On Thu, 2013-02-21 at 15:00 +0800, Michael Wang wrote: > 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?
If we didn't ever ask if it's ok, we'd always pull, and stack load up on one package if there's the tiniest of holes to stuff a task into, periodic balance forcibly rips buddies back apart, repeat. At least with wake_affine() in the loop, there's somewhat of a damper. > >> 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... We've always used wake_affine() before, yet been able to schedule at high frequency, so I don't see that it can be _that_ expensive. I haven't actually measured lately (loooong time) though. WRT cost/benefit of migration, yeah, it depends entirely on the tasks, some will gain, some will lose. On a modern single processor box, it just doesn't matter, there's only one llc (two s_i_s() calls = oopsie), but on my beloved old Q6600 or a big box, it'll matter a lot to something. NUMA balance will deal with big boxen, my trusty old Q6600 will likely get all upset with some localhost stuff. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
