Re: High load average on idle machine running 2.6.32
From: James Pearson
Date: Mon Dec 14 2009 - 12:49:33 EST
James Pearson wrote:
I've booted a 64 bit 2.6.32 kernel on dual processor, quad core Xeon
E5440 machine. The load average when the machine is idle varies
between 2 and 3.
When using a 2.6.31 kernel on the same machine, the load average when
idle is nearly 0
The kernel doesn't use modules - all that is needed is compiled in.
The machine uses NFS-root
Strangely, when I run 'iftop' (from
http://www.ex-parrot.com/pdw/iftop/) using the 2.6.32 kernel, the load
average drops to below 0.5 - stop running iftop, and the load average
climbs again ...
Any idea what might be causing this?
It looks like whatever is causing this happened between 2.6.31-git7 and
2.6.31-git8 - unfortunately I don't know how to find out what change
caused this ...
Also, if I 'hot-unplug' CPUs 1 to 7, the load average drops to 0 - when
I re-enable theses CPUs, the load average climbs.
I guess this is a problem with my particular config - or maybe because
I'm using NFS-root (the root file system is readonly), or using a
non-module kernel?
I gave 'git bisect' a go - which appears to suggest that my problem
started at:
% git bisect bad
d7c33c4930f569caf6b2ece597432853c4151a45 is first bad commit
commit d7c33c4930f569caf6b2ece597432853c4151a45
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date: Fri Sep 11 12:45:38 2009 +0200
sched: Fix task affinity for select_task_rq_fair
While merging select_task_rq_fair() and sched_balance_self() I made
a mistake that leads to testing the wrong task affinty.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
:040000 040000 3d7aa3e193c7faf9c7ebbb1443c6f63269d86d04
9cfb647eb5d80f156fd8a495da68f765c3fdd772 M kernel
However, while running the bisects, it became harder to decide what was
a 'bad' and a 'good' idle load average - for example the kernel with the
above patch gave an idle load average of about 1.5 - which is not as
high as the idle load average seen with a 2.6.32 kernel and the kernel
without this patch gave an idle load average of about 0.7 - which is not
as low as the idle load average with a 2.6.31 kernel ...
So I guess, it is not just one patch that has caused the issue I'm
seeing, which I guess is to be expected as the above patch was part of
the 'scheduler updates for v2.6.32' patch set
<http://marc.info/?l=linux-kernel&m=125322428306777&w=2>
I guess as no one else has reported this issue - it must be something to
do with my set up - could using NFS-root affect how the load average is
calculated?
Or, do I have something strange or missing in my kernel config that
could cause this issue?
Thanks
James Pearson
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