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
32 3. The Governor Interface in the CPUfreq Core
36 1. What Is A CPUFreq Governor?
37 ==============================
39 Most cpufreq drivers (except the intel_pstate and longrun) or even most
40 cpu frequency scaling algorithms only offer the CPU to be set to one
41 frequency. In order to offer dynamic frequency scaling, the cpufreq
42 core must be able to tell these drivers of a "target frequency". So
43 these specific drivers will be transformed to offer a "->target/target_index"
44 call instead of the existing "->setpolicy" call. For "longrun", all
45 stays the same, though.
47 How to decide what frequency within the CPUfreq policy should be used?
48 That's done using "cpufreq governors". Two are already in this patch
49 -- they're the already existing "powersave" and "performance" which
50 set the frequency statically to the lowest or highest frequency,
51 respectively. At least two more such governors will be ready for
52 addition in the near future, but likely many more as there are various
53 different theories and models about dynamic frequency scaling
54 around. Using such a generic interface as cpufreq offers to scaling
55 governors, these can be tested extensively, and the best one can be
56 selected for each specific use.
58 Basically, it's the following flow graph:
60 CPU can be set to switch independently | CPU can only be set
61 within specific "limits" | to specific frequencies
64 consists of frequency limits (policy->{min,max})
65 and CPUfreq governor to be used
68 / the cpufreq governor decides
69 / (dynamically or statically)
70 / what target_freq to set within
71 / the limits of policy->{min,max}
74 Using the ->setpolicy call, Using the ->target/target_index call,
75 the limits and the the frequency closest
76 "policy" is set. to target_freq is set.
78 is within policy->{min,max}
81 2. Governors In the Linux Kernel
82 ================================
87 The CPUfreq governor "performance" sets the CPU statically to the
88 highest frequency within the borders of scaling_min_freq and
95 The CPUfreq governor "powersave" sets the CPU statically to the
96 lowest frequency within the borders of scaling_min_freq and
103 The CPUfreq governor "userspace" allows the user, or any userspace
104 program running with UID "root", to set the CPU to a specific frequency
105 by making a sysfs file "scaling_setspeed" available in the CPU-device
112 The CPUfreq governor "ondemand" sets the CPU depending on the
113 current usage. To do this the CPU must have the capability to
114 switch the frequency very quickly.
120 Measured in uS (10^-6 seconds), this is how often you want the kernel
121 to look at the CPU usage and to make decisions on what to do about the
122 frequency. Typically this is set to values of around '10000' or more.
123 It's default value is (cmp. with users-guide.txt): transition_latency
124 * 1000. Be aware that transition latency is in ns and sampling_rate
125 is in us, so you get the same sysfs value by default. Sampling rate
126 should always get adjusted considering the transition latency to set
127 the sampling rate 750 times as high as the transition latency in the
128 bash (as said, 1000 is default), do:
130 $ echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) > ondemand/sampling_rate
134 The sampling rate is limited by the HW transition latency:
135 transition_latency * 100
137 Or by kernel restrictions:
138 - If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
139 - If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is
140 used, the limits depend on the CONFIG_HZ option:
141 HZ=1000: min=20000us (20ms)
142 HZ=250: min=80000us (80ms)
143 HZ=100: min=200000us (200ms)
145 The highest value of kernel and HW latency restrictions is shown and
146 used as the minimum sampling rate.
150 This defines what the average CPU usage between the samplings of
151 'sampling_rate' needs to be for the kernel to make a decision on
152 whether it should increase the frequency. For example when it is set
153 to its default value of '95' it means that between the checking
154 intervals the CPU needs to be on average more than 95% in use to then
155 decide that the CPU frequency needs to be increased.
159 This parameter takes a value of '0' or '1'. When set to '0' (its
160 default), all processes are counted towards the 'cpu utilisation'
161 value. When set to '1', the processes that are run with a 'nice'
162 value will not count (and thus be ignored) in the overall usage
163 calculation. This is useful if you are running a CPU intensive
164 calculation on your laptop that you do not care how long it takes to
165 complete as you can 'nice' it and prevent it from taking part in the
166 deciding process of whether to increase your CPU frequency.
168 * sampling_down_factor:
170 This parameter controls the rate at which the kernel makes a decision
171 on when to decrease the frequency while running at top speed. When set
172 to 1 (the default) decisions to reevaluate load are made at the same
173 interval regardless of current clock speed. But when set to greater
174 than 1 (e.g. 100) it acts as a multiplier for the scheduling interval
175 for reevaluating load when the CPU is at its top speed due to high
176 load. This improves performance by reducing the overhead of load
177 evaluation and helping the CPU stay at its top speed when truly busy,
178 rather than shifting back and forth in speed. This tunable has no
179 effect on behavior at lower speeds/lower CPU loads.
183 This parameter takes a value between 0 to 1000. It defines the
184 percentage (times 10) value of the target frequency that will be
185 shaved off of the target. For example, when set to 100 -- 10%, when
186 ondemand governor would have targeted 1000 MHz, it will target
187 1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
188 (disabled) by default.
190 When AMD frequency sensitivity powersave bias driver --
191 drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
192 defines the workload frequency sensitivity threshold in which a lower
193 frequency is chosen instead of ondemand governor's original target.
194 The frequency sensitivity is a hardware reported (on AMD Family 16h
195 Processors and above) value between 0 to 100% that tells software how
196 the performance of the workload running on a CPU will change when
197 frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
198 will not perform any better on higher core frequency, whereas a
199 workload with sensitivity of 100% (CPU-bound) will perform better
200 higher the frequency. When the driver is loaded, this is set to 400 by
201 default -- for CPUs running workloads with sensitivity value below
202 40%, a lower frequency is chosen. Unloading the driver or writing 0
203 will disable this feature.
209 The CPUfreq governor "conservative", much like the "ondemand"
210 governor, sets the CPU depending on the current usage. It differs in
211 behaviour in that it gracefully increases and decreases the CPU speed
212 rather than jumping to max speed the moment there is any load on the
213 CPU. This behaviour more suitable in a battery powered environment.
214 The governor is tweaked in the same manner as the "ondemand" governor
215 through sysfs with the addition of:
219 This describes what percentage steps the cpu freq should be increased
220 and decreased smoothly by. By default the cpu frequency will increase
221 in 5% chunks of your maximum cpu frequency. You can change this value
222 to anywhere between 0 and 100 where '0' will effectively lock your CPU
223 at a speed regardless of its load whilst '100' will, in theory, make
224 it behave identically to the "ondemand" governor.
228 Same as the 'up_threshold' found for the "ondemand" governor but for
229 the opposite direction. For example when set to its default value of
230 '20' it means that if the CPU usage needs to be below 20% between
231 samples to have the frequency decreased.
233 * sampling_down_factor:
235 Similar functionality as in "ondemand" governor. But in
236 "conservative", it controls the rate at which the kernel makes a
237 decision on when to decrease the frequency while running in any speed.
238 Load for frequency increase is still evaluated every sampling rate.
240 3. The Governor Interface in the CPUfreq Core
241 =============================================
243 A new governor must register itself with the CPUfreq core using
244 "cpufreq_register_governor". The struct cpufreq_governor, which has to
245 be passed to that function, must contain the following values:
247 governor->name - A unique name for this governor
248 governor->governor - The governor callback function
249 governor->owner - .THIS_MODULE for the governor module (if
252 The governor->governor callback is called with the current (or to-be-set)
253 cpufreq_policy struct for that CPU, and an unsigned int event. The
254 following events are currently defined:
256 CPUFREQ_GOV_START: This governor shall start its duty for the CPU
258 CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
260 CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
261 policy->min and policy->max.
263 If you need other "events" externally of your driver, _only_ use the
264 cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
265 CPUfreq core to ensure proper locking.
268 The CPUfreq governor may call the CPU processor driver using one of
271 int cpufreq_driver_target(struct cpufreq_policy *policy,
272 unsigned int target_freq,
273 unsigned int relation);
275 int __cpufreq_driver_target(struct cpufreq_policy *policy,
276 unsigned int target_freq,
277 unsigned int relation);
279 target_freq must be within policy->min and policy->max, of course.
280 What's the difference between these two functions? When your governor
281 still is in a direct code path of a call to governor->governor, the
282 per-CPU cpufreq lock is still held in the cpufreq core, and there's
283 no need to lock it again (in fact, this would cause a deadlock). So
284 use __cpufreq_driver_target only in these cases. In all other cases
285 (for example, when there's a "daemonized" function that wakes up
286 every second), use cpufreq_driver_target to lock the cpufreq per-CPU
287 lock before the command is passed to the cpufreq processor driver.