Re: [PATCH] sched: smart wake-affine

From: Michael Wang
Date: Thu Jul 04 2013 - 22:48:11 EST


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?

My test could not get a stable differ, this time a little bit lose but
next time a little bit win, it's always floating, may caused by the
different chip cache-behaviour I suppose...

>
> 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...

But the task_struct is really a little big now, may be we could put the
'cold' members into a new structure and just record the pointer, that
may increase the chances of cache-hit the hot members, but it's platform
related and not so easy to be detect...

Regards,
Michael Wang

>
> -Mike
>
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