While the majority of server OS distributions are deployed with the
"performance" governor as the default, some distributions like Ubuntu use
the "powersave" governor by default.

While using the "powersave" governor in its default configuration on
Sapphire Rapids systems leads to much lower power, the performance is
lower by more than 25% for several workloads relative to the
"performance" governor.

A 37% difference has been reported by www.Phoronix.com [1].

This is a consequence of using a relatively high EPP value in the
default configuration of the "powersave" governor and the performance
can be made much closer to the "performance" governor's level by
adjusting the default EPP value. Based on experiments, with EPP of 0x00,
0x10, 0x20, the performance delta between the "powersave" governor and
the "performance" one is around 12%. However, the EPP of 0x20 reduces
average power by 18% with respect to the lower EPP values.

[Note that raising min_perf_pct in sysfs as high as 50% in addition to
 adjusting EPP does not improve the performance any further.]

For this reason, change the EPP value corresponding to the the default
balance_performance setting for Sapphire Rapids to 0x20, which is
straightforward, because analogous default EPP adjustment has been
applied to Alder Lake and there is a way to set the balance_performance
EPP value in intel_pstate based on the processor model already.

The goal here is to limit the mean performance delta between the
"powersave" governor in the default configuration and the "performance"
governor for a wide variety of server workloadsto to around 10-12%. For
some bursty workloads, this delta can be still large, as the frequency
ramp-up will still lag when the "powersave" governor is in use
irrespective of the EPP setting, because the performance governor always
requests the maximum possible frequency.

Link: https://www.phoronix.com/review/centos-clear-spr/6 # [1]
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>