lkml.org 
[lkml]   [2009]   [Jul]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE
On Sat, 18 Jul 2009 23:00:55 -0400
Chris Snook <chris.snook@gmail.com> wrote:

> On Fri, Jul 17, 2009 at 7:39 PM, john stultz<johnstul@us.ibm.com>
> wrote:
> > -       if (likely(gtod->sysctl_enabled && gtod->clock.vread))
> > +       if (likely(gtod->sysctl_enabled))
>
> This irks me. If the sysctl is enabled and the codepath is getting
> used often enough that we care about performance, branch prediction
> should do the right thing without compiler hints. On the other hand,
> if the sysctl is disabled, and the compiler is telling the cpu to
> ignore its branch predictor, it'll hurt. I don't think we should be
> wrapping (un)likely annotations around configuration options, unless
> we're biasing against debug conditions where we definitely don't care
> about performance. The patch is certainly no worse than the existing
> code, but while we have the hood up, it might be nice to remove the
> annotation, unless we're sure that it does no harm, and does some
> good.

it's on x86.. likely/unlikely don't impact the CPU (since there are no
"ignore the branch predictor" hints), only the code placement.....

(and that's probably a good thing; CPU branch predictors are pretty
good, I'd not be surprised if they're at least as good as the
programmers who think how they code is used)


--
Arjan van de Ven Intel Open Source Technology Centre
For development, discussion and tips for power savings,
visit http://www.lesswatts.org
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2009-07-19 08:15    [W:0.092 / U:0.468 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site