Messages in this thread | | | Date | Thu, 21 Feb 2013 12:51:20 +0800 | From | Michael Wang <> | Subject | Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair() |
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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 >
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