Re: UDP-U stream performance regression on 32-rc1 kernel

From: Zhang, Yanmin
Date: Mon Nov 02 2009 - 23:32:30 EST

On Tue, 2009-11-03 at 11:47 +0800, Alex Shi wrote:
> We found the UDP-U 1k/4k stream of netperf benchmark have some
> performance regression from 10% to 20% on our Tulsa and some NHM
> machines.
perf events shows function find_busiest_group consumes about 4.5% cpu time
with the patch while it only consumes 0.5% cpu time without the patch.

The communication between netperf client and netserver is very fast.
When netserver receives a message and there is no new message available,
it goes to sleep and scheduler calls idle_balance => load_balance_newidle.
load_balance_newidle spends too much time and a new message arrives quickly
before load_balance_newidle ends.

As the comments in the patch say hackbench benefits from it, I tested hackbench
on Nehalem and core2 machines. hackbench does benefit from it, about 6% on
nehalem machines, but doesn't benefit on core2 machines.


> Bisecting found it is due to the following commitment.
> commit 840a0653100dbde599ae8ddf83fa214dfa5fd1aa
> Author: Ingo Molnar <mingo@xxxxxxx>
> Date: Fri Sep 4 11:32:54 2009 +0200

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