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From "Sergey Shelukhin (JIRA)" <>
Subject [jira] [Commented] (HIVE-10600) optimize group by for GC
Date Tue, 11 Aug 2015 22:42:46 GMT


Sergey Shelukhin commented on HIVE-10600:

[~mmccline] could one say you are doing this as part of vectorized group by? :)

> optimize group by for GC
> ------------------------
>                 Key: HIVE-10600
>                 URL:
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Sergey Shelukhin
> Quoting [~gopalv]:
> {noformat}
> So, something like a sum() GROUP BY will create a few hundred thousand
> AbstractAggregationBuffer objects all of which will suddenly go out of
> scope when the map.aggr flushes it down to the sort buffer.
> That particular GC collection takes forever because the tiny buffers take
> a lot of time to walk over and then they leave the memory space
> fragmented, which requires a compaction pass (which btw, writes to a
> page-interleaved NUMA zone).
> And to make things worse, the pre-allocated sort buffers with absolutely
> zero data in them take up most of the tenured regions causing these chunks
> of memory to be visited more and more often as they are part of the Eden
> space.
> {noformat}
> We need flat data structures to be GC friendly.

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