1 CPU frequency and voltage scaling code in the Linux(TM) kernel
4 L i n u x C P U F r e q
6 C P U F r e q G o v e r n o r s
8 - information for users and developers -
11 Dominik Brodowski <linux@brodo.de>
12 some additions and corrections by Nico Golde <nico@ngolde.de>
16 Clock scaling allows you to change the clock speed of the CPUs on the
17 fly. This is a nice method to save battery power, because the lower
18 the clock speed, the less power the CPU consumes.
23 1. What is a CPUFreq Governor?
25 2. Governors In the Linux Kernel
33 3. The Governor Interface in the CPUfreq Core
37 1. What Is A CPUFreq Governor?
38 ==============================
40 Most cpufreq drivers (except the intel_pstate and longrun) or even most
41 cpu frequency scaling algorithms only offer the CPU to be set to one
42 frequency. In order to offer dynamic frequency scaling, the cpufreq
43 core must be able to tell these drivers of a "target frequency". So
44 these specific drivers will be transformed to offer a "->target/target_index"
45 call instead of the existing "->setpolicy" call. For "longrun", all
46 stays the same, though.
48 How to decide what frequency within the CPUfreq policy should be used?
49 That's done using "cpufreq governors". Two are already in this patch
50 -- they're the already existing "powersave" and "performance" which
51 set the frequency statically to the lowest or highest frequency,
52 respectively. At least two more such governors will be ready for
53 addition in the near future, but likely many more as there are various
54 different theories and models about dynamic frequency scaling
55 around. Using such a generic interface as cpufreq offers to scaling
56 governors, these can be tested extensively, and the best one can be
57 selected for each specific use.
59 Basically, it's the following flow graph:
61 CPU can be set to switch independently | CPU can only be set
62 within specific "limits" | to specific frequencies
65 consists of frequency limits (policy->{min,max})
66 and CPUfreq governor to be used
69 / the cpufreq governor decides
70 / (dynamically or statically)
71 / what target_freq to set within
72 / the limits of policy->{min,max}
75 Using the ->setpolicy call, Using the ->target/target_index call,
76 the limits and the the frequency closest
77 "policy" is set. to target_freq is set.
79 is within policy->{min,max}
82 2. Governors In the Linux Kernel
83 ================================
88 The CPUfreq governor "performance" sets the CPU statically to the
89 highest frequency within the borders of scaling_min_freq and
96 The CPUfreq governor "powersave" sets the CPU statically to the
97 lowest frequency within the borders of scaling_min_freq and
104 The CPUfreq governor "userspace" allows the user, or any userspace
105 program running with UID "root", to set the CPU to a specific frequency
106 by making a sysfs file "scaling_setspeed" available in the CPU-device
113 The CPUfreq governor "ondemand" sets the CPU depending on the
114 current usage. To do this the CPU must have the capability to
115 switch the frequency very quickly. There are a number of sysfs file
116 accessible parameters:
118 sampling_rate: measured in uS (10^-6 seconds), this is how often you
119 want the kernel to look at the CPU usage and to make decisions on
120 what to do about the frequency. Typically this is set to values of
121 around '10000' or more. It's default value is (cmp. with users-guide.txt):
122 transition_latency * 1000
123 Be aware that transition latency is in ns and sampling_rate is in us, so you
124 get the same sysfs value by default.
125 Sampling rate should always get adjusted considering the transition latency
126 To set the sampling rate 750 times as high as the transition latency
127 in the bash (as said, 1000 is default), do:
128 echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
129 >ondemand/sampling_rate
132 The sampling rate is limited by the HW transition latency:
133 transition_latency * 100
134 Or by kernel restrictions:
135 If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
136 If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is used, the
137 limits depend on the CONFIG_HZ option:
138 HZ=1000: min=20000us (20ms)
139 HZ=250: min=80000us (80ms)
140 HZ=100: min=200000us (200ms)
141 The highest value of kernel and HW latency restrictions is shown and
142 used as the minimum sampling rate.
144 up_threshold: defines what the average CPU usage between the samplings
145 of 'sampling_rate' needs to be for the kernel to make a decision on
146 whether it should increase the frequency. For example when it is set
147 to its default value of '95' it means that between the checking
148 intervals the CPU needs to be on average more than 95% in use to then
149 decide that the CPU frequency needs to be increased.
151 ignore_nice_load: this parameter takes a value of '0' or '1'. When
152 set to '0' (its default), all processes are counted towards the
153 'cpu utilisation' value. When set to '1', the processes that are
154 run with a 'nice' value will not count (and thus be ignored) in the
155 overall usage calculation. This is useful if you are running a CPU
156 intensive calculation on your laptop that you do not care how long it
157 takes to complete as you can 'nice' it and prevent it from taking part
158 in the deciding process of whether to increase your CPU frequency.
160 sampling_down_factor: this parameter controls the rate at which the
161 kernel makes a decision on when to decrease the frequency while running
162 at top speed. When set to 1 (the default) decisions to reevaluate load
163 are made at the same interval regardless of current clock speed. But
164 when set to greater than 1 (e.g. 100) it acts as a multiplier for the
165 scheduling interval for reevaluating load when the CPU is at its top
166 speed due to high load. This improves performance by reducing the overhead
167 of load evaluation and helping the CPU stay at its top speed when truly
168 busy, rather than shifting back and forth in speed. This tunable has no
169 effect on behavior at lower speeds/lower CPU loads.
171 powersave_bias: this parameter takes a value between 0 to 1000. It
172 defines the percentage (times 10) value of the target frequency that
173 will be shaved off of the target. For example, when set to 100 -- 10%,
174 when ondemand governor would have targeted 1000 MHz, it will target
175 1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
176 (disabled) by default.
177 When AMD frequency sensitivity powersave bias driver --
178 drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
179 defines the workload frequency sensitivity threshold in which a lower
180 frequency is chosen instead of ondemand governor's original target.
181 The frequency sensitivity is a hardware reported (on AMD Family 16h
182 Processors and above) value between 0 to 100% that tells software how
183 the performance of the workload running on a CPU will change when
184 frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
185 will not perform any better on higher core frequency, whereas a
186 workload with sensitivity of 100% (CPU-bound) will perform better
187 higher the frequency. When the driver is loaded, this is set to 400
188 by default -- for CPUs running workloads with sensitivity value below
189 40%, a lower frequency is chosen. Unloading the driver or writing 0
190 will disable this feature.
196 The CPUfreq governor "conservative", much like the "ondemand"
197 governor, sets the CPU depending on the current usage. It differs in
198 behaviour in that it gracefully increases and decreases the CPU speed
199 rather than jumping to max speed the moment there is any load on the
200 CPU. This behaviour more suitable in a battery powered environment.
201 The governor is tweaked in the same manner as the "ondemand" governor
202 through sysfs with the addition of:
204 freq_step: this describes what percentage steps the cpu freq should be
205 increased and decreased smoothly by. By default the cpu frequency will
206 increase in 5% chunks of your maximum cpu frequency. You can change this
207 value to anywhere between 0 and 100 where '0' will effectively lock your
208 CPU at a speed regardless of its load whilst '100' will, in theory, make
209 it behave identically to the "ondemand" governor.
211 down_threshold: same as the 'up_threshold' found for the "ondemand"
212 governor but for the opposite direction. For example when set to its
213 default value of '20' it means that if the CPU usage needs to be below
214 20% between samples to have the frequency decreased.
216 sampling_down_factor: similar functionality as in "ondemand" governor.
217 But in "conservative", it controls the rate at which the kernel makes
218 a decision on when to decrease the frequency while running in any
219 speed. Load for frequency increase is still evaluated every
225 The CPUfreq governor "interactive" is designed for latency-sensitive,
226 interactive workloads. This governor sets the CPU speed depending on
227 usage, similar to "ondemand" and "conservative" governors. However,
228 the governor is more aggressive about scaling the CPU speed up in
229 response to CPU-intensive activity.
231 Sampling the CPU load every X ms can lead to under-powering the CPU
232 for X ms, leading to dropped frames, stuttering UI, etc. Instead of
233 sampling the cpu at a specified rate, the interactive governor will
234 check whether to scale the cpu frequency up soon after coming out of
235 idle. When the cpu comes out of idle, a timer is configured to fire
236 within 1-2 ticks. If the cpu is very busy between exiting idle and
237 when the timer fires then we assume the cpu is underpowered and ramp
240 If the cpu was not sufficiently busy to immediately ramp to MAX speed,
241 then governor evaluates the cpu load since the last speed adjustment,
242 choosing the highest value between that longer-term load or the
243 short-term load since idle exit to determine the cpu speed to ramp to.
245 The tuneable values for this governor are:
247 min_sample_time: The minimum amount of time to spend at the current
248 frequency before ramping down. This is to ensure that the governor has
249 seen enough historic cpu load data to determine the appropriate
250 workload. Default is 80000 uS.
252 hispeed_freq: An intermediate "hi speed" at which to initially ramp
253 when CPU load hits the value specified in go_hispeed_load. If load
254 stays high for the amount of time specified in above_hispeed_delay,
255 then speed may be bumped higher. Default is maximum speed.
257 go_hispeed_load: The CPU load at which to ramp to the intermediate "hi
258 speed". Default is 85%.
260 above_hispeed_delay: Once speed is set to hispeed_freq, wait for this
261 long before bumping speed higher in response to continued high load.
264 timer_rate: Sample rate for reevaluating cpu load when the system is
265 not idle. Default is 20000 uS.
267 input_boost: If non-zero, boost speed of all CPUs to hispeed_freq on
268 touchscreen activity. Default is 0.
270 boost: If non-zero, immediately boost speed of all CPUs to
271 hispeed_freq. If zero, allow CPU speeds to drop below hispeed_freq.
274 3. The Governor Interface in the CPUfreq Core
275 =============================================
277 A new governor must register itself with the CPUfreq core using
278 "cpufreq_register_governor". The struct cpufreq_governor, which has to
279 be passed to that function, must contain the following values:
281 governor->name - A unique name for this governor
282 governor->governor - The governor callback function
283 governor->owner - .THIS_MODULE for the governor module (if
286 The governor->governor callback is called with the current (or to-be-set)
287 cpufreq_policy struct for that CPU, and an unsigned int event. The
288 following events are currently defined:
290 CPUFREQ_GOV_START: This governor shall start its duty for the CPU
292 CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
294 CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
295 policy->min and policy->max.
297 If you need other "events" externally of your driver, _only_ use the
298 cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
299 CPUfreq core to ensure proper locking.
302 The CPUfreq governor may call the CPU processor driver using one of
305 int cpufreq_driver_target(struct cpufreq_policy *policy,
306 unsigned int target_freq,
307 unsigned int relation);
309 int __cpufreq_driver_target(struct cpufreq_policy *policy,
310 unsigned int target_freq,
311 unsigned int relation);
313 target_freq must be within policy->min and policy->max, of course.
314 What's the difference between these two functions? When your governor
315 still is in a direct code path of a call to governor->governor, the
316 per-CPU cpufreq lock is still held in the cpufreq core, and there's
317 no need to lock it again (in fact, this would cause a deadlock). So
318 use __cpufreq_driver_target only in these cases. In all other cases
319 (for example, when there's a "daemonized" function that wakes up
320 every second), use cpufreq_driver_target to lock the cpufreq per-CPU
321 lock before the command is passed to the cpufreq processor driver.