity that you describe are very rare?  The
usual design is a queued ingestion pipeline, where a pool of indexer threads take docs out
of a queue and feed them to an IndexWriter, I think?  In such a world the threads wouldn't
have an affinity for similar docs.

And if a user really has so different docs, maybe the right answer would be to have more than
one single index?  Even if today an app utilizes the thread affinity, this only results in
maybe somewhat faster indexing performance, but the benefits would be lost after flusing/merging.

If we assign docs randomly to available DocumentsWriterPerThreads, then we should on average
make good use of the overall memory?  Alternatively we could also select the DWPT from the
pool of available DWPTs that has the highest amount of free memory?  

Having a fully decoupled memory management is compelling I think, mainly because it makes
everything so much simpler.  A DWPT could decide itself when it's time to flush, and the other
ones can keep going independently.  

If you do have a global RAM management, how would the flushing work?  E.g. when a global flush
is triggered because all RAM is consumed, and we pick the DWPT with the highest amount of
allocated memory for flushing, what will the other DWPTs do during that flush?  Wouldn't we
have to pause the other DWPTs to make sure we don't exceed the maxRAMBufferSize?
Of course we could say "always flush when 90% of the overall memory is consumed", but how
would we know that the remaining 10% won't fill up during the time the flush takes?  

> 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: 3.1
>         Attachments: 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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