[Impala-CR](cdh5-trunk) IMPALA-3019: Fix unnecessary resets of iterator

["Lars Volker (Code Review)" <ge...@cloudera.org>]

Hello Internal Jenkins,

I'd like you to do a code review.  Please visit

    http://gerrit.cloudera.org:8080/3031

to review the following change.

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-on: http://gerrit.cloudera.org:8080/2330
Reviewed-by: Lars Volker <lv...@cloudera.com>
Tested-by: Internal Jenkins
---
M be/src/scheduling/simple-scheduler.cc
1 file changed, 6 insertions(+), 1 deletion(-)


  git pull ssh://gerrit.cloudera.org:29418/Impala refs/changes/31/3031/1
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Gerrit-MessageType: newchange
Gerrit-Change-Id: I831d485b46c7d9460fb014a302a26864b6bd573e
Gerrit-PatchSet: 1
Gerrit-Project: Impala
Gerrit-Branch: cdh5-trunk
Gerrit-Owner: Lars Volker <lv...@cloudera.com>
Gerrit-Reviewer: Internal Jenkins