lkml.org 
[lkml]   [2018]   [Feb]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v3 1/3] sched/fair: add util_est on top of PELT
On 30-Jan 15:01, Peter Zijlstra wrote:
> On Tue, Jan 30, 2018 at 02:04:32PM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 30, 2018 at 12:46:33PM +0000, Patrick Bellasi wrote:
> > > > Aside from that being whitespace challenged, did you also try:
> > > >
> > > > if ((unsigned)((util_est - util_last) + LIM - 1) < (2 * LIM - 1))
> > >
> > > No, since the above code IMO is so much "easy to parse for humans" :)
> >
> > Heh, true. Although that's fixable by wrapping it in some helper with a
> > comment.
> >
> > > But, mainly because since the cache alignment update, also while testing on a
> > > "big" Intel machine I cannot see regressions on hackbench.
> > >
> > > This is the code I get on my Xeon E5-2690 v2:
> > >
> > > if (abs(util_est - util_last) <= (SCHED_CAPACITY_SCALE / 100))
> > > 6ba0: 8b 86 7c 02 00 00 mov 0x27c(%rsi),%eax
> > > 6ba6: 48 29 c8 sub %rcx,%rax
> > > 6ba9: 48 99 cqto
> > > 6bab: 48 31 d0 xor %rdx,%rax
> > > 6bae: 48 29 d0 sub %rdx,%rax
> > > 6bb1: 48 83 f8 0a cmp $0xa,%rax
> > > 6bb5: 7e 1d jle 6bd4 <dequeue_task_fair+0x7e4>
> > >
> > > Does it look so bad?
> >
> > Its not terrible, and I think your GCC is far more clever than the one I
>
> To clarify; my GCC at the time generated conditional branches to compute
> the absolute value; and in that case the thing I proposed wins hands
> down because its unconditional.
>
> However the above is also unconditional and then the difference is much
> less important.

I've finally convinced myself that we can live with the "parsing
complexity" of your proposal... and wrapped into an inline it turned
out to be not so bad.

> > used at the time. But that's 4 dependent instructions (cqto,xor,sub,cmp)
> > whereas the one I proposed uses only 2 (add,cmp).

The ARM64 generated code is also simpler.

> > Now, my proposal is, as you say, somewhat hard to read, and it also
> > doesn't work right when our values are 'big' (which they will not be in
> > our case, because util has a very definite bound), and I suspect you're
> > right that ~2 cycles here will not be measurable.

Indeed, I cannot see noticeable differences if not just a slightly
improvement...

> >
> > So yeah.... whatever ;-)

... I'm going to post a v4 using your proposal ;-)

Thanks Patrick

--
#include <best/regards.h>

Patrick Bellasi

\
 
 \ /
  Last update: 2018-02-05 18:51    [W:0.037 / U:4.748 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site