On 02/20/2013 06:49 PM, Ingo Molnar wrote: [snip] > > The changes look clean and reasoable, any ideas exactly *why* it > speeds up? > > I.e. are there one or two key changes in the before/after logic > and scheduling patterns that you can identify as causing the > speedup?
Hi, Ingo Thanks for your reply, please let me point out the key changes here (forgive me for haven't wrote a good description in cover). The performance improvement from this patch set is: 1. delay the invoke on wake_affine(). 2. save the circle to gain proper sd. The second point is obviously, and will benefit a lot when the sd topology is deep (NUMA is suppose to make it deeper on large system). So in my testing on a 12 cpu box, actually most of the benefit comes from the first point, and please let me introduce it in detail. The old logical when locate affine_sd is: if prev_cpu != curr_cpu if wake_affine() prev_cpu = curr_cpu new_cpu = select_idle_sibling(prev_cpu) return new_cpu The new logical is same to the old one if prev_cpu == curr_cpu, so let's simplify the old logical like: 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? So the new logical in this patch set is: new_cpu = select_idle_sibling(prev_cpu) if idle_cpu(new_cpu) return new_cpu new_cpu = select_idle_sibling(curr_cpu) if idle_cpu(new_cpu) { if wake_affine() return new_cpu } return prev_cpu And now, unless we are really going to move load from prev_cpu to curr_cpu, we won't use wake_affine() any more. So we avoid wake_affine() when system load is low or high, for middle load, the worst cases is when failed to locate idle cpu in prev_cpu topology but succeed to locate one in curr_cpu's, but that's rarely happen and the benchmark results proved that point. Some comparison below: 1. system load is low old logical cost: wake_affine() select_idle_sibling() new logical cost: select_idle_sibling() 2. system load is high old logical cost: wake_affine() select_idle_sibling() new logical cost: select_idle_sibling() select_idle_sibling() 3. system load is middle don't know 1 save the cost of wake_affine(), 3 could be proved by benchmark that no regression at least. For 2, it's the comparison between wake_affine() and select_idle_sibling(), since the system load is high, wake_affine() cost far more than select_idle_sibling(), and we saved many according to the benchmark results. > > Such changes also typically have a chance to cause regressions > in other workloads - when that happens we need this kind of > information to be able to enact plan-B. The benefit comes from avoiding unnecessary works, and the patch set is suppose to only reduce the cost of key function with least logical changing, I could not promise it benefit all the workloads, but till now, I've not found regression. Regards, Michael Wang > > Thanks, > > Ingo > -- 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/