Mailing-List: contact java-dev-help@lucene.apache.org; run by ezmlm
Precedence: bulk
Reply-To: java-dev@lucene.apache.org
Message-ID: <617990923.48401267653747298.JavaMail.jira@brutus.apache.org>
Date: Wed, 3 Mar 2010 22:02:27 +0000 (UTC)
From: "Michael Busch (JIRA)" <jira@apache.org>
To: java-dev@lucene.apache.org
Subject: [jira] Commented: (LUCENE-2293) IndexWriter has hard limit on max
 concurrency
In-Reply-To: <396950764.46741267649907121.JavaMail.jira@brutus.apache.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit


    [ https://issues.apache.org/jira/browse/LUCENE-2293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12840911#action_12840911 ] 

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

Good timing - a couple days ago I was thinking about how threading could be changed in the indexer.

The other downside is that you would have to buffer deleted docs and queries separately for each thread state, because you have to keep the private docID? So that would nee a bit more memory.

Couldn't we make the DocumentsWriter and all related down-stream classes single-threaded then? The IndexWriter (or a new class) would have the doc queue, basically a load balancer, that multiple DocumentsWriter instances would pull from as soon as they are done inverting the previous document?

This would allow us to simplify the indexer chain a lot - we could get rid of all the *PerThread classes. We'd also have to separate then the docstores from the DocumentsWriter, so that multiple DocumentsWriter instances could share it. (what I'd like to do anyway for LUCENE-2026 anyway).

> 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.

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.


---------------------------------------------------------------------
To unsubscribe, e-mail: java-dev-unsubscribe@lucene.apache.org
For additional commands, e-mail: java-dev-help@lucene.apache.org