Re: [PATCH] sched: smart wake-affine
From: Peter Zijlstra
Date: Thu Jul 04 2013 - 05:14:24 EST
On Tue, Jul 02, 2013 at 05:35:33PM +0800, Michael Wang wrote:
>
>> + if (jiffies > current->last_switch_decay + HZ) {
>
>> + current->nr_wakee_switch = 0;
>
>> + current->last_switch_decay = jiffies;
>
>> + }
>
>
>
> This isn't so much a decay as it is wiping state. Did you try an actual
>
> decay -- something like: current->nr_wakee_switch >>= 1; ?
>
>
>
> I suppose you wanted to avoid something like:
>
>
>
> now = jiffies;
>
> while (now > current->last_switch_decay + HZ) {
>
> current->nr_wakee_switch >>= 1;
>
> current->last_switch_decay += HZ;
>
> }
>
>
Right, actually I have though about the decay problem with some testing,
>
including some similar implementations like this, but one issue I could
>
not solve is:
>
>
the task waken up after dequeue 10secs and the task waken up
>
after dequeue 1sec will suffer the same decay.
>
>
Thus, in order to keep fair, we have to do some calculation here to make
>
the decay correct, but that means cost...
Right, but something like the below is limited in cost to at most 32/64 (I
forgot the type) shifts. Now its probably not worth doing, but it shows
things like that can be done in 'constant' time.
now = jiffies;
if (now - p->last_switch_decay > 8*sizeof(p->nr_wakee_switch)*HZ) {
p->nr_wakee_switch = 0;
p->last_switch_decay = now;
} else while (now > p->last_switch_decay + HZ) {
p->nr_wakee_switch >>= 1;
p->last_switch_decay += HZ;
}
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