Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core

Giovanni Gherdovich
2020-10-22T20:10:52Z

Hello Peter, Rafael,

back in August I tested a v5.8 kernel adding Rafael's patches from v5.9 that
make schedutil and HWP works together, i.e. f6ebbcf08f37 ("cpufreq: intel_pstate:
Implement passive mode with HWP enabled").

The main point I took from the exercise is that tbench (network benchmark
in localhost) is problematic for schedutil and only with HWP (thanks to
Rafael's patch above) it reaches the throughput of the other governors.
When HWP isn't available, the penalty is 5-10% and I need to understand if
the cause is something that can affect other applications too (or just a
quirk of this test).

I ran this campaign this summer when Rafal CC'ed me to f6ebbcf08f37
("cpufreq: intel_pstate: Implement passive mode with HWP enabled"),
I didn't reply as the patch was a win anyways (my bad, I should have posted
the positive results). The regression of tbench with schedutil w/o HWP,
that went unnoticed for long, got the best of my attention.

Other remarks

* on gitsource (running the git unit test suite, measures elapsed time)
schedutil is a lot better than Intel's powersave but not as good as the
performance governor.

* for the AMD EPYC machines we haven't yet implemented frequency invariant
accounting, which might explain why schedutil looses to ondemand on all
the benchmarks.

* on dbench (filesystem, measures latency) and kernbench (kernel compilation),
sugov is as good as the Intel performance governor. You can add or remove
HWP (to either sugov or perfgov), it doesn't make a difference. Intel's
powersave in general trails behind.

* generally my main concern is performance, not power efficiency, but I was
a little disappointed to see schedutil being just as efficient as
perfgov (the performance-per-watt ratios): there are even a few cases
where (on tbench) the performance governor is both faster and more
efficient. From previous conversations with Rafael I recall that
switching frequency has an energy cost, so it could be that schedutil
switches too often to amortize it. I haven't checked.

To read the tables:

Tilde (~) means the result is the same as baseline (or, the ratio is close
to 1). The double asterisk (**) is a visual aid and means the result is
worse than baseline (higher or lower depending on the case).

For an overview of the possible configurations (intel_psate passive,
active, HWP on/off etc) I made the diagram at
https://beta.suse.com/private/ggherdovich/cpufreq/x86-cpufreq.png

1) INTEL, HWP-CAPABLE MACHINES
2) INTEL, NON-HWP-CAPABLE MACHINES
3) AMD EPYC

1) INTEL, HWP-CAPABLE MACHINES:

64x_SKYLAKE_NUMA: Intel Skylake SP, 32 cores / 64 threads, NUMA, SATA SSD storage
------------------------------------------------------------------------------
sugov-HWP sugov-no-HWP powersave-HWP perfgov-HWP better if
------------------------------------------------------------------------------
PERFORMANCE RATIOS
tbench 1.00 0.68 ~ 1.03** higher
dbench 1.00 ~ 1.03 ~ lower
kernbench 1.00 ~ 1.11 ~ lower
gitsource 1.00 1.03 2.26 0.82** lower
------------------------------------------------------------------------------
PERFORMANCE-PER-WATT RATIOS
tbench 1.00 0.74 ~ ~ higher
dbench 1.00 ~ ~ ~ higher
kernbench 1.00 ~ 0.96 ~ higher
gitsource 1.00 0.96 0.45 1.15** higher

8x_SKYLAKE_UMA: Intel Skylake (client), 4 cores / 8 threads, UMA, SATA SSD storage
------------------------------------------------------------------------------
sugov-HWP sugov-no-HWP powersave-HWP perfgov-HWP better if
------------------------------------------------------------------------------
PERFORMANCE RATIOS
tbench 1.00 0.91 ~ ~ higher
dbench 1.00 ~ ~ ~ lower
kernbench 1.00 ~ ~ ~ lower
gitsource 1.00 1.04 1.77 ~ lower
------------------------------------------------------------------------------
PERFORMANCE-PER-WATT RATIOS
tbench 1.00 0.95 ~ ~ higher
dbench 1.00 ~ ~ ~ higher
kernbench 1.00 ~ ~ ~ higher
gitsource 1.00 ~ 0.74 ~ higher

8x_COFFEELAKE_UMA: Intel Coffee Lake, 4 cores / 8 threads, UMA, NVMe SSD storage
---------------------------------------------------------------
sugov-HWP powersave-HWP perfgov-HWP better if
---------------------------------------------------------------
PERFORMANCE RATIOS
tbench 1.00 ~ ~ higher
dbench 1.00 1.12 ~ lower
kernbench 1.00 ~ ~ lower
gitsource 1.00 2.05 ~ lower
---------------------------------------------------------------
PERFORMANCE-PER-WATT RATIOS
tbench 1.00 ~ ~ higher
dbench 1.00 1.80** ~ higher
kernbench 1.00 ~ ~ higher
gitsource 1.00 1.52** ~ higher

2) INTEL, NON-HWP-CAPABLE MACHINES:

80x_BROADWELL_NUMA: Intel Broadwell EP, 40 cores / 80 threads, NUMA, SATA SSD storage
---------------------------------------------------------------
sugov powersave perfgov better if
---------------------------------------------------------------
PERFORMANCE RATIOS
tbench 1.00 1.11** 1.10** higher
dbench 1.00 1.10 ~ lower
kernbench 1.00 1.10 ~ lower
gitsource 1.00 2.27 0.95** lower
---------------------------------------------------------------
PERFORMANCE-PER-WATT RATIOS
tbench 1.00 1.05** 1.04** higher
dbench 1.00 1.24** 0.95 higher
kernbench 1.00 ~ ~ higher
gitsource 1.00 0.86 1.04** higher

48x_HASWELL_NUMA: Intel Haswell EP, 24 cores / 48 threads, NUMA, HDD storage
---------------------------------------------------------------
sugov powersave perfgov better if
---------------------------------------------------------------
PERFORMANCE RATIOS
tbench 1.00 1.25** 1.27** higher
dbench 1.00 1.17 ~ lower
kernbench 1.00 1.04 ~ lower
gitsource 1.00 1.54 0.79** lower
---------------------------------------------------------------
PERFORMANCE-PER-WATT RATIOS
tbench 1.00 1.18** 1.11** higher
dbench 1.00 1.25** ~ higher
kernbench 1.00 1.04** 0.97 higher
gitsource 1.00 0.77 ~ higher

3) AMD EPYC:

256x_ROME_NUMA: AMD Rome , 128 cores / 256 threads, NUMA, SATA SSD storage
---------------------------------------------------------------
sugov ondemand perfgov better if
---------------------------------------------------------------
PERFORMANCE RATIOS
tbench 1.00 1.11** 1.58** higher
dbench 1.00 0.44** 0.40** lower
kernbench 1.00 ~ 0.91** lower
gitsource 1.00 0.96** 0.65** lower

128x_NAPLES_NUMA: AMD Naples , 64 cores / 128 threads, NUMA, SATA SSD storage
---------------------------------------------------------------
sugov ondemand perfgov better if
---------------------------------------------------------------
PERFORMANCE RATIOS
tbench 1.00 1.10** 1.19** higher
dbench 1.00 1.05 0.95** lower
kernbench 1.00 ~ 0.95** lower
gitsource 1.00 0.93** 0.55** lower

Giovanni