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https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13127185#comment-13127185
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Matt Corgan commented on HBASE-4218:
------------------------------------
I'm trying to hook the prefix trie code into this, which is going well enough.
Testing on some HFileV1 data, i think i'm seeing some double-decoding in
HFileReaderV1.java:328. You encode the block to put in the block cache in
blockDeltaEncoder.beforeBlockCache(..), but then go back to using the unencoded
version, which triggers a second encoding a few lines later at
blockDeltaEncoder.afterReadFromDiskAndPuttingInCache(..). Possible change:
{code}
// Cache the block
if (cacheBlock && blockCache != null) {
HFileBlock cachedBlock = blockDeltaEncoder.beforeBlockCache(hfileBlock);
blockCache.cacheBlock(cacheKey, cachedBlock, inMemory);
}
hfileBlock = blockDeltaEncoder.afterReadFromDiskAndPuttingInCache(
hfileBlock, isCompaction);
{code}
{code}
// Cache the block
if (cacheBlock && blockCache != null) {
hfileBlock = blockDeltaEncoder.beforeBlockCache(hfileBlock);
blockCache.cacheBlock(cacheKey, hfileBlock, inMemory);
}
hfileBlock = blockDeltaEncoder.afterReadFromDiskAndPuttingInCache(
hfileBlock, isCompaction);
{code}
A few other comments:
* I wonder if we could make some of the naming more general than "Delta"
encoding since that's not the only type it can support. I added a TRIE entry
to DeltaEncoderAlgorithms. Maybe we could call it KeyValueEncoding,
DataBlockEncoding, HCellEncoding, BlockEncoding, etc...
* saw "comparator" spelled "comperator" several places
* seems like PREFIX is always the winner. are the others better at certain
datasets, or are they just there for comparison?
* i've been running the tests on different block sizes from 1KB to 1MB and
seeing seeks/s decline from ~300,000/s to 3,000/s because of the sequential
access inside a block. even using 64KB block is ~6x slower than 1KB blocks
{code}
table,encoding,blockSize,numCells,avgKeyBytes,avgValueBytes,sequentialMB/s,seeks/s,~cycles/seek
Count5s,PREFIX,1KB ,1338940,85,9,167,323685, 6178
Count5s,PREFIX,4KB ,1338627,85,9,281,334873, 5972
Count5s,PREFIX,16KB ,1338420,85,9,381,168987, 11835
Count5s,PREFIX,64KB ,1338016,85,9,380, 52781, 37891
Count5s,PREFIX,256KB,1339210,85,9,392, 14203,140810
Count5s,PREFIX,1MB ,1337318,85,9,371, 3703,539958
{code}
> Delta Encoding of KeyValues (aka prefix compression)
> -----------------------------------------------------
>
> Key: HBASE-4218
> URL: https://issues.apache.org/jira/browse/HBASE-4218
> Project: HBase
> Issue Type: Improvement
> Components: io
> Affects Versions: 0.94.0
> Reporter: Jacek Migdal
> Labels: compression
> Attachments: open-source.diff
>
>
> A compression for keys. Keys are sorted in HFile and they are usually very
> similar. Because of that, it is possible to design better compression than
> general purpose algorithms,
> It is an additional step designed to be used in memory. It aims to save
> memory in cache as well as speeding seeks within HFileBlocks. It should
> improve performance a lot, if key lengths are larger than value lengths. For
> example, it makes a lot of sense to use it when value is a counter.
> Initial tests on real data (key length = ~ 90 bytes , value length = 8 bytes)
> shows that I could achieve decent level of compression:
> key compression ratio: 92%
> total compression ratio: 85%
> LZO on the same data: 85%
> LZO after delta encoding: 91%
> While having much better performance (20-80% faster decompression ratio than
> LZO). Moreover, it should allow far more efficient seeking which should
> improve performance a bit.
> It seems that a simple compression algorithms are good enough. Most of the
> savings are due to prefix compression, int128 encoding, timestamp diffs and
> bitfields to avoid duplication. That way, comparisons of compressed data can
> be much faster than a byte comparator (thanks to prefix compression and
> bitfields).
> In order to implement it in HBase two important changes in design will be
> needed:
> -solidify interface to HFileBlock / HFileReader Scanner to provide seeking
> and iterating; access to uncompressed buffer in HFileBlock will have bad
> performance
> -extend comparators to support comparison assuming that N first bytes are
> equal (or some fields are equal)
> Link to a discussion about something similar:
> http://search-hadoop.com/m/5aqGXJEnaD1/hbase+windows&subj=Re+prefix+compression
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