Re: High load average on idle machine running 2.6.32

[Andrea Suisani]

[cc:ed Peter Zijlstra]

James Pearson wrote:
> 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
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/