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https://issues.apache.org/jira/browse/CASSANDRA-14654?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Benedict updated CASSANDRA-14654:
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Status: Open (was: Ready to Commit)
> Reduce heap pressure during compactions
> ---------------------------------------
>
> Key: CASSANDRA-14654
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14654
> Project: Cassandra
> Issue Type: Improvement
> Components: Local/Compaction
> Reporter: Chris Lohfink
> Assignee: Chris Lohfink
> Priority: Normal
> Labels: Performance, pull-request-available
> Fix For: 4.x
>
> Attachments: screenshot-1.png, screenshot-2.png, screenshot-3.png,
> screenshot-4.png
>
> Time Spent: 40m
> Remaining Estimate: 0h
>
> Small partition compactions are painfully slow with a lot of overhead per
> partition. There also tends to be an excess of objects created (ie
> 200-700mb/s) per compaction thread.
> The EncoderStats walks through all the partitions and with mergeWith it will
> create a new one per partition as it walks the potentially millions of
> partitions. In a test scenario of about 600byte partitions and a couple 100mb
> of data this consumed ~16% of the heap pressure. Changing this to instead
> mutably track the min values and create one in a EncodingStats.Collector
> brought this down considerably (but not 100% since the
> UnfilteredRowIterator.stats() still creates 1 per partition).
> The KeyCacheKey makes a full copy of the underlying byte array in
> ByteBufferUtil.getArray in its constructor. This is the dominating heap
> pressure as there are more sstables. By changing this to just keeping the
> original it completely eliminates the current dominator of the compactions
> and also improves read performance.
> Minor tweak included for this as well for operators when compactions are
> behind on low read clusters is to make the preemptive opening setting a
> hotprop.
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