Re: smpnice loadbalancing with high priority tasks

[Peter Williams <pwil3058@bigpond.net.au>]

Peter Williams wrote:
> Siddha, Suresh B wrote:
> > On Sun, Apr 02, 2006 at 05:08:33PM +1000, Peter Williams wrote:
> > > Siddha, Suresh B wrote:
> > > > Peter,
> > > >
> > > > There are still issues which we need to address.. These are surfacing
> > > > as we are patching issue by issue(instead of addressing the root 
> > > > issue, which
> > > > is: presence of high priority tasks messes up load balancing of normal
> > > > priority tasks..)
> > > >
> > > > for example
> > > >
> > > > a) on a simple 4-way MP system, if we have one high priority and 4 
> > > > normal
> > > > priority tasks, with smpnice we would like to see the high priority 
> > > > task
> > > > scheduled on one cpu, two other cpus getting one normal task each 
> > > > and the
> > > > fourth cpu getting the remaining two normal tasks. but with smpnice 
> > > > that extra normal priority task keeps jumping from one cpu to 
> > > > another cpu having
> > > > the normal priority task.
> > > >
> > > > This is because of the busiest_has_loaded_cpus, nr_loaded_cpus 
> > > > logic.. We
> > > > are not including the cpu with high priority task in max_load 
> > > > calculations
> > > > but including that in total and avg_load calcuations.. leading to 
> > > > max_load <
> > > > avg_load and load balance between cpus running normal priority 
> > > > tasks(2 Vs 1)
> > > > will always show imbalanace as one normal priority and the extra normal
> > > > priority task will keep moving from one cpu to another cpu having
> > > > normal priority task..
> > > I can't see anything like this in the code.  
> >
> > Don't you see a condition where max_load < avg_load(as mentioned in the
> > above example) and in this case, code ignores avg_load and imbalance
> > will aways be the extra normal priority task( coming from
> > "max_load - busiest_load_per_task") and this normal priority task 
> > keeps hopping from one cpu to another cpu having normal priority task..
> >
> > > Can you send a patch to fix what you think the problem in the is?
> >
> > I am looking at ways in fixing all these issues cleanly... I don't have
> > a clean solution yet...
>
> OK.  I think this means some fiddling with avg_load may be necessary in 
> some cases but this will be complex.  I'm not really happy about making 
> this code more complex until some of the current unnecessary complexity 
> is removed.  I.e. until a proper solution to the problem of triggering 
> active_load_balance() is implemented.

I forgot to mention that I've been looking at whether mucking around 
with avg_load is necessary and so far have been unable to convince 
myself that it is.  Your argument above hasn't changed that opinion.

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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