On Wed, 22 Dec 2021, Rafael J. Wysocki wrote:

On Wed, Dec 22, 2021 at 12:57 AM Francisco Jerez [off-list ref] wrote:

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"Rafael J. Wysocki" [off-list ref] writes:

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On Sun, Dec 19, 2021 at 11:10 PM Francisco Jerez [off-list ref] wrote:

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Julia Lawall [off-list ref] writes:

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On Sat, 18 Dec 2021, Francisco Jerez wrote:

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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