On Thu, 2013-07-04 at 17:38 +0800, Michael Wang wrote:
> On 07/04/2013 05:13 PM, Peter Zijlstra wrote:
> [snip]
> >
> > 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;
> > }
>
> Hmm...interesting, some kind of cataclysm decay, not sure how it works
> but yes, the cost was capped.
>
> Well, seems like we still have many follow-up research works after fix
> the issue ;-)
Yeah. Like how to how to exterminate the plus sign, they munch cache
lines, and have a general tendency to negatively impact benchmarks.
Q6600 box, hackbench -l 1000
avg
3.10.0-regress 2.293 2.297 2.313 2.291 2.295 2.297
1.000
3.10.0-regressx 2.560 2.524 2.427 2.599 2.602 2.542
1.106
pahole said...
marge:/usr/local/src/kernel/linux-3.x.git # tail virgin
long unsigned int timer_slack_ns; /* 1512 8 */
long unsigned int default_timer_slack_ns; /* 1520 8 */
atomic_t ptrace_bp_refcnt; /* 1528 4 */
/* size: 1536, cachelines: 24, members: 125 */
/* sum members: 1509, holes: 6, sum holes: 23 */
/* bit holes: 1, sum bit holes: 26 bits */
/* padding: 4 */
/* paddings: 1, sum paddings: 4 */
};
marge:/usr/local/src/kernel/linux-3.x.git # tail michael
long unsigned int default_timer_slack_ns; /* 1552 8 */
atomic_t ptrace_bp_refcnt; /* 1560 4 */
/* size: 1568, cachelines: 25, members: 128 */
/* sum members: 1533, holes: 8, sum holes: 31 */
/* bit holes: 1, sum bit holes: 26 bits */
/* padding: 4 */
/* paddings: 1, sum paddings: 4 */
/* last cacheline: 32 bytes */
};
..but plugging holes, didn't help, moving this/that around neither, nor
did letting pahole go wild to get the line back. It's plus signs I tell
ya, the evil things must die ;-)
-Mike
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