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https://issues.apache.org/jira/browse/LUCENE-888?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12499224
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Eks Dev commented on LUCENE-888:
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we did some time ago a few tests and simply concluded, it boils down to what
Doug said, "It does incur extra system calls, but uses less memory, which is a
tradeoff."
in *our* setup 4k was kind of magic number , ca 5-8% faster. I guess it is
actually a compromise between time spent in extra os calls vs probability of
reading more than you will really use (waste time on it). Why 4k number happens
often to be good compromise is probably the difference in speed of buffer copy
for 4k vs 1k being negligible compared to time spent on system calls.
The only conclusion we came up to is that you have to measure it and find good
compromise. Our case is a bit atypical, short documents (1G index, 60Mio docs)
and queries with a lot of terms (80-200), Win 2003 Server, single disk.
And I do not remember was it before or after we started using MMAP, so no idea
really if this affects MMAP setup, guess not.
> Improve indexing performance by increasing internal buffer sizes
> ----------------------------------------------------------------
>
> Key: LUCENE-888
> URL: https://issues.apache.org/jira/browse/LUCENE-888
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Affects Versions: 2.1
> Reporter: Michael McCandless
> Assigned To: Michael McCandless
> Priority: Minor
> Attachments: LUCENE-888.patch
>
>
> In working on LUCENE-843, I noticed that two buffer sizes have a
> substantial impact on overall indexing performance.
> First is BufferedIndexOutput.BUFFER_SIZE (also used by
> BufferedIndexInput). Second is CompoundFileWriter's buffer used to
> actually build the compound file. Both are now 1 KB (1024 bytes).
> I ran the same indexing test I'm using for LUCENE-843. I'm indexing
> ~5,500 byte plain text docs derived from the Europarl corpus
> (English). I index 200,000 docs with compound file enabled and term
> vector positions & offsets stored plus stored fields. I flush
> documents at 16 MB RAM usage, and I set maxBufferedDocs carefully to
> not hit LUCENE-845. The resulting index is 1.7 GB. The index is not
> optimized in the end and I left mergeFactor @ 10.
> I ran the tests on a quad-core OS X 10 machine with 4-drive RAID 0 IO
> system.
> At 1 KB (current Lucene trunk) it takes 622 sec to build the index; if
> I increase both buffers to 8 KB it takes 554 sec to build the index,
> which is an 11% overall gain!
> I will run more tests to see if there is a natural knee in the curve
> (buffer size above which we don't really gain much more performance).
> I'm guessing we should leave BufferedIndexInput's default BUFFER_SIZE
> at 1024, at least for now. During searching there can be quite a few
> of this class instantiated, and likely a larger buffer size for the
> freq/prox streams could actually hurt search performance for those
> searches that use skipping.
> The CompoundFileWriter buffer is created only briefly, so I think we
> can use a fairly large (32 KB?) buffer there. And there should not be
> too many BufferedIndexOutputs alive at once so I think a large-ish
> buffer (16 KB?) should be OK.
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