diff options
Diffstat (limited to 'Documentation/cpu-freq/governors.txt')
| -rw-r--r-- | Documentation/cpu-freq/governors.txt | 282 |
1 files changed, 282 insertions, 0 deletions
diff --git a/Documentation/cpu-freq/governors.txt b/Documentation/cpu-freq/governors.txt new file mode 100644 index 00000000..51b1cd36 --- /dev/null +++ b/Documentation/cpu-freq/governors.txt @@ -0,0 +1,282 @@ + 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 +2.6 Interactive + +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 + +show_sampling_rate_min: +The sampling rate is limited by the HW transition latency: +transition_latency * 100 +Or by kernel restrictions: +If CONFIG_NO_HZ is set, the limit is 10ms fixed. +If CONFIG_NO_HZ is not set or no_hz=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. + +show_sampling_rate_max: THIS INTERFACE IS DEPRECATED, DON'T USE IT. + +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. + + +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. + + +2.6 Interactive +--------------- + +The CPUfreq governor "interactive" is designed for latency-sensitive, +interactive workloads. This governor sets the CPU speed depending on +usage, similar to "ondemand" and "conservative" governors. However, +the governor is more aggressive about scaling the CPU speed up in +response to CPU-intensive activity. + +Sampling the CPU load every X ms can lead to under-powering the CPU +for X ms, leading to dropped frames, stuttering UI, etc. Instead of +sampling the cpu at a specified rate, the interactive governor will +check whether to scale the cpu frequency up soon after coming out of +idle. When the cpu comes out of idle, a timer is configured to fire +within 1-2 ticks. If the cpu is very busy between exiting idle and +when the timer fires then we assume the cpu is underpowered and ramp +to MAX speed. + +If the cpu was not sufficiently busy to immediately ramp to MAX speed, +then governor evaluates the cpu load since the last speed adjustment, +choosing the highest value between that longer-term load or the +short-term load since idle exit to determine the cpu speed to ramp to. + +The tuneable values for this governor are: + +min_sample_time: The minimum amount of time to spend at the current +frequency before ramping down. This is to ensure that the governor has +seen enough historic cpu load data to determine the appropriate +workload. Default is 80000 uS. + +go_maxspeed_load: The CPU load at which to ramp to max speed. Default +is 85. + +timer_rate: Sample rate for reevaluating cpu load when the system is +not idle. Default is 30000 uS. + +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. + |