Re: [RFC PATCH v2 0/5] sched: modular find_busiest_group()

Vaidyanathan Srinivasan
2008-10-24T10:02:08Z

* Peter Zijlstra <peterz@infradead.org> [2008-10-14 15:25:03]:

> On Tue, 2008-10-14 at 18:37 +0530, Vaidyanathan Srinivasan wrote:
> > * Peter Zijlstra <peterz@infradead.org> [2008-10-14 14:09:13]:
> >
> > >
> > > Hi,
> > >
> > > So the basic issue is sched_group::cpu_power should become more dynamic.
> >
> > Hi Peter,
> >
> > This is a good idea. Dynamically increasing cpu power to some groups
> > will automatically help power savings when we want to consolidate
> > better to one cpu package when overall system utilisation is very low.
>
> Ah, yes another use case of this ;-)
>
> > > Dynamic Speed Technology
> > > ------------------------
> > >
> > > With cpus actively fiddling with their processing capacity we get into
> > > similar issues. Again we can measure this, but this would require the
> > > addition of a clock that measures work instead of time.
> > >
> > > Having that, we can even acturately measure the old SMT case, which has
> > > always been approximated by a static percentage - even though the actual
> > > gain is very workload dependent.
> > >
> > > The idea is to introduce sched_work_clock() so that:
> > >
> > > work_delta / time_delta gives the power for a cpu. <1 means we
> > > did less work than a dedicated pipeline, >1 means we did more.
> >
> > The challenge here is measurement of 'work'. What will be the
> > parameter that will be fair for most workloads and easy to measure on
> > most systems?
> >
> > * Instructions completion count
> > * APERF or similar CPU specific counter on x86
> > * POWER has PURR and SPURR to have a measure of relative work done
>
> Right - I was hoping for some feedback from the arch folks (maybe I
> should have CC'ed linux-arch) on this issue.

Hi Peter,

Do you want to post this RFD again to get more feedback?

--Vaidy