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https://issues.apache.org/jira/browse/LUCENE-2293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12845159#action_12845159
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Michael Busch commented on LUCENE-2293:
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bq. How about a new issue?
OK, will open one.
bq. (if Zipf's law is applying, half the terms should be singletons; if it's
not, you could have many more singleton terms...)
Yeah we should utilize our knowledge of term distribution to optimize in-memory
postings. For example, currently a nice optimization would be to store the
first posting in the PostingList object and only allocate slices once you see
the second occurrence (similar to the pulsing codec)?
bq. Though... to reduce our per-unique-term RAM cost, we may want to move away
from separate postings object per term to parallel arrays.
What exactly do you mean with parallel arrays? Parallel to the termHash array?
Then the termsHash array would not be an array of PostingList objects anymore,
but an array of pointers into the char[] array? And you'd have e.g. a parallel
int[] array for df, another int[] for pointers into the postings byte pool,
etc? Something like that?
> 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
>
> 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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