Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 24 Dec 2021 12:08:53 +0100 (CET) | From | Julia Lawall <> | Subject | Re: cpufreq: intel_pstate: map utilization into the pstate range |
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On Wed, 22 Dec 2021, Rafael J. Wysocki wrote:
> On Wed, Dec 22, 2021 at 12:57 AM Francisco Jerez <currojerez@riseup.net> wrote: > > > > "Rafael J. Wysocki" <rafael@kernel.org> writes: > > > > > On Sun, Dec 19, 2021 at 11:10 PM Francisco Jerez <currojerez@riseup.net> wrote: > > >> > > >> Julia Lawall <julia.lawall@inria.fr> writes: > > >> > > >> > On Sat, 18 Dec 2021, Francisco Jerez wrote: > > > > > > [cut] > > > > > >> > I did some experiements with forcing different frequencies. I haven't > > >> > finished processing the results, but I notice that as the frequency goes > > >> > up, the utilization (specifically the value of > > >> > map_util_perf(sg_cpu->util) at the point of the call to > > >> > cpufreq_driver_adjust_perf in sugov_update_single_perf) goes up as well. > > >> > Is this expected? > > >> > > > >> > > >> Actually, it *is* expected based on our previous hypothesis that these > > >> workloads are largely latency-bound: In cases where a given burst of CPU > > >> work is not parallelizable with any other tasks the thread needs to > > >> complete subsequently, its overall runtime will decrease monotonically > > >> with increasing frequency, therefore the number of instructions executed > > >> per unit of time will increase monotonically with increasing frequency, > > >> and with it its frequency-invariant utilization. > > > > > > But shouldn't these two effects cancel each other if the > > > frequency-invariance mechanism works well? > > > > No, they won't cancel each other out under our hypothesis that these > > workloads are largely latency-bound, since the performance of the > > application will increase steadily with increasing frequency, and with > > it the amount of computational resources it utilizes per unit of time on > > the average, and therefore its frequency-invariant utilization as well. > > OK, so this is a workload in which the maximum performance is only > achieved at the maximum available frequency. IOW, there's no > performance saturation point and increasing the frequency (if > possible) will always cause more work to be done per unit of time. > > For this type of workloads, requirements regarding performance (for > example, upper bound on the expected time of computations) need to be > known in order to determine the "most suitable" frequency to run them > and I agree that schedutil doesn't help much in that respect. > > It is probably better to run them with intel_pstate in the active mode > (ie. "pure HWP") or decrease EPP via sysfs to allow HWP to ramp up > turbo more aggressively.
active mode + powersave indeed both gives faster runtimes and less energy consumption for these examples.
thanks, julia
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