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From "Tim Armstrong (Code Review)" <>
Subject [Impala-ASF-CR] IMPALA-5713: always reserve memory for preaggs
Date Wed, 23 Aug 2017 23:31:08 GMT
Tim Armstrong has posted comments on this change.

Change subject: IMPALA-5713: always reserve memory for preaggs

Patch Set 3:

File fe/src/main/java/org/apache/impala/planner/

PS3, Line 327: 16b
> I vote for omitting that last calculation.

Line 330:         perInstanceMinReservation = (bufferSize + 64 * 1024) * PARTITION_FANOUT;
> Tim once suggested that the pre-agg should really be a separate exec node. 
>  Why even have 16 partitions etc.

I tried changing that once and it didn't go well :). It's actually much slower to have a single
partition for some aggregations. There's a strong pre-partitioning effect that improves memory
locality in the merge aggregation.

But yeah it's ripe for a lot of cleanup - we could separate out the ExecNode implementations
and figure out a better way to exploit the pre-partitioning (since the effect is now accidental
and undocumented).

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Gerrit-MessageType: comment
Gerrit-Change-Id: I2b544f9ec1a906719e2bbb074743926b95a3a573
Gerrit-PatchSet: 3
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Tim Armstrong <>
Gerrit-Reviewer: Alex Behm <>
Gerrit-Reviewer: Dan Hecht <>
Gerrit-Reviewer: Mostafa Mokhtar <>
Gerrit-Reviewer: Tim Armstrong <>
Gerrit-HasComments: Yes

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