Messages in this thread | | | Date | Thu, 21 Feb 2013 15:00:23 +0800 | From | Michael Wang <> | Subject | Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair() |
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On 02/21/2013 02:11 PM, Mike Galbraith wrote: > On Thu, 2013-02-21 at 12:51 +0800, Michael Wang wrote: >> On 02/20/2013 06:49 PM, Ingo Molnar wrote: >> [snip] [snip] >> >> if wake_affine() >> new_cpu = select_idle_sibling(curr_cpu) >> else >> new_cpu = select_idle_sibling(prev_cpu) >> >> return new_cpu >> >> Actually that doesn't make sense. >> >> I think wake_affine() is trying to check whether move a task from >> prev_cpu to curr_cpu will break the balance in affine_sd or not, but why >> won't break balance means curr_cpu is better than prev_cpu for searching >> the idle cpu? > > You could argue that it's impossible to break balance by moving any task > to any idle cpu, but that would mean bouncing tasks cross node on every > wakeup is fine, which it isn't.
I don't get it... could you please give me more detail on how wake_affine() related with bouncing?
> >> So the new logical in this patch set is: >> >> new_cpu = select_idle_sibling(prev_cpu) >> if idle_cpu(new_cpu) >> return new_cpu > > So you tilted the scales in favor of leaving tasks in their current > package, which should benefit large footprint tasks, but should also > penalize light communicating tasks.
Yes, I'd prefer to wakeup the task on a cpu which: 1. idle 2. close to prev_cpu
So if both curr_cpu and prev_cpu have idle cpu in their topology, which one is better? that depends on how task benefit from cache and the balance situation, whatever, I don't think the benefit worth the high cost of wake_affine() in most cases...
Regards, Michael Wang
> > I suspect that much of the pgbench improvement comes from the preemption > mitigation from keeping 1:N load maximally spread, which is the perfect > thing to do with such loads. In all the testing I ever did with it in > 1:N mode, preemption dominated performance numbers. Keep server away > from clients, it has fewer fair competition worries, can consume more > CPU preemption free, pushing the load collapse point strongly upward. > > -Mike > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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