* Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx> [2009-12-15 15:59:09]: > * Salman Qazi <sqazi@xxxxxxxxxx> [2009-12-14 16:36:20]: > > > On Mon, Dec 14, 2009 at 4:19 PM, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote: > > > On Mon, 14 Dec 2009 15:11:47 -0800 > > > Salman Qazi <sqazi@xxxxxxxxxx> 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 _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
