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From "Michael Busch (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2293) IndexWriter has hard limit on max concurrency
Date Thu, 04 Mar 2010 09:12:27 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12841135#action_12841135
] 

Michael Busch commented on LUCENE-2293:
---------------------------------------

Sorry - after reading my comment again I can see why it was confusing. Loadbalancer wasn't
a very good analogy.

I totally agree that Lucene should still piggyback on the application's threads and not start
its own thread for document inversion.

Today, as you said, does the DocumentsWriter manage a certain number of thread states, has
the WaitQueue, and its own memory management.

What I was thinking was that it would be simpler if the DocumentsWriter was only used by a
single thread. The IndexWriter would have multiple DocumentsWriters and do the thread binding
(+waitqueue). This would make the code in DocumentsWriter and the downstream classes simpler.
The side-effect is that each DocumentsWriter would manage its own memory. 

{quote}
Also, I thought that each thread writes to different ThreadState does not ensure documents
are written in order, but that finally when DW flushes, the different ThreadStates are merged
together and one segment is written, somehow restores the orderness ...
{quote}

Stored fields are written to an on-disk stream (docstore) in order. The WaitQueue takes care
of finishing the docs in the right order. 
The postings are written into TermHashes per threadstate in parallel. The doc ids are in increasing
order, but can have gaps. E.g. Threadstate 1 inverts doc 1 and 3, Threadstate 2 inverts doc
2. When it's time to flush the whole buffer these different TermHash postingslists get interleaved.

> IndexWriter has hard limit on max concurrency
> ---------------------------------------------
>
>                 Key: LUCENE-2293
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2293
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Index
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>             Fix For: 3.1
>
>
> DocumentsWriter has this nasty hardwired constant:
> {code}
> private final static int MAX_THREAD_STATE = 5;
> {code}
> which probably I should have attached a //nocommit to the moment I
> wrote it ;)
> That constant sets the max number of thread states to 5.  This means,
> if more than 5 threads enter IndexWriter at once, they will "share"
> only 5 thread states, meaning we gate CPU concurrency to 5 running
> threads inside IW (each thread must first wait for the last thread to
> finish using the thread state before grabbing it).
> This is bad because modern hardware can make use of more than 5
> threads.  So I think an immediate fix is to make this settable
> (expert), and increase the default (8?).
> It's tricky, though, because the more thread states, the less RAM
> efficiency you have, meaning the worse indexing throughput.  So you
> shouldn't up and set this to 50: you'll be flushing too often.
> But... I think a better fix is to re-think how threads write state
> into DocumentsWriter.  Today, a single docID stream is assigned across
> threads (eg one thread gets docID=0, next one docID=1, etc.), and each
> thread writes to a private RAM buffer (living in the thread state),
> and then on flush we do a merge sort.  The merge sort is inefficient
> (does not currently use a PQ)... and, wasteful because we must
> re-decode every posting byte.
> I think we could change this, so that threads write to private RAM
> buffers, with a private docID stream, but then instead of merging on
> flush, we directly flush each thread as its own segment (and, allocate
> private docIDs to each thread).  We can then leave merging to CMS
> which can already run merges in the BG without blocking ongoing
> indexing (unlike the merge we do in flush, today).
> This would also allow us to separately flush thread states.  Ie, we
> need not flush all thread states at once -- we can flush one when it
> gets too big, and then let the others keep running.  This should be a
> good concurrency gain since is uses IO & CPU resources "throughout"
> indexing instead of "big burst of CPU only" then "big burst of IO
> only" that we have today (flush today "stops the world").
> One downside I can think of is... docIDs would now be "less
> monotonic", meaning if N threads are indexing, you'll roughly get
> in-time-order assignment of docIDs.  But with this change, all of one
> thread state would get 0..N docIDs, the next thread state'd get
> N+1...M docIDs, etc.  However, a single thread would still get
> monotonic assignment of docIDs.

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