Re: [regression] cross core scheduling frequency drop bisected to 0c313cb20732

Rik van Riel
2016-04-11T12:38:21Z

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On Mon, 2016-04-11 at 05:04 +0200, Mike Galbraith wrote:
> On Sun, 2016-04-10 at 16:24 -0400, Rik van Riel wrote:
> >
> > On Sun, 2016-04-10 at 17:39 +0200, Mike Galbraith wrote:
> >
> > >
> > > Should the default idle state not then be governor
> > > dependent?  When I
> > > set gov=performance, I'm expecting box to go just as fast as it
> > > can
> > > go
> > > without melting.  Does polling risk CPU -> lava conversion?
> > Current CPUs can only have some cores run at full speed
> > (turbo mode) if other cores are idling and/or running at
> > lower speeds.
> The real world is very unlikely to miss the prettier numbers I'm
> grieving over one tiny bit.  Knowing that doesn't make giving them up
> any easier though.. byebye cycles (sniff) ;-)

I suspect your pipe benchmark could be very relevant to
network performance numbers, too.

I would like to go into polling a little bit more aggressively
in a future kernel, and I think we can get away with it if we
teach the polling loop to exit after we have spent enough time
there that the menu governor will pick HLT after a few timed
out poll loops.

That way while we run a workload that actually benefits from
polling, we will get polling, but once we run a workload that
actually sleeps longer than the HLT threshold, we will quickly
fall back to HLT.

With 10Gbps network traffic, it could make a real difference
whether or not the CPU can wake up immediately, or takes a
microsecond to wake up...

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