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@xxxxxxxxxxxxxxx> --- v2: * Largely re-wrote the documentation .../pm/intel_uncore_frequency_scaling.rst | 59 +++++++++++++++++++ 1 file changed, 59 insertions(+) diff --git a/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst b/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst index 5ab3440e6cee..26ded32b06f5 100644 --- a/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst +++ b/Documentation/admin-guide/pm/intel_uncore_frequency_scaling.rst @@ -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 +frequency 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 -- 2.43.1
