Messages in this thread | | | Date | Sat, 18 Jul 2009 23:14:29 -0700 | From | Arjan van de Ven <> | Subject | Re: [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/
| |