[jira] [Created] (CASSANDRA-12961) LCS needlessly checks for L0 STCS candidates multiple times

["Jeff Jirsa (JIRA)" <ji...@apache.org>]

Jeff Jirsa created CASSANDRA-12961:
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             Summary: 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



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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