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From "Jeffrey Zhong (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-13389) [REGRESSION] HBASE-12600 undoes skip-mvcc parse optimizations
Date Fri, 03 Apr 2015 06:05:53 GMT

    [ https://issues.apache.org/jira/browse/HBASE-13389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14394111#comment-14394111
] 

Jeffrey Zhong commented on HBASE-13389:
---------------------------------------

[~stack] The performance regression is due to we keep mvcc values longer(HBASE-11315) so comes
the later change https://github.com/apache/hbase/commit/2c280e62530777ee43e6148fd6fcf6dac62881c0#diff-07c7ac0a9179cedff02112489a20157fR96.


I'm surprised the extra mvcc value caused so much perf regression. Here is the code which
calculates minSeqId to keep in file Compactor.java during compaction. 

{code}
        // when isAllFiles is true, all files are compacted so we can calculate the smallest

        // MVCC value to keep
        if(fd.minSeqIdToKeep < file.getMaxMemstoreTS()) {
          fd.minSeqIdToKeep = file.getMaxMemstoreTS();
        }
....
        // output to writer:
        for (Cell c : cells) {
          if (cleanSeqId && c.getSequenceId() <= smallestReadPoint) {
            CellUtil.setSequenceId(c, 0);
          }
{code}

> [REGRESSION] HBASE-12600 undoes skip-mvcc parse optimizations
> -------------------------------------------------------------
>
>                 Key: HBASE-13389
>                 URL: https://issues.apache.org/jira/browse/HBASE-13389
>             Project: HBase
>          Issue Type: Sub-task
>          Components: Performance
>            Reporter: stack
>
> HBASE-12600 moved the edit sequenceid from tags to instead exploit the mvcc/sequenceid
slot in a key. Now Cells near-always have an associated mvcc/sequenceid where previous it
was rare or the mvcc was kept up at the file level. This is sort of how it should be many
of us would argue but as a side-effect of this change, read-time optimizations that helped
speed scans were undone by this change.
> In this issue, lets see if we can get the optimizations back -- or just remove the optimizations
altogether.
> The parse of mvcc/sequenceid is expensive. It was noticed over in HBASE-13291.
> The optimizations undone by this changes are (to quote the optimizer himself, Mr [~lhofhansl]):
> {quote}
> Looks like this undoes all of HBASE-9751, HBASE-8151, and HBASE-8166.
> We're always storing the mvcc readpoints, and we never compare them against the actual
smallestReadpoint, and hence we're always performing all the checks, tests, and comparisons
that these jiras removed in addition to actually storing the data - which with up to 8 bytes
per Cell is not trivial.
> {quote}
> This is the 'breaking' change: https://github.com/apache/hbase/commit/2c280e62530777ee43e6148fd6fcf6dac62881c0#diff-07c7ac0a9179cedff02112489a20157fR96



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