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