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https://issues.apache.org/jira/browse/LUCENE-888?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12499206
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Michael Busch commented on LUCENE-888:
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> I'm a little nervous about that: I don't know the impact it will have
> on searching, especially queries that heavily use skipping?
Doesn't the OS always read at least a whole block from the disk (usually
4 KB)? If the answer is yes, then we don't safe IO by limiting the buffer
size to 1 KB, right? Of course it makes sense to limit the size for
streams that we clone often (like the freq stream) to safe memory and
array copies. But we never clone the base stream in the cfs reader.
> Hmmm, actually, a CSIndexInput potentially goes through 2 buffers when
> it does a read -- its own (since each CSIndexInput subclasses from
> BufferedIndexInput) and then the main stream of the
> CompoundFileReader. It seems like we shouldn't do this? We should
> not do a double copy.
>
> It almost seems like the double copy would not occur becaase
> readBytes() has logic to read directly from the underlying stream if
> the sizes is >= bufferSize. However, I see at least one case during
> merging where this logic doesn't kick in (and we do a double buffer
> copy) because the buffers become "skewed". I will open a separate
> issue for this; I think we should fix it and gain some more
> performance.
Good catch! Reminds me a bit of LUCENE-431 where we also did double
buffering for the RAMInputStream and RAMOutputStream. Yes, I think
we should fix this.
> 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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