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