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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6544) Reduce GC activity during compaction
Date Wed, 08 Jan 2014 13:01:52 GMT


Benedict commented on CASSANDRA-6544:

Good point. I was looking at the 2.0 branch I briefly had checked out.

That said, it could still be beneficial, and would require an even smaller buffer so less
downside as well. No real need to move the buffer off heap, but re-using a buffer could help
avoid both young-gen churn and also, if the tables being merged are large and have long intervals
where neither overlap, could help mitigate promotion from eden so help keep pauses low.

That all said, though, for small rows the general object overhead of LCR, LCR.Reducer, ColumnStats
etc are likely to outweigh any BB costs now.

> Reduce GC activity during compaction
> ------------------------------------
>                 Key: CASSANDRA-6544
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Vijay
>            Assignee: Vijay
>             Fix For: 2.1
> We are noticing increase in P99 while the compactions are running at full stream. Most
of it is because of the increased GC activity (followed by full GC).
> The obvious solution/work around is to throttle the compactions, but with SSD's we can
get more disk bandwidth for reads and compactions.
> It will be nice to move the compaction object allocations off heap. First thing to do
might be create a Offheap Slab allocator with the size as the compaction in memory size and
recycle it. 
> Also we might want to make it configurable so folks can disable it when they don't have
off-heap memory to reserve.

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