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From "Carl Yeksigian (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-12961) LCS needlessly checks for L0 STCS candidates multiple times
Date Mon, 28 Nov 2016 19:08:59 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-12961?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15702811#comment-15702811
] 

Carl Yeksigian commented on CASSANDRA-12961:
--------------------------------------------

I don't think we want to move this check outside of the loop, as we only want to do STCS in
L0 if we would otherwise do a compaction in a higher level - if we have the space in L1, we
would prefer to do a normal L0&rarr;L1 compaction to bring sstables out of L0, not continue
to compact them with each other in L0. But, we should definitely short circuit this search
after we've searched for the STCS candidates once.

> LCS needlessly checks for L0 STCS candidates multiple times
> -----------------------------------------------------------
>
>                 Key: CASSANDRA-12961
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12961
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Compaction
>            Reporter: Jeff Jirsa
>            Priority: Trivial
>              Labels: lhf
>
> It's very likely that the check for L0 STCS candidates (if L0 is falling behind) can
be moved outside of the loop, or at very least made so that it's not called on each loop iteration:
> {code}
>         for (int i = generations.length - 1; i > 0; i--)
>         {
>             List<SSTableReader> sstables = getLevel(i);
>             if (sstables.isEmpty())
>                 continue; // mostly this just avoids polluting the debug log with zero
scores
>             // we want to calculate score excluding compacting ones
>             Set<SSTableReader> sstablesInLevel = Sets.newHashSet(sstables);
>             Set<SSTableReader> remaining = Sets.difference(sstablesInLevel, cfs.getTracker().getCompacting());
>             double score = (double) SSTableReader.getTotalBytes(remaining) / (double)maxBytesForLevel(i,
maxSSTableSizeInBytes);
>             logger.trace("Compaction score for level {} is {}", i, score);
>             if (score > 1.001)
>             {
>                 // before proceeding with a higher level, let's see if L0 is far enough
behind to warrant STCS
>                 CompactionCandidate l0Compaction = getSTCSInL0CompactionCandidate();
>                 if (l0Compaction != null)
>                     return l0Compaction;
> ......
> {code}



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