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SubjectRe: [PATCH] sched: smart wake-affine
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On Fri, 2013-07-05 at 10:47 +0800, Michael Wang wrote: 
> On 07/04/2013 06:33 PM, Mike Galbraith wrote:
> [snip]
> >> 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
>
> Wow, I used to think such issue is very hard to be tracked by
> benchmarks, is this regression stable?

Yeah, seems to be. I was curious as to why you saw an improvement to
hackbench, didn't seem there should be any, so though I'd try it on my
little box on the way to a long weekend. The unexpected happened.

> > 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 ;-)
>
> Hmm...so the new members kicked some tail members to a new line...or may
> be totally different when compiler take part in...
>
> It's really hard to estimate the influence, especially when the
> task_struct is still keep changing...

Yeah, could be memory layout crud that disappears with the next
pull/build. Wouldn't be the first time.

-Mike



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