Re: ppc32: Weird process scheduling behaviour with 2.6.24-rc

On Mon, 2008-01-28 at 09:50 +0100, Peter Zijlstra wrote:

On Sun, 2008-01-27 at 17:13 +0100, Michel Dänzer wrote:

quoted

In summary, there are two separate problems with similar symptoms, which
had me confused at times:

      * With CONFIG_FAIR_USER_SCHED disabled, there are severe
        interactivity hickups with a niced CPU hog and top running. This
        started with commit 810e95ccd58d91369191aa4ecc9e6d4a10d8d0c8. 

The revert at the bottom causes the wakeup granularity to shrink for +
nice and to grow for - nice. That is, it becomes easier to preempt a +
nice task, and harder to preempt a - nice task.

Yeah, that matches my observations. :)

I think we originally had that; didn't comment it, forgot the reason
changed it because the units didn't match. Another reason might have
been the more difficult preemption of - nice tasks. That might - niced
tasks to cause horrible latencies - Ingo, any recollection?

Are you perhaps running with a very low HZ (HZ=100)? (If wakeup
preemption fails, tick preemption will take over).

I haven't had it below 250 since that became an option, and I'm
currently at 1000 (and NO_HZ, but disabling that didn't seem to make a
difference).

Also, could you try lowering:
  /proc/sys/kernel/sched_wakeup_granularity_ns

Will try.

quoted

      * With CONFIG_FAIR_USER_SCHED enabled, X becomes basically
        unusable with a niced CPU hog, with or without top running. I
        don't know when this started, possibly when this option was
        first introduced.

Srivatsa found an issue that might explain the very bad behaviour under
group scheduling. But I gather you're not at all interested in this
feature?

That's right, but it's good to hear you have a lead there as well, and
if you can't find any interested testers, let me know and I'll try.


-- 
Earthling Michel Dänzer           |          http://tungstengraphics.com
Libre software enthusiast         |          Debian, X and DRI developer