Re: [PATCH] Avoid moving tasks when a schedule can be made.

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

Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
> 
>>i think the right approach would be to split up this work into smaller 
>>chunks. Or rather, lets first see how this can happen: why is 
>>can_migrate() false for so many tasks? Are they all cpu-hot? If yes, 
>>shouldnt we simply skip only up to a limit of tasks in this case - 
>>it's not like we want to spend 1.5 msecs searching for a cache-cold 
>>task which might give us a 50 usecs advantage over cache-hot tasks ...
> 
> 
> the only legimate case where we have to skip alot of tasks is the case 
> when there are alot of CPU-bound (->cpus_alowed) tasks in the runqueue.  
> In that case the scheduler really has to skip that task. But that is not 
> an issue in your workload.

The new SMP nice load balancing may cause some tasks to be skipped while 
it's looking for ones with small enough bias_prios but I doubt that this 
would cause many to be skipped on a real system.

BTW why do you assume that this problem is caused by can_migrate() 
failing and is not just simply due to large numbers of tasks needing to 
be moved (which is highly likely to be true when hackbench is running)?

If it is a "skip" problem that would make the "busting it up into 
smaller chunks" solution more complex as it would make it desirable to 
remember where you were up to between chunks.

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

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