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

Dimitrios Apostolou
2007-08-06T18:28:01Z

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