Documentation/cpu-freq/governors.txt

   1      CPU frequency and voltage scaling code in the Linux(TM) kernel
   2
   3
   4                          L i n u x    C P U F r e q
   5
   6                       C P U F r e q   G o v e r n o r s
   7
   8                    - information for users and developers -
   9
  10
  11                     Dominik Brodowski  <linux@brodo.de>
  12             some additions and corrections by Nico Golde <nico@ngolde.de>
  13
  14
  15
  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.
  19
  20
  21 Contents:
  22 ---------
  23 1.   What is a CPUFreq Governor?
  24
  25 2.   Governors In the Linux Kernel
  26 2.1  Performance
  27 2.2  Powersave
  28 2.3  Userspace
  29 2.4  Ondemand
  30 2.5  Conservative
  31 2.6  Interactive
  32
  33 3.   The Governor Interface in the CPUfreq Core
  34
  35
  36
  37 1. What Is A CPUFreq Governor?
  38 ==============================
  39
  40 Most cpufreq drivers (in fact, all except one, 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"
  45 call instead of the existing "->setpolicy" call. For "longrun", all
  46 stays the same, though.
  47
  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.
  58
  59 Basically, it's the following flow graph:
  60
  61 CPU can be set to switch independently   |         CPU can only be set
  62       within specific "limits"           |       to specific frequencies
  63
  64                                  "CPUfreq policy"
  65                 consists of frequency limits (policy->{min,max})
  66                      and CPUfreq governor to be used
  67                          /                    \
  68                         /                      \
  69                        /                       the cpufreq governor decides
  70                       /                        (dynamically or statically)
  71                      /                         what target_freq to set within
  72                     /                          the limits of policy->{min,max}
  73                    /                                \
  74                   /                                  \
  75         Using the ->setpolicy call,              Using the ->target call,
  76             the limits and the                    the frequency closest
  77              "policy" is set.                     to target_freq is set.
  78                                                   It is assured that it
  79                                                   is within policy->{min,max}
  80
  81
  82 2. Governors In the Linux Kernel
  83 ================================
  84
  85 2.1 Performance
  86 ---------------
  87
  88 The CPUfreq governor "performance" sets the CPU statically to the
  89 highest frequency within the borders of scaling_min_freq and
  90 scaling_max_freq.
  91
  92
  93 2.2 Powersave
  94 -------------
  95
  96 The CPUfreq governor "powersave" sets the CPU statically to the
  97 lowest frequency within the borders of scaling_min_freq and
  98 scaling_max_freq.
  99
 100
 101 2.3 Userspace
 102 -------------
 103
 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
 107 directory.
 108
 109
 110 2.4 Ondemand
 111 ------------
 112
 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:
 117
 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
 130
 131 sampling_rate_min:
 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.
 143
 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.
 150
 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.
 159
 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.
 170
 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.
 191
 192
 193 2.5 Conservative
 194 ----------------
 195
 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:
 203
 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.
 210
 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.
 215
 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
 220 sampling rate.
 221
 222 2.6 Interactive
 223 ---------------
 224
 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, but with a
 228 different set of configurable behaviors.
 229
 230 The tuneable values for this governor are:
 231
 232 target_loads: CPU load values used to adjust speed to influence the
 233 current CPU load toward that value.  In general, the lower the target
 234 load, the more often the governor will raise CPU speeds to bring load
 235 below the target.  The format is a single target load, optionally
 236 followed by pairs of CPU speeds and CPU loads to target at or above
 237 those speeds.  Colons can be used between the speeds and associated
 238 target loads for readability.  For example:
 239
 240    85 1000000:90 1700000:99
 241
 242 targets CPU load 85% below speed 1GHz, 90% at or above 1GHz, until
 243 1.7GHz and above, at which load 99% is targeted.  If speeds are
 244 specified these must appear in ascending order.  Higher target load
 245 values are typically specified for higher speeds, that is, target load
 246 values also usually appear in an ascending order. The default is
 247 target load 90% for all speeds.
 248
 249 min_sample_time: The minimum amount of time to spend at the current
 250 frequency before ramping down. Default is 80000 uS.
 251
 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 the maximum speed
 256 allowed by the policy at governor initialization time.
 257
 258 go_hispeed_load: The CPU load at which to ramp to hispeed_freq.
 259 Default is 99%.
 260
 261 above_hispeed_delay: When speed is at or above hispeed_freq, wait for
 262 this long before raising speed in response to continued high load.
 263 The format is a single delay value, optionally followed by pairs of
 264 CPU speeds and the delay to use at or above those speeds.  Colons can
 265 be used between the speeds and associated delays for readability.  For
 266 example:
 267
 268    80000 1300000:200000 1500000:40000
 269
 270 uses delay 80000 uS until CPU speed 1.3 GHz, at which speed delay
 271 200000 uS is used until speed 1.5 GHz, at which speed (and above)
 272 delay 40000 uS is used.  If speeds are specified these must appear in
 273 ascending order.  Default is 20000 uS.
 274
 275 timer_rate: Sample rate for reevaluating CPU load when the CPU is not
 276 idle.  A deferrable timer is used, such that the CPU will not be woken
 277 from idle to service this timer until something else needs to run.
 278 (The maximum time to allow deferring this timer when not running at
 279 minimum speed is configurable via timer_slack.)  Default is 20000 uS.
 280
 281 timer_slack: Maximum additional time to defer handling the governor
 282 sampling timer beyond timer_rate when running at speeds above the
 283 minimum.  For platforms that consume additional power at idle when
 284 CPUs are running at speeds greater than minimum, this places an upper
 285 bound on how long the timer will be deferred prior to re-evaluating
 286 load and dropping speed.  For example, if timer_rate is 20000uS and
 287 timer_slack is 10000uS then timers will be deferred for up to 30msec
 288 when not at lowest speed.  A value of -1 means defer timers
 289 indefinitely at all speeds.  Default is 80000 uS.
 290
 291 boost: If non-zero, immediately boost speed of all CPUs to at least
 292 hispeed_freq until zero is written to this attribute.  If zero, allow
 293 CPU speeds to drop below hispeed_freq according to load as usual.
 294 Default is zero.
 295
 296 boostpulse: On each write, immediately boost speed of all CPUs to
 297 hispeed_freq for at least the period of time specified by
 298 boostpulse_duration, after which speeds are allowed to drop below
 299 hispeed_freq according to load as usual.
 300
 301 boostpulse_duration: Length of time to hold CPU speed at hispeed_freq
 302 on a write to boostpulse, before allowing speed to drop according to
 303 load as usual.  Default is 80000 uS.
 304
 305
 306 3. The Governor Interface in the CPUfreq Core
 307 =============================================
 308
 309 A new governor must register itself with the CPUfreq core using
 310 "cpufreq_register_governor". The struct cpufreq_governor, which has to
 311 be passed to that function, must contain the following values:
 312
 313 governor->name -            A unique name for this governor
 314 governor->governor -        The governor callback function
 315 governor->owner -           .THIS_MODULE for the governor module (if
 316                             appropriate)
 317
 318 The governor->governor callback is called with the current (or to-be-set)
 319 cpufreq_policy struct for that CPU, and an unsigned int event. The
 320 following events are currently defined:
 321
 322 CPUFREQ_GOV_START:   This governor shall start its duty for the CPU
 323                      policy->cpu
 324 CPUFREQ_GOV_STOP:    This governor shall end its duty for the CPU
 325                      policy->cpu
 326 CPUFREQ_GOV_LIMITS:  The limits for CPU policy->cpu have changed to
 327                      policy->min and policy->max.
 328
 329 If you need other "events" externally of your driver, _only_ use the
 330 cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
 331 CPUfreq core to ensure proper locking.
 332
 333
 334 The CPUfreq governor may call the CPU processor driver using one of
 335 these two functions:
 336
 337 int cpufreq_driver_target(struct cpufreq_policy *policy,
 338                                  unsigned int target_freq,
 339                                  unsigned int relation);
 340
 341 int __cpufreq_driver_target(struct cpufreq_policy *policy,
 342                                    unsigned int target_freq,
 343                                    unsigned int relation);
 344
 345 target_freq must be within policy->min and policy->max, of course.
 346 What's the difference between these two functions? When your governor
 347 still is in a direct code path of a call to governor->governor, the
 348 per-CPU cpufreq lock is still held in the cpufreq core, and there's
 349 no need to lock it again (in fact, this would cause a deadlock). So
 350 use __cpufreq_driver_target only in these cases. In all other cases
 351 (for example, when there's a "daemonized" function that wakes up
 352 every second), use cpufreq_driver_target to lock the cpufreq per-CPU
 353 lock before the command is passed to the cpufreq processor driver.
 354