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https://issues.apache.org/jira/browse/LUCENE-10112?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17417684#comment-17417684
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Uwe Schindler edited comment on LUCENE-10112 at 9/20/21, 3:06 PM:
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Hi,
I had to revert the read/write of shorts, because this seems to cause errors.
It looks like Lucene 9 sometimes uses a EndiannessReverserDataInput which
causes test failures when reading 8.x indexes (which have different
endianness). This endianness reversal cause the LZ4 stream to have wrong shorts.
So we can't rely on endianness of DataInput passed to our LZ4 method and should
hardcode it when writing/reading the LZ4 format. So the PR only has the central
readInt method for hashing.
was (Author: thetaphi):
Hi,
I had to revert the read/write of shorts, because this seems to cause errors.
It looks like Lucene 9 sometimes uses a EndiannessReverserDataInput which
causes test failures when reading 8.x indexes (which have different
endianness). This endianness reversal cause the LZ4 stream to have wrong shorts.
So we can't rely on endianness and should hardcode it when writing/reading the
LZ4 format. So the PR only has the central readInt method for hashing.
> Improve LZ4 Compression performance with direct primitive read/writes
> ---------------------------------------------------------------------
>
> Key: LUCENE-10112
> URL: https://issues.apache.org/jira/browse/LUCENE-10112
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Tim Brooks
> Assignee: Uwe Schindler
> Priority: Minor
> Attachments: LUCENE-10112.patch
>
> Time Spent: 40m
> Remaining Estimate: 0h
>
> *Summary*
> Java9 introduced VarHandles as a tool to quickly read and write primitive
> types directly to byte arrays without bound checks. The LZ4 compressor class
> must consistently read ints from a byte array to analyze matches. The
> performance can be improved by reading these using a VarHandle.
> Additionally, the LZ4 compressor/decompressor methods currently individually
> read/write the bytes for LE shorts. Lucene's DataOutput/DataInput
> abstractions already have dedicated methods for reading/writing LE shorts.
> These methods are selectively optimized in certain implementations and will
> provide superior performance than individual byte reads.
> *Concerns*
> The DataOutput/DataInput readShort() and writeShort() methods do not call out
> that they are LE. It just looks to me that the DataOutput/DataInput are LE?
> Since this particular change does not appear to provide significant
> performance wins, maybe the patch is better leaving the explicit individual
> byte reads?
> Additionally, this patch changes read ints to read them in the platform
> native order which should be fine since it is just matching bytes. But I can
> change it to only read in the order the previous version did.
> *Benchmarks*
> I created JMH benchmarks which compresses 1MB of highly compressible JSON
> observability data. And compresses it 64KB at a time. In order to simulate
> the "short" changes, I use a forked version `ByteArrayDataOutput` which
> writes shorts using a VarHandle (to simulate fast writes that the ByteBuffer
> versions would get.) I also ran a benchmark without the short changes, just
> the reading ints using a VarHandle.
>
>
> {noformat}
> Benchmark Mode Cnt Score Error
> Units
> MyBenchmark.testCompressLuceneLZ4 thrpt 9 712.430 ± 3.616
> ops/s
> MyBenchmark.testCompressLuceneLZ4Forked thrpt 9 945.380 ± 4.776
> ops/s
> MyBenchmark.testCompressLuceneLZ4ForkedNoShort thrpt 9 940.812 ± 3.868
> ops/s
> MyBenchmark.testCompressLuceneLZ4HC thrpt 9 147.432 ± 4.730
> ops/s
> MyBenchmark.testCompressLuceneLZ4HCForked thrpt 9 183.954 ± 2.534
> ops/s
> MyBenchmark.testCompressLuceneLZ4HCForkedNoShort thrpt 9 188.065 ± 0.727
> ops/s{noformat}
>
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