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https://issues.apache.org/jira/browse/CASSANDRA-18773?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17766678#comment-17766678
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Cameron Zemek commented on CASSANDRA-18773:
-------------------------------------------
[~blambov] have updated the pull request with your feedback and is ready for
review.
> Compactions are slow
> --------------------
>
> Key: CASSANDRA-18773
> URL: https://issues.apache.org/jira/browse/CASSANDRA-18773
> Project: Cassandra
> Issue Type: Improvement
> Components: Local/Compaction
> Reporter: Cameron Zemek
> Priority: Normal
> Fix For: 4.0.x, 4.1.x, 5.0.x, 5.x
>
> Attachments: 18773.patch, compact-poc.patch, flamegraph.png,
> stress.yaml
>
> Time Spent: 1h 10m
> Remaining Estimate: 0h
>
> I have noticed that compactions involving a lot of sstables are very slow
> (for example major compactions). I have attached a cassandra stress profile
> that can generate such a dataset under ccm. In my local test I have 2567
> sstables at 4Mb each.
> I added code to track wall clock time of various parts of the code. One
> problematic part is ManyToOne constructor. Tracing through the code for every
> partition creating a ManyToOne for all the sstable iterators for each
> partition. In my local test get a measy 60Kb/sec read speed, and bottlenecked
> on single core CPU (since this code is single threaded) with it spending 85%
> of the wall clock time in ManyToOne constructor.
> As another datapoint to show its the merge iterator part of the code using
> the cfstats from [https://github.com/instaclustr/cassandra-sstable-tools/]
> which reads all the sstables but does no merging gets 26Mb/sec read speed.
> Tracking back from ManyToOne call I see this in
> UnfilteredPartitionIterators::merge
> {code:java}
> for (int i = 0; i < toMerge.size(); i++)
> {
> if (toMerge.get(i) == null)
> {
> if (null == empty)
> empty = EmptyIterators.unfilteredRow(metadata,
> partitionKey, isReverseOrder);
> toMerge.set(i, empty);
> }
> }
> {code}
> Not sure what purpose of creating these empty rows are. But on a whim I
> removed all these empty iterators before passing to ManyToOne and then all
> the wall clock time shifted to CompactionIterator::hasNext() and read speed
> increased to 1.5Mb/s.
> So there are further bottlenecks in this code path it seems, but the first is
> this ManyToOne and having to build it for every partition read.
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