linux-kernel - Re: [PATCH] sched: smart wake-affine

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Date:	Thu, 04 Jul 2013 12:33:33 +0200
From:	Mike Galbraith <efault@....de>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Alex Shi <alex.shi@...el.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Turner <pjt@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Ram Pai <linuxram@...ibm.com>
Subject: Re: [PATCH] sched: smart wake-affine

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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