[CASSANDRA-12961] LCS needlessly checks for L0 STCS candidates multiple times

• Type: Improvement
• Status: Resolved
• Priority: Low
• Resolution: Fixed
• Fix Version/s: 4.0-alpha1, 4.0
• Component/s: Local/Compaction
• Labels: lhf
• Assignee: Vusal Ahmadoglu
• Reporter: Jeff Jirsa

Authors

Vusal Ahmadoglu

Reviewers

Jeff Jirsa

• Created: 28/Nov/16 03:36
• Updated: 15/May/20 08:03
• Resolved: 03/Oct/17 00:53

Description

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:

        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;
......