Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS

[Srinivas Pandruvada]

On Fri, 2018-02-02 at 19:48 +0000, Mel Gorman wrote:
> On Fri, Feb 02, 2018 at 06:54:24AM -0800, Srinivas Pandruvada wrote:
> > > > > No idea, desired would be the one I would start with, it
> > > > > matches
> > > > > with
> > > > > the intent here. But I've no idea what our current HWP
> > > > > implementation
> > > > > actually does with it.
> > > > 
> > > > Desired !=0 will disable HWP autonomous mode of frequency
> > > > selection.
> > > 
> > > But I don't think it will just run at "desired" then, will it?
> > 
> > HWP all are these hints only not a guarantee.
> 
> Sure, but the lack on detection when tasks are low utilisation but
> still
> latency/throughput sensitive is problematic. Users shouldn't have to
> know they need to disable HWP or set performance goernor out of the
> box.
> It's only going to get worse as sockets get larger.
I am not saying that we shouldn't do anything. Can you give me some
workloads which you care the most?

> 
> > There are totally different way HWP is handled in client an
> > servers.
> > If you set desired all heuristics they collected will be dumped, so
> > they suggest don't set desired when you are in autonomous mode. If
> > we
> > really want a boost set the EPP. We know that EPP makes lots of
> > measurable difference.
> > 
> 
> Sure boosting EPP makes a difference -- it's essentially what the
> performance
> goveror does and I know that can be done by a user but it's still
> basically a
> cop-out. Default performance for low utilisation or lightly loaded
> machines
> is poor. Maybe it should be set based on the ACPI preferred profile
> but
> that information is not always available. It would be nice if *some*
> sort of hint about new migrations or tasks waking from IO would be
> desirable.
EPP is a range not a single value. So you don't need to make EPP=0 as a
performance governor. PeterZ gave me some scheduler change to
experiment, which can be used as hint to play with EPP. 

Thanks,
Srinivas