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https://issues.apache.org/jira/browse/LUCENE-2293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12841744#action_12841744
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Michael Busch commented on LUCENE-2293:
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{quote}
But if each DW maintains its own doc IDs, separately from the others, what will
be stored in the int[]? DW1 deleted docID 0 (its 0) and DW4 deleted the same.
The two documents are not the same one ... no?
{quote}
In DW you don't delete by docID. You can only delete by term or query. You have
to run the (term)query in all DWs to determine if any of the DWs have one or
more matching docs that have to be deleted.
Today the queries and/or terms are buffered, along with the maxDocID at the
time the delete or update was called. They are applied just after the DW buffer
was flushed to a segment, be cause that's the first time the docs are
searchable and the delete queries can be executed.
In the future, when we can search the DW buffer(s), you can apply the deletes
right away. Using this int[] approach for deletes will avoid the need of
cloning bitsets in each reopen.
> 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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