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https://issues.apache.org/jira/browse/LUCENE-2293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12841388#action_12841388
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Michael Busch commented on LUCENE-2293:
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{quote}
But, I was proposing a bigger change (call it "private RAM segments"):
there would be multiple DWs, each one writing to its own private RAM
segment (each one getting private docID assignment) and its own doc
stores.
{quote}
Cool! I wasn't sure if you wanted to give them private doc stores too. +1, I
like it.
> 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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