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From "Jeff Jirsa (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-14654) Reduce heap pressure during compactions
Date Fri, 17 Aug 2018 22:30:00 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-14654?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jeff Jirsa updated CASSANDRA-14654:
-----------------------------------
    Fix Version/s: 4.x

> Reduce heap pressure during compactions
> ---------------------------------------
>
>                 Key: CASSANDRA-14654
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14654
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Compaction
>            Reporter: Chris Lohfink
>            Assignee: Chris Lohfink
>            Priority: Major
>              Labels: Performance
>             Fix For: 4.x
>
>
> 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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