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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 EST.

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.
21	Contents:
22	---------
23	1.   What is a CPUFreq Governor?
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
32	3.   The Governor Interface in the CPUfreq Core
36	1. What Is A CPUFreq Governor?
37	==============================
39	Most cpufreq drivers (in fact, all except one, 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
63	                                 "CPUfreq policy"
64			consists of frequency limits (policy->{min,max})
65	  		     and CPUfreq governor to be used
66				 /		      \
67				/		       \
68			       /		       the cpufreq governor decides
69			      /			       (dynamically or statically)
70			     /			       what target_freq to set within
71			    /			       the limits of policy->{min,max}
72			   /			            \
73			  /				     \
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.
77							  It is assured that it
78							  is within policy->{min,max}
81	2. Governors In the Linux Kernel
82	================================
84	2.1 Performance
85	---------------
87	The CPUfreq governor "performance" sets the CPU statically to the
88	highest frequency within the borders of scaling_min_freq and
89	scaling_max_freq.
92	2.2 Powersave
93	-------------
95	The CPUfreq governor "powersave" sets the CPU statically to the
96	lowest frequency within the borders of scaling_min_freq and
97	scaling_max_freq.
100	2.3 Userspace
101	-------------
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
106	directory.
109	2.4 Ondemand
110	------------
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.  There are a number of sysfs file
115	accessible parameters:
117	sampling_rate: measured in uS (10^-6 seconds), this is how often you
118	want the kernel to look at the CPU usage and to make decisions on
119	what to do about the frequency.  Typically this is set to values of
120	around '10000' or more. It's default value is (cmp. with users-guide.txt):
121	transition_latency * 1000
122	Be aware that transition latency is in ns and sampling_rate is in us, so you
123	get the same sysfs value by default.
124	Sampling rate should always get adjusted considering the transition latency
125	To set the sampling rate 750 times as high as the transition latency
126	in the bash (as said, 1000 is default), do:
127	echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
128	    >ondemand/sampling_rate
130	sampling_rate_min:
131	The sampling rate is limited by the HW transition latency:
132	transition_latency * 100
133	Or by kernel restrictions:
134	If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
135	If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is used, the
136	limits depend on the CONFIG_HZ option:
137	HZ=1000: min=20000us  (20ms)
138	HZ=250:  min=80000us  (80ms)
139	HZ=100:  min=200000us (200ms)
140	The highest value of kernel and HW latency restrictions is shown and
141	used as the minimum sampling rate.
143	up_threshold: defines what the average CPU usage between the samplings
144	of 'sampling_rate' needs to be for the kernel to make a decision on
145	whether it should increase the frequency.  For example when it is set
146	to its default value of '95' it means that between the checking
147	intervals the CPU needs to be on average more than 95% in use to then
148	decide that the CPU frequency needs to be increased.  
150	ignore_nice_load: this parameter takes a value of '0' or '1'. When
151	set to '0' (its default), all processes are counted towards the
152	'cpu utilisation' value.  When set to '1', the processes that are
153	run with a 'nice' value will not count (and thus be ignored) in the
154	overall usage calculation.  This is useful if you are running a CPU
155	intensive calculation on your laptop that you do not care how long it
156	takes to complete as you can 'nice' it and prevent it from taking part
157	in the deciding process of whether to increase your CPU frequency.
159	sampling_down_factor: this parameter controls the rate at which the
160	kernel makes a decision on when to decrease the frequency while running
161	at top speed. When set to 1 (the default) decisions to reevaluate load
162	are made at the same interval regardless of current clock speed. But
163	when set to greater than 1 (e.g. 100) it acts as a multiplier for the
164	scheduling interval for reevaluating load when the CPU is at its top
165	speed due to high load. This improves performance by reducing the overhead
166	of load evaluation and helping the CPU stay at its top speed when truly
167	busy, rather than shifting back and forth in speed. This tunable has no
168	effect on behavior at lower speeds/lower CPU loads.
170	powersave_bias: this parameter takes a value between 0 to 1000. It
171	defines the percentage (times 10) value of the target frequency that
172	will be shaved off of the target. For example, when set to 100 -- 10%,
173	when ondemand governor would have targeted 1000 MHz, it will target
174	1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
175	(disabled) by default.
176	When AMD frequency sensitivity powersave bias driver --
177	drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
178	defines the workload frequency sensitivity threshold in which a lower
179	frequency is chosen instead of ondemand governor's original target.
180	The frequency sensitivity is a hardware reported (on AMD Family 16h
181	Processors and above) value between 0 to 100% that tells software how
182	the performance of the workload running on a CPU will change when
183	frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
184	will not perform any better on higher core frequency, whereas a
185	workload with sensitivity of 100% (CPU-bound) will perform better
186	higher the frequency. When the driver is loaded, this is set to 400
187	by default -- for CPUs running workloads with sensitivity value below
188	40%, a lower frequency is chosen. Unloading the driver or writing 0
189	will disable this feature.
192	2.5 Conservative
193	----------------
195	The CPUfreq governor "conservative", much like the "ondemand"
196	governor, sets the CPU depending on the current usage.  It differs in
197	behaviour in that it gracefully increases and decreases the CPU speed
198	rather than jumping to max speed the moment there is any load on the
199	CPU.  This behaviour more suitable in a battery powered environment.
200	The governor is tweaked in the same manner as the "ondemand" governor
201	through sysfs with the addition of:
203	freq_step: this describes what percentage steps the cpu freq should be
204	increased and decreased smoothly by.  By default the cpu frequency will
205	increase in 5% chunks of your maximum cpu frequency.  You can change this
206	value to anywhere between 0 and 100 where '0' will effectively lock your
207	CPU at a speed regardless of its load whilst '100' will, in theory, make
208	it behave identically to the "ondemand" governor.
210	down_threshold: same as the 'up_threshold' found for the "ondemand"
211	governor but for the opposite direction.  For example when set to its
212	default value of '20' it means that if the CPU usage needs to be below
213	20% between samples to have the frequency decreased.
215	sampling_down_factor: similar functionality as in "ondemand" governor.
216	But in "conservative", it controls the rate at which the kernel makes
217	a decision on when to decrease the frequency while running in any
218	speed. Load for frequency increase is still evaluated every
219	sampling rate.
221	3. The Governor Interface in the CPUfreq Core
222	=============================================
224	A new governor must register itself with the CPUfreq core using
225	"cpufreq_register_governor". The struct cpufreq_governor, which has to
226	be passed to that function, must contain the following values:
228	governor->name -	    A unique name for this governor
229	governor->governor -	    The governor callback function
230	governor->owner	-	    .THIS_MODULE for the governor module (if 
231				    appropriate)
233	The governor->governor callback is called with the current (or to-be-set)
234	cpufreq_policy struct for that CPU, and an unsigned int event. The
235	following events are currently defined:
237	CPUFREQ_GOV_START:   This governor shall start its duty for the CPU
238			     policy->cpu
239	CPUFREQ_GOV_STOP:    This governor shall end its duty for the CPU
240			     policy->cpu
241	CPUFREQ_GOV_LIMITS:  The limits for CPU policy->cpu have changed to
242			     policy->min and policy->max.
244	If you need other "events" externally of your driver, _only_ use the
245	cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
246	CPUfreq core to ensure proper locking.
249	The CPUfreq governor may call the CPU processor driver using one of
250	these two functions:
252	int cpufreq_driver_target(struct cpufreq_policy *policy,
253	                                 unsigned int target_freq,
254	                                 unsigned int relation);
256	int __cpufreq_driver_target(struct cpufreq_policy *policy,
257	                                   unsigned int target_freq,
258	                                   unsigned int relation);
260	target_freq must be within policy->min and policy->max, of course.
261	What's the difference between these two functions? When your governor
262	still is in a direct code path of a call to governor->governor, the
263	per-CPU cpufreq lock is still held in the cpufreq core, and there's
264	no need to lock it again (in fact, this would cause a deadlock). So
265	use __cpufreq_driver_target only in these cases. In all other cases 
266	(for example, when there's a "daemonized" function that wakes up 
267	every second), use cpufreq_driver_target to lock the cpufreq per-CPU
268	lock before the command is passed to the cpufreq processor driver.
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