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Documentation: admin-guide: pm: Add efficiency vs. latency tradeoff to uncore documentation

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Added documentation about the functionality of efficiency vs. latency tradeoff
control in intel Xeon processors, and how this is configured via sysfs.

Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240828153657.1296410-2-tero.kristo@linux.intel.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
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Tero Kristo 2024-08-28 18:34:53 +03:00 committed by Hans de Goede
parent d9dca21570
commit 8022ae2c43
1 changed files with 59 additions and 0 deletions
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59
Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst
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@ -113,3 +113,62 @@ to apply at each uncore* level.
Support for "current_freq_khz" is available only at each fabric cluster
level (i.e., in uncore* directory).
Efficiency vs. Latency Tradeoff
-------------------------------
The Efficiency Latency Control (ELC) feature improves performance
per watt. With this feature hardware power management algorithms
optimize trade-off between latency and power consumption. For some
latency sensitive workloads further tuning can be done by SW to
get desired performance.
The hardware monitors the average CPU utilization across all cores
in a power domain at regular intervals and decides an uncore frequency.
While this may result in the best performance per watt, workload may be
expecting higher performance at the expense of power. Consider an
application that intermittently wakes up to perform memory reads on an
otherwise idle system. In such cases, if hardware lowers uncore
frequency, then there may be delay in ramp up of frequency to meet
target performance.
The ELC control defines some parameters which can be changed from SW.
If the average CPU utilization is below a user-defined threshold
(elc_low_threshold_percent attribute below), the user-defined uncore
floor frequency will be used (elc_floor_freq_khz attribute below)
instead of hardware calculated minimum.
Similarly in high load scenario where the CPU utilization goes above
the high threshold value (elc_high_threshold_percent attribute below)
instead of jumping to maximum uncore frequency, frequency is increased
in 100MHz steps. This avoids consuming unnecessarily high power
immediately with CPU utilization spikes.
Attributes for efficiency latency control:
``elc_floor_freq_khz``
This attribute is used to get/set the efficiency latency floor frequency.
If this variable is lower than the 'min_freq_khz', it is ignored by
the firmware.
``elc_low_threshold_percent``
This attribute is used to get/set the efficiency latency control low
threshold. This attribute is in percentages of CPU utilization.
``elc_high_threshold_percent``
This attribute is used to get/set the efficiency latency control high
threshold. This attribute is in percentages of CPU utilization.
``elc_high_threshold_enable``
This attribute is used to enable/disable the efficiency latency control
high threshold. Write '1' to enable, '0' to disable.
Example system configuration below, which does following:
* when CPU utilization is less than 10%: sets uncore frequency to 800MHz
* when CPU utilization is higher than 95%: increases uncore frequency in
100MHz steps, until power limit is reached
elc_floor_freq_khz:800000
elc_high_threshold_percent:95
elc_high_threshold_enable:1
elc_low_threshold_percent:10