[jira] [Commented] (HBASE-27370) Avoid decompressing blocks when reading from bucket cache prefetch threads

["Hudson (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-27370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17607350#comment-17607350 ] 

Hudson commented on HBASE-27370:
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Results for branch master
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> Avoid decompressing blocks when reading from bucket cache prefetch threads 
> ---------------------------------------------------------------------------
>
>                 Key: HBASE-27370
>                 URL: https://issues.apache.org/jira/browse/HBASE-27370
>             Project: HBase
>          Issue Type: Improvement
>    Affects Versions: 3.0.0-alpha-4
>            Reporter: Wellington Chevreuil
>            Assignee: Wellington Chevreuil
>            Priority: Major
>             Fix For: 2.6.0, 2.5.1, 3.0.0-alpha-4, 2.4.15
>
>
> When prefetching blocks into bucket cache, we had observed a consistent CPU usage around 70% with no other workloads ongoing. For large bucket caches (i.e. when using file based bucket cache), the prefetch can last for sometime and having such a high CPU usage may impact the database usage by client applications.
> Further analysis of the prefetch threads stack trace showed that very often, decompress logic is being executed by these threads:
> {noformat}
> "hfile-prefetch-1654895061122" #234 daemon prio=5 os_prio=0 tid=0x0000557bb2907000 nid=0x406d runnable [0x00007f294a504000]
>    java.lang.Thread.State: RUNNABLE
>         at org.apache.hadoop.io.compress.snappy.SnappyDecompressor.decompressBytesDirect(Native Method)
>         at org.apache.hadoop.io.compress.snappy.SnappyDecompressor.decompress(SnappyDecompressor.java:235)
>         at org.apache.hadoop.io.compress.BlockDecompressorStream.decompress(BlockDecompressorStream.java:88)
>         at org.apache.hadoop.io.compress.DecompressorStream.read(DecompressorStream.java:105)
>         at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
>         at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
>         - locked <0x00000002d24c0ae8> (a java.io.BufferedInputStream)
>         at org.apache.hadoop.hbase.io.util.BlockIOUtils.readFullyWithHeapBuffer(BlockIOUtils.java:105)
>         at org.apache.hadoop.hbase.io.compress.Compression.decompress(Compression.java:465)
>         at org.apache.hadoop.hbase.io.encoding.HFileBlockDefaultDecodingContext.prepareDecoding(HFileBlockDefaultDecodingContext.java:90)
>         at org.apache.hadoop.hbase.io.hfile.HFileBlock.unpack(HFileBlock.java:650)
>         at org.apache.hadoop.hbase.io.hfile.HFileReaderImpl.readBlock(HFileReaderImpl.java:1342) {noformat}
> This is because *HFileReaderImpl.readBlock* is always decompressing blocks even when *hbase.block.data.cachecompressed* is set to true. 
> This patch proposes an alternative flag to differentiate prefetch from normal reads, so that doesn't decompress DATA blocks when prefetching with  *hbase.block.data.cachecompressed* set to true. 



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