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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