Documentation/cpu-freq/governors.txt - maze/linux - Git at Google

      CPU frequency and voltage scaling code in the Linux(TM) kernel


 		         L i n u x    C P U F r e q

 		      C P U F r e q   G o v e r n o r s

 		   - information for users and developers -


 		    Dominik Brodowski  <linux@brodo.de>
             some additions and corrections by Nico Golde <nico@ngolde.de>


    Clock scaling allows you to change the clock speed of the CPUs on the
     fly. This is a nice method to save battery power, because the lower
             the clock speed, the less power the CPU consumes.


 Contents:
 ---------
 1.   What is a CPUFreq Governor?

 2.   Governors In the Linux Kernel
 2.1  Performance
 2.2  Powersave
 2.3  Userspace
 2.4  Ondemand
 2.5  Conservative

 3.   The Governor Interface in the CPUfreq Core


 1. What Is A CPUFreq Governor?
 ==============================

 Most cpufreq drivers (in fact, all except one, longrun) or even most
 cpu frequency scaling algorithms only offer the CPU to be set to one
 frequency. In order to offer dynamic frequency scaling, the cpufreq
 core must be able to tell these drivers of a "target frequency". So
 these specific drivers will be transformed to offer a "->target"
 call instead of the existing "->setpolicy" call. For "longrun", all
 stays the same, though.

 How to decide what frequency within the CPUfreq policy should be used?
 That's done using "cpufreq governors". Two are already in this patch
 -- they're the already existing "powersave" and "performance" which
 set the frequency statically to the lowest or highest frequency,
 respectively. At least two more such governors will be ready for
 addition in the near future, but likely many more as there are various
 different theories and models about dynamic frequency scaling
 around. Using such a generic interface as cpufreq offers to scaling
 governors, these can be tested extensively, and the best one can be
 selected for each specific use.

 Basically, it's the following flow graph:

 CPU can be set to switch independently	 |	   CPU can only be set
       within specific "limits"		 |       to specific frequencies

                                  "CPUfreq policy"
 		consists of frequency limits (policy->{min,max})
   		     and CPUfreq governor to be used
 			 /		      \
 			/		       \
 		       /		       the cpufreq governor decides
 		      /			       (dynamically or statically)
 		     /			       what target_freq to set within
 		    /			       the limits of policy->{min,max}
 		   /			            \
 		  /				     \
 	Using the ->setpolicy call,		 Using the ->target call,
 	    the limits and the			  the frequency closest
 	     "policy" is set.			  to target_freq is set.
 						  It is assured that it
 						  is within policy->{min,max}


 2. Governors In the Linux Kernel
 ================================

 2.1 Performance
 ---------------

 The CPUfreq governor "performance" sets the CPU statically to the
 highest frequency within the borders of scaling_min_freq and
 scaling_max_freq.


 2.2 Powersave
 -------------

 The CPUfreq governor "powersave" sets the CPU statically to the
 lowest frequency within the borders of scaling_min_freq and
 scaling_max_freq.


 2.3 Userspace
 -------------

 The CPUfreq governor "userspace" allows the user, or any userspace
 program running with UID "root", to set the CPU to a specific frequency
 by making a sysfs file "scaling_setspeed" available in the CPU-device
 directory.


 2.4 Ondemand
 ------------

 The CPUfreq governor "ondemand" sets the CPU depending on the
 current usage. To do this the CPU must have the capability to
 switch the frequency very quickly.  There are a number of sysfs file
 accessible parameters:

 sampling_rate: measured in uS (10^-6 seconds), this is how often you
 want the kernel to look at the CPU usage and to make decisions on
 what to do about the frequency.  Typically this is set to values of
 around '10000' or more. It's default value is (cmp. with users-guide.txt):
 transition_latency * 1000
 Be aware that transition latency is in ns and sampling_rate is in us, so you
 get the same sysfs value by default.
 Sampling rate should always get adjusted considering the transition latency
 To set the sampling rate 750 times as high as the transition latency
 in the bash (as said, 1000 is default), do:
 echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
     >ondemand/sampling_rate

 sampling_rate_min:
 The sampling rate is limited by the HW transition latency:
 transition_latency * 100
 Or by kernel restrictions:
 If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
 If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is used, the
 limits depend on the CONFIG_HZ option:
 HZ=1000: min=20000us  (20ms)
 HZ=250:  min=80000us  (80ms)
 HZ=100:  min=200000us (200ms)
 The highest value of kernel and HW latency restrictions is shown and
 used as the minimum sampling rate.

 up_threshold: defines what the average CPU usage between the samplings
 of 'sampling_rate' needs to be for the kernel to make a decision on
 whether it should increase the frequency.  For example when it is set
 to its default value of '95' it means that between the checking
 intervals the CPU needs to be on average more than 95% in use to then
 decide that the CPU frequency needs to be increased.

 ignore_nice_load: this parameter takes a value of '0' or '1'. When
 set to '0' (its default), all processes are counted towards the
 'cpu utilisation' value.  When set to '1', the processes that are
 run with a 'nice' value will not count (and thus be ignored) in the
 overall usage calculation.  This is useful if you are running a CPU
 intensive calculation on your laptop that you do not care how long it
 takes to complete as you can 'nice' it and prevent it from taking part
 in the deciding process of whether to increase your CPU frequency.

 sampling_down_factor: this parameter controls the rate at which the
 kernel makes a decision on when to decrease the frequency while running
 at top speed. When set to 1 (the default) decisions to reevaluate load
 are made at the same interval regardless of current clock speed. But
 when set to greater than 1 (e.g. 100) it acts as a multiplier for the
 scheduling interval for reevaluating load when the CPU is at its top
 speed due to high load. This improves performance by reducing the overhead
 of load evaluation and helping the CPU stay at its top speed when truly
 busy, rather than shifting back and forth in speed. This tunable has no
 effect on behavior at lower speeds/lower CPU loads.

 powersave_bias: this parameter takes a value between 0 to 1000. It
 defines the percentage (times 10) value of the target frequency that
 will be shaved off of the target. For example, when set to 100 -- 10%,
 when ondemand governor would have targeted 1000 MHz, it will target
 1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
 (disabled) by default.
 When AMD frequency sensitivity powersave bias driver --
 drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
 defines the workload frequency sensitivity threshold in which a lower
 frequency is chosen instead of ondemand governor's original target.
 The frequency sensitivity is a hardware reported (on AMD Family 16h
 Processors and above) value between 0 to 100% that tells software how
 the performance of the workload running on a CPU will change when
 frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
 will not perform any better on higher core frequency, whereas a
 workload with sensitivity of 100% (CPU-bound) will perform better
 higher the frequency. When the driver is loaded, this is set to 400
 by default -- for CPUs running workloads with sensitivity value below
 40%, a lower frequency is chosen. Unloading the driver or writing 0
 will disable this feature.


 2.5 Conservative
 ----------------

 The CPUfreq governor "conservative", much like the "ondemand"
 governor, sets the CPU depending on the current usage.  It differs in
 behaviour in that it gracefully increases and decreases the CPU speed
 rather than jumping to max speed the moment there is any load on the
 CPU.  This behaviour more suitable in a battery powered environment.
 The governor is tweaked in the same manner as the "ondemand" governor
 through sysfs with the addition of:

 freq_step: this describes what percentage steps the cpu freq should be
 increased and decreased smoothly by.  By default the cpu frequency will
 increase in 5% chunks of your maximum cpu frequency.  You can change this
 value to anywhere between 0 and 100 where '0' will effectively lock your
 CPU at a speed regardless of its load whilst '100' will, in theory, make
 it behave identically to the "ondemand" governor.

 down_threshold: same as the 'up_threshold' found for the "ondemand"
 governor but for the opposite direction.  For example when set to its
 default value of '20' it means that if the CPU usage needs to be below
 20% between samples to have the frequency decreased.

 sampling_down_factor: similar functionality as in "ondemand" governor.
 But in "conservative", it controls the rate at which the kernel makes
 a decision on when to decrease the frequency while running in any
 speed. Load for frequency increase is still evaluated every
 sampling rate.

 3. The Governor Interface in the CPUfreq Core
 =============================================

 A new governor must register itself with the CPUfreq core using
 "cpufreq_register_governor". The struct cpufreq_governor, which has to
 be passed to that function, must contain the following values:

 governor->name -	    A unique name for this governor
 governor->governor -	    The governor callback function
 governor->owner	-	    .THIS_MODULE for the governor module (if
 			    appropriate)

 The governor->governor callback is called with the current (or to-be-set)
 cpufreq_policy struct for that CPU, and an unsigned int event. The
 following events are currently defined:

 CPUFREQ_GOV_START:   This governor shall start its duty for the CPU
 		     policy->cpu
 CPUFREQ_GOV_STOP:    This governor shall end its duty for the CPU
 		     policy->cpu
 CPUFREQ_GOV_LIMITS:  The limits for CPU policy->cpu have changed to
 		     policy->min and policy->max.

 If you need other "events" externally of your driver, _only_ use the
 cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
 CPUfreq core to ensure proper locking.


 The CPUfreq governor may call the CPU processor driver using one of
 these two functions:

 int cpufreq_driver_target(struct cpufreq_policy *policy,
                                  unsigned int target_freq,
                                  unsigned int relation);

 int __cpufreq_driver_target(struct cpufreq_policy *policy,
                                    unsigned int target_freq,
                                    unsigned int relation);

 target_freq must be within policy->min and policy->max, of course.
 What's the difference between these two functions? When your governor
 still is in a direct code path of a call to governor->governor, the
 per-CPU cpufreq lock is still held in the cpufreq core, and there's
 no need to lock it again (in fact, this would cause a deadlock). So
 use __cpufreq_driver_target only in these cases. In all other cases
 (for example, when there's a "daemonized" function that wakes up
 every second), use cpufreq_driver_target to lock the cpufreq per-CPU
 lock before the command is passed to the cpufreq processor driver.
	CPU frequency and voltage scaling code in the Linux(TM) kernel


	L i n u x C P U F r e q

	C P U F r e q G o v e r n o r s

	- information for users and developers -


	Dominik Brodowski <linux@brodo.de>
	some additions and corrections by Nico Golde <nico@ngolde.de>



	Clock scaling allows you to change the clock speed of the CPUs on the
	fly. This is a nice method to save battery power, because the lower
	the clock speed, the less power the CPU consumes.


	Contents:
	---------
	1. What is a CPUFreq Governor?

	2. Governors In the Linux Kernel
	2.1 Performance
	2.2 Powersave
	2.3 Userspace
	2.4 Ondemand
	2.5 Conservative

	3. The Governor Interface in the CPUfreq Core



	1. What Is A CPUFreq Governor?
	==============================

	Most cpufreq drivers (in fact, all except one, longrun) or even most
	cpu frequency scaling algorithms only offer the CPU to be set to one
	frequency. In order to offer dynamic frequency scaling, the cpufreq
	core must be able to tell these drivers of a "target frequency". So
	these specific drivers will be transformed to offer a "->target"
	call instead of the existing "->setpolicy" call. For "longrun", all
	stays the same, though.

	How to decide what frequency within the CPUfreq policy should be used?
	That's done using "cpufreq governors". Two are already in this patch
	-- they're the already existing "powersave" and "performance" which
	set the frequency statically to the lowest or highest frequency,
	respectively. At least two more such governors will be ready for
	addition in the near future, but likely many more as there are various
	different theories and models about dynamic frequency scaling
	around. Using such a generic interface as cpufreq offers to scaling
	governors, these can be tested extensively, and the best one can be
	selected for each specific use.

	Basically, it's the following flow graph:

	CPU can be set to switch independently \| CPU can only be set
	within specific "limits" \| to specific frequencies

	"CPUfreq policy"
	consists of frequency limits (policy->{min,max})
	and CPUfreq governor to be used
	/ \
	/ \
	/ the cpufreq governor decides
	/ (dynamically or statically)
	/ what target_freq to set within
	/ the limits of policy->{min,max}
	/ \
	/ \
	Using the ->setpolicy call, Using the ->target call,
	the limits and the the frequency closest
	"policy" is set. to target_freq is set.
	It is assured that it
	is within policy->{min,max}


	2. Governors In the Linux Kernel
	================================

	2.1 Performance
	---------------

	The CPUfreq governor "performance" sets the CPU statically to the
	highest frequency within the borders of scaling_min_freq and
	scaling_max_freq.


	2.2 Powersave
	-------------

	The CPUfreq governor "powersave" sets the CPU statically to the
	lowest frequency within the borders of scaling_min_freq and
	scaling_max_freq.


	2.3 Userspace
	-------------

	The CPUfreq governor "userspace" allows the user, or any userspace
	program running with UID "root", to set the CPU to a specific frequency
	by making a sysfs file "scaling_setspeed" available in the CPU-device
	directory.


	2.4 Ondemand
	------------

	The CPUfreq governor "ondemand" sets the CPU depending on the
	current usage. To do this the CPU must have the capability to
	switch the frequency very quickly. There are a number of sysfs file
	accessible parameters:

	sampling_rate: measured in uS (10^-6 seconds), this is how often you
	want the kernel to look at the CPU usage and to make decisions on
	what to do about the frequency. Typically this is set to values of
	around '10000' or more. It's default value is (cmp. with users-guide.txt):
	transition_latency * 1000
	Be aware that transition latency is in ns and sampling_rate is in us, so you
	get the same sysfs value by default.
	Sampling rate should always get adjusted considering the transition latency
	To set the sampling rate 750 times as high as the transition latency
	in the bash (as said, 1000 is default), do:
	echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
	>ondemand/sampling_rate

	sampling_rate_min:
	The sampling rate is limited by the HW transition latency:
	transition_latency * 100
	Or by kernel restrictions:
	If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
	If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is used, the
	limits depend on the CONFIG_HZ option:
	HZ=1000: min=20000us (20ms)
	HZ=250: min=80000us (80ms)
	HZ=100: min=200000us (200ms)
	The highest value of kernel and HW latency restrictions is shown and
	used as the minimum sampling rate.

	up_threshold: defines what the average CPU usage between the samplings
	of 'sampling_rate' needs to be for the kernel to make a decision on
	whether it should increase the frequency. For example when it is set
	to its default value of '95' it means that between the checking
	intervals the CPU needs to be on average more than 95% in use to then
	decide that the CPU frequency needs to be increased.

	ignore_nice_load: this parameter takes a value of '0' or '1'. When
	set to '0' (its default), all processes are counted towards the
	'cpu utilisation' value. When set to '1', the processes that are
	run with a 'nice' value will not count (and thus be ignored) in the
	overall usage calculation. This is useful if you are running a CPU
	intensive calculation on your laptop that you do not care how long it
	takes to complete as you can 'nice' it and prevent it from taking part
	in the deciding process of whether to increase your CPU frequency.

	sampling_down_factor: this parameter controls the rate at which the
	kernel makes a decision on when to decrease the frequency while running
	at top speed. When set to 1 (the default) decisions to reevaluate load
	are made at the same interval regardless of current clock speed. But
	when set to greater than 1 (e.g. 100) it acts as a multiplier for the
	scheduling interval for reevaluating load when the CPU is at its top
	speed due to high load. This improves performance by reducing the overhead
	of load evaluation and helping the CPU stay at its top speed when truly
	busy, rather than shifting back and forth in speed. This tunable has no
	effect on behavior at lower speeds/lower CPU loads.

	powersave_bias: this parameter takes a value between 0 to 1000. It
	defines the percentage (times 10) value of the target frequency that
	will be shaved off of the target. For example, when set to 100 -- 10%,
	when ondemand governor would have targeted 1000 MHz, it will target
	1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
	(disabled) by default.
	When AMD frequency sensitivity powersave bias driver --
	drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
	defines the workload frequency sensitivity threshold in which a lower
	frequency is chosen instead of ondemand governor's original target.
	The frequency sensitivity is a hardware reported (on AMD Family 16h
	Processors and above) value between 0 to 100% that tells software how
	the performance of the workload running on a CPU will change when
	frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
	will not perform any better on higher core frequency, whereas a
	workload with sensitivity of 100% (CPU-bound) will perform better
	higher the frequency. When the driver is loaded, this is set to 400
	by default -- for CPUs running workloads with sensitivity value below
	40%, a lower frequency is chosen. Unloading the driver or writing 0
	will disable this feature.


	2.5 Conservative
	----------------

	The CPUfreq governor "conservative", much like the "ondemand"
	governor, sets the CPU depending on the current usage. It differs in
	behaviour in that it gracefully increases and decreases the CPU speed
	rather than jumping to max speed the moment there is any load on the
	CPU. This behaviour more suitable in a battery powered environment.
	The governor is tweaked in the same manner as the "ondemand" governor
	through sysfs with the addition of:

	freq_step: this describes what percentage steps the cpu freq should be
	increased and decreased smoothly by. By default the cpu frequency will
	increase in 5% chunks of your maximum cpu frequency. You can change this
	value to anywhere between 0 and 100 where '0' will effectively lock your
	CPU at a speed regardless of its load whilst '100' will, in theory, make
	it behave identically to the "ondemand" governor.

	down_threshold: same as the 'up_threshold' found for the "ondemand"
	governor but for the opposite direction. For example when set to its
	default value of '20' it means that if the CPU usage needs to be below
	20% between samples to have the frequency decreased.

	sampling_down_factor: similar functionality as in "ondemand" governor.
	But in "conservative", it controls the rate at which the kernel makes
	a decision on when to decrease the frequency while running in any
	speed. Load for frequency increase is still evaluated every
	sampling rate.

	3. The Governor Interface in the CPUfreq Core
	=============================================

	A new governor must register itself with the CPUfreq core using
	"cpufreq_register_governor". The struct cpufreq_governor, which has to
	be passed to that function, must contain the following values:

	governor->name - A unique name for this governor
	governor->governor - The governor callback function
	governor->owner - .THIS_MODULE for the governor module (if
	appropriate)

	The governor->governor callback is called with the current (or to-be-set)
	cpufreq_policy struct for that CPU, and an unsigned int event. The
	following events are currently defined:

	CPUFREQ_GOV_START: This governor shall start its duty for the CPU
	policy->cpu
	CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
	policy->cpu
	CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
	policy->min and policy->max.

	If you need other "events" externally of your driver, _only_ use the
	cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
	CPUfreq core to ensure proper locking.


	The CPUfreq governor may call the CPU processor driver using one of
	these two functions:

	int cpufreq_driver_target(struct cpufreq_policy *policy,
	unsigned int target_freq,
	unsigned int relation);

	int __cpufreq_driver_target(struct cpufreq_policy *policy,
	unsigned int target_freq,
	unsigned int relation);

	target_freq must be within policy->min and policy->max, of course.
	What's the difference between these two functions? When your governor
	still is in a direct code path of a call to governor->governor, the
	per-CPU cpufreq lock is still held in the cpufreq core, and there's
	no need to lock it again (in fact, this would cause a deadlock). So
	use __cpufreq_driver_target only in these cases. In all other cases
	(for example, when there's a "daemonized" function that wakes up
	every second), use cpufreq_driver_target to lock the cpufreq per-CPU
	lock before the command is passed to the cpufreq processor driver.