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

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

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.

Yanmin

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