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From "Lars Volker (Code Review)" <>
Subject [Impala-CR](cdh5-trunk) IMPALA-3019: Fix unnecessary resets of iterator
Date Thu, 12 May 2016 00:20:52 GMT
Lars Volker has uploaded a new patch set (#2).

Change subject: IMPALA-3019: Fix unnecessary resets of iterator

IMPALA-3019: Fix unnecessary resets of iterator

In order to perform round-robin backend selection, the simple
scheduler uses an iterator to the next backend entry to be selected.
This iterator needs to be reset whenever it is invalidated by changes
to the underlying map. The current behavior resets the pointer on
every message of the statestored, even if the message was empty and
thus did not result in any changes to the map.

After every reset of the iterator round-robin selection starts from
the first backend in the scheduler's backend map. As the statestored
sends empty keepalive messages every couple of seconds, this
effectively limits scheduling of remote reads to only a few backends.

This change introduces a check to prevent those unnecessary iterator
resets, which will spread remote reads more evenly over all backends.

Change-Id: I831d485b46c7d9460fb014a302a26864b6bd573e
Reviewed-by: Lars Volker <>
Tested-by: Internal Jenkins
(cherry picked from commit fc743d69f8ed671cebaca89f41f5373e326ffee5)
M be/src/scheduling/
1 file changed, 6 insertions(+), 1 deletion(-)

  git pull ssh:// refs/changes/31/3031/2
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Gerrit-MessageType: newpatchset
Gerrit-Change-Id: I831d485b46c7d9460fb014a302a26864b6bd573e
Gerrit-PatchSet: 2
Gerrit-Project: Impala
Gerrit-Branch: cdh5-trunk
Gerrit-Owner: Lars Volker <>
Gerrit-Reviewer: Internal Jenkins

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