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https://issues.apache.org/jira/browse/CASSANDRA-12961?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16056563#comment-16056563
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Jeff Jirsa commented on CASSANDRA-12961:
----------------------------------------
[~calonso] - if you're still interested, I think it's simpler than that.
In the above code block, look at:
{code}
CompactionCandidate l0Compaction =
getSTCSInL0CompactionCandidate();
{code}
If you move that before the loop, we can still keep the check where it is:
{code}
if (l0Compaction != null)
return l0Compaction;
{code}
At each level, without having to actually call all the way down to the
(relatively expensive)
{{SizeTieredCompactionStrategy.createSSTableAndLengthPairs}} /
{{SizeTieredCompactionStrategy.getBuckets}}
> 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: Improvement
> 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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