Re: [tip:x86/urgent] x86 idle: Repair large-server 50-watt idle-power regression

[Peter Zijlstra]

On Tue, Mar 18, 2014 at 07:14:04PM -0700, Jason Low wrote:
> On Tue, Mar 18, 2014 at 2:16 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Mon, Mar 17, 2014 at 05:20:10PM -0700, Davidlohr Bueso wrote:
> >> On Thu, 2013-12-19 at 11:51 -0800, tip-bot for Len Brown wrote:
> >> > Commit-ID:  40e2d7f9b5dae048789c64672bf3027fbb663ffa
> >> > Gitweb:     http://git.kernel.org/tip/40e2d7f9b5dae048789c64672bf3027fbb663ffa
> >> > Author:     Len Brown <len.brown@xxxxxxxxx>
> >> > AuthorDate: Wed, 18 Dec 2013 16:44:57 -0500
> >> > Committer:  H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
> >> > CommitDate: Thu, 19 Dec 2013 11:47:39 -0800
> >> >
> >> > x86 idle: Repair large-server 50-watt idle-power regression
> >>
> >> FYI this commit can cause some non trivial performance regressions for
> >> larger core count systems. While not surprising because of the nature of
> >> the change, having intel_idle do more cacheline invalidations, I still
> >> wanted to let you guys know. For instance, on a 160 core Westmere
> >> system, aim7 throughput can go down in a number of tests, anywhere from
> >> -10% to -25%.
> >>
> >> I guess it comes down to one of those performance vs energy things. And
> >> sure, max_cstate can be set to overcome this, but it's still something
> >> that was previously taken for granted.
> >
> > -10% to -25% seems a lot for a single cacheline flush. Also I would
> > expect the expected idle time to be very short while running aim7. So
> > could it be the cacheflush is actually taking longer than the expected
> > idle time?
> 
> Can we consider conditionally skipping the cacheline flush if the
> approximate average CPU idle time is very short, for instance,
> something along the lines of skipping if CPU average idle time <
> (sched migration cost or "cacheline_flush_penalty")?

I'd say its too early for proposing solutions; first we need to find
facts about what is happening.
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