Re: Scheduler fairness problem on 2.6 series (Attn: Nick Piggin and others)

[Lee Revell <rlrevell@joe-job.com>]

On Sat, 2004-08-28 at 20:25, Lee Revell wrote:
> On Sat, 2004-08-28 at 20:21, Peter Williams wrote:
> > spaminos-ker@yahoo.com wrote:
> > > --- Peter Williams <pwil3058@bigpond.net.au> wrote:
> 
> > >     -----------------
> > >  => started at: kernel_fpu_begin+0x21/0x60
> > >  => ended at:   _mmx_memcpy+0x131/0x180
> > > =======>
> > > 00000001 0.000ms (+0.000ms): kernel_fpu_begin (_mmx_memcpy)
> > > 00000001 0.730ms (+0.730ms): sub_preempt_count (_mmx_memcpy)
> > > 00000001 0.730ms (+0.000ms): _mmx_memcpy (check_preempt_timing)
> > > 00000001 0.730ms (+0.000ms): kernel_fpu_begin (_mmx_memcpy)
> > > 
> > 
> > As far as I can see sub_preempt_count() is part of the latency measuring 
> > component of the voluntary preempt patch so, like you, I'm not sure 
> > whether this report makes any sense.
> 
> Is this an SMP machine?  There were problems with that version of the
> voluntary preemption patches on SMP.  The latest version, Q3, should fix
> these.
> 

Hmm, after rereading the entire thread, I am not sure that voluntary
preemption will help you here.  Voluntary preemption (and preemption in
general) deals with the situation in which you have a high priority
task, often the highest priority task on the system, that spends most of
its time sleeping on some resource, and this task needs to run as soon
as possible once it becomes runnable.  In that situation the scheduler
doesn't have a very difficult decision, there is no question that it
should run this task ASAP.  How long 'ASAP' is depends on how long it
takes whatever task was running when our high priority task became
runnable to yield the processor.  The scheduler has a very easy job
here, there is only one right thing to do.  Also the intervals involved
are very small, usually less than 1ms, whereas you are talking about a
variance of several seconds.

In the situation you describe, you have two tasks running at the same
base priority, and the scheduler does not seem to be doing a good job
balancing them.  This is a different situation, and much more dependent
on the scheduling policy.

Lee