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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-2293) IndexWriter has hard limit on max concurrency
Date Sun, 14 Mar 2010 09:45:27 GMT


Michael McCandless commented on LUCENE-2293:

bq. Or do you want to commit the simple patch, close this one and open a new issue?

How about a new issue?

bq. Also, I've mostly seen gc performance problems so far if there were a big number of long-living
objects - So what do you think about removing the pooling of the PostingList objects?

It's not only the GC cost, it's also the cost of init'ing these objects.  EG filling in 0s
for all the fields, when we're gonna overwrite them anyway.

But, let's test on modern JREs to confirm this?  I do agree pooling adds code complexity,
so, if it's not buying us anything (or very little) we should remove it.

The worst case should be docs with many unique terms...  Though... to reduce our per-unique-term
RAM cost, we may want to move away from separate postings object per term to parallel arrays.
 We also could do something different for singleton terms vs the rest (if Zipf's law is applying,
half the terms should be singletons; if it's not, you could have many more singleton terms...).
 I'd do this as an experimental indexing chain :)

> IndexWriter has hard limit on max concurrency
> ---------------------------------------------
>                 Key: LUCENE-2293
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Index
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>             Fix For: 3.1
>         Attachments: LUCENE-2293.patch
> 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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