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SubjectRe: RFC: A proposal for power capping through forced idle in the Linux Kernel
* Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2009-12-15 15:59:09]:

> * Salman Qazi <sqazi@google.com> [2009-12-14 16:36:20]:
>
> > On Mon, Dec 14, 2009 at 4:19 PM, Arjan van de Ven <arjan@infradead.org> wrote:
> > > On Mon, 14 Dec 2009 15:11:47 -0800
> > > Salman Qazi <sqazi@google.com> wrote:
> > >
> > >
> > > I like the general idea, I have one request (that I didn't see quite in
> > > your explanation): Please make sure that all cpus in the system do
> > > their idle injection at the same time, so that memory can go into power
> > > saving mode as well during this time etc etc...
> > >
>
> The value of the overall idea is well understood but the
> implementation and benefits in terms of power savings was the major
> point of discussion earlier.
>
> > With the current interface, the forced idle percentages on the CPUs
> > are controlled independently. There's a trade-off here. If we inject
> > idle cycles on all the CPU at the same time, our machine
> > responsiveness also degrades: essentially every CPU becomes equally
> > bad for an interactive task to run on. Our aim at the moment is to
> > try to concentrate the idle cycles on a small set of CPUs, to strive
> > to leave some CPUs where interactive tasks can run unhindered. But,
> > given a different workload and goals the correct policy may be
> > different.
> >
> > Simultaneously idling multiple "cores" becomes necessary in the SMT
> > case: as there is no point in idling a single thread, while the other
> > thread is running full tilt. So, in such a case it is necessary to
> > idle all the threads making up the physical core. This feature has
> > not been implemented yet.
> >
> > I think the best approach may be to provide a way to specify the
> > policy from the user space. Basically let the user decide at what
> > level of CPU hierarchy the forced idle percentages are specified.
> > Then, in the levels below, we simply inject at the same time.
>
> Synchronising the idle times across multiple cores and also selecting
> sibling threads belonging to the same core is important. The current
> ACPI forced idle driver can inject idle time but not synchronized
> across multiple cores.
>
> Allowing the scheduler load balancer to avoid using a part of the
> sched domain tree will allow easy grouping of sibling threads and
> sibling cores if that saves more power.
>
> However as Arjan mentioned, new architectures have significant power
> savings at full system idle where memory power is reduced. Injecting
> idle time in any of the core will actually increase the utilisation on
> the other cores (unless the system is full loaded) and reduce the full
> system idle time opportunity. Basically injecting idle time on some
> of the cores in the system goes against the race-to-idle policy
> thereby decreasing overall system operating efficiency.
>
> Can you please clarify the following questions:
>
> * What is the typical duration of idle time injected?
> - 10s of milli seconds? CPUs are expected to goto lowest
> power idle state within this time?
>
> * You mentioned that natural idle time in the system is taken into
> account before injecting forced idle time, which is a good feature
> to have.
> - In most workloads, as the utilisation drops, all the cpus
> have similar idle times. This is favourable for exploiting
> memory power saving.
> - Now when more idle time need to be inserted, is it
> uniformly spread across all CPUs?

* How is the fairness issue in the scheduler handled? Inserting idle
time may affect interactivity and fairness badly.

--Vaidy


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