linux-kernel - Re: high system cpu load during intense disk i/o

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Date:	Mon, 06 Aug 2007 21:27:56 +0200
From:	Dimitrios Apostolou <jimis@....net>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: high system cpu load during intense disk i/o

Hi,

Andrew Morton wrote:
> I suspect I was fooled by the oprofile output, which showed tremendous
> amounts of load in schedule() and switch_to().  The percentages which
> opreport shows are the percentage of non-halted CPU time.  So if you have a
> function in the kernel which is using 1% of the total CPU, and the CPU is
> halted for 95% of the time, it appears that the function is taking 20% of
> CPU!
> 
> The fix for that is to boot with the "idle=poll" boot parameter, to make
> the CPU spin when it has nothing else to do.

I'll test again the two_discs_bad situation after booting with that 
parameter. Thanks.

> 
> I'm suspecting that your machine is just stuck in D state waiting for disk.
>  Did we have a sysrq-T trace? 

The amazing thing is that this doesn't happen! Every single cron jobs 
that keeps running  (I intentionally said that before too) and never 
ends is in R state. By strace'ing the processes they just seem to be 
going *extremely* slow. I also changed the I/O elevator of hdb (the OS 
disk) to deadline from cfq, unfortunately with no results. That is why I 
've been considering it a CPU scheduler issue.

Dimitris
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