Most of the processor have mechanism to save power by scaling the frequency & voltage. Now, the question arise, why these two?
Existing CMOS technology consumes power in mostly three major areas :-
1. Leakage current - It is basically due to the underlying circuitry in the schematics.
P(lc) = I,load * Vc = squr(Vc)/RL
2. Recharging Current - It is mainly the parasitic capacitance inducing current.
P(rc) = squr(Vc)/Rp = squr(Vc) * Cp * F
3. Shoot through current - Sometimes when the transistor in the circuit is opening up and the opposite to it is just started to close, there is some amount of current.
P(sc) = squr(Vc) * F / Rs
So, the total power equals to P(lc) + P(rc) + P(sc)
Therefore, the power is directly proportional to the square of Voltage and the constant or the linear function of the Frequency.
So, here we are with the answer to our basic question of why scaling only freq & volt.
How does Linux Kernel provide support for this? Answer lies in the CPU-Freq framework provided by the kernel.
It is the linux subsystem to set cpu frequency. The details of this can be read from "Documentation/cpu-freq/".
Lets see how to use this framework from the User-Space.
The cpufreq framework is mostly divided in three parts -
1. cpufreq module
2. cpu-specific drivers
3. in-kernel governors
Linux kernel provide three kinds of governor which can be used for any kind of CPU. They are
1. performance governor
2. powersave governor
3. userspace governor
apart from these, there are two new governors namely,
1. Ondemand governor
2. conservative governor
On the lowest level of this architecture, there lies ACPI processor driver. This is used by the CPU specific drivers like powernow-k8 etc. These provide various /sys/ and /proc/ interfaces. above it lies in-kernel governors. User-level governors uses userspace governor to modify cpu frequencies.
To understand how we use these sys attributes, go to /sys/devices/system/cpu/. This lists the available cpus and cpu-freq governor details.
arya@maya:/$ ls /sys/devices/system/cpu/
cpu0 cpu1 cpufreq cpuidle kernel_max microcode modalias offline online possible power present probe release uevent
To check the governor,
arya@maya:/sys/devices/system/cpu$ ls cpufreq/
boost ondemand
Ok. Now lets go inside one of the cpus.
root@maya:/sys/devices/system/cpu/cpu0# ll cpufreq/
total 0
drwxr-xr-x 3 root root 0 Apr 1 22:53 ./
drwxr-xr-x 8 root root 0 Apr 2 2013 ../
-r--r--r-- 1 root root 4096 Apr 1 23:02 affected_cpus
-r--r--r-- 1 root root 4096 Apr 1 23:02 bios_limit
-rw-r--r-- 1 root root 4096 Apr 1 23:02 cpb
-r-------- 1 root root 4096 Apr 1 23:02 cpuinfo_cur_freq
-r--r--r-- 1 root root 4096 Apr 1 23:02 cpuinfo_max_freq
-r--r--r-- 1 root root 4096 Apr 1 23:02 cpuinfo_min_freq
-r--r--r-- 1 root root 4096 Apr 1 23:02 cpuinfo_transition_latency
-r--r--r-- 1 root root 4096 Apr 1 23:02 related_cpus
-r--r--r-- 1 root root 4096 Apr 1 22:53 scaling_available_frequencies
-r--r--r-- 1 root root 4096 Apr 1 22:53 scaling_available_governors
-r--r--r-- 1 root root 4096 Apr 1 22:53 scaling_cur_freq
-r--r--r-- 1 root root 4096 Apr 1 23:02 scaling_driver
-rw-r--r-- 1 root root 4096 Apr 1 22:53 scaling_governor
-rw-r--r-- 1 root root 4096 Apr 1 23:02 scaling_max_freq
-rw-r--r-- 1 root root 4096 Apr 1 23:02 scaling_min_freq
-rw-r--r-- 1 root root 4096 Apr 1 22:53 scaling_setspeed
drwxr-xr-x 2 root root 0 Apr 1 23:02 stats/
1. current cpu frequency
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/cpuinfo_cur_freq
800000
2. max supported cpu frequency
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/cpuinfo_max_freq
1600000
3. min supported cpu frequency
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/cpuinfo_min_freq
800000
4. list all available cpu frequency
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/scaling_available_frequencies
1600000 1280000 800000
5. list all available governors
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/scaling_available_governors
conservative ondemand userspace powersave performance
6. return cached value of cpu frequency from cpu-freq driver
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/scaling_cur_freq
800000
7. return cpu specific driver
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/scaling_driver
acpi-cpufreq
8. user controlled lower cpu frequency
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/scaling_min_freq
800000
9. user controlled higher cpu frequency
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/scaling_max_freq
1600000
The interfaces under the stats/ directory provide the statistics about the usage of frequency changes on any particular CPU.
1. details of cpu frequency transitions
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/stats/trans_table
From :To : 1600000 1280000 800000
1600000 : 0 1582 2938
1280000 : 384 0 1198
800000 : 4135 0 0
2. details of total time spent in a particular state.
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/stats/time_in_state
1600000 46780
1280000 4558
800000 421491
3. details of total number of transition by cpu.
root@maya:/sys/devices/system/cpu/cpu0# cat cpufreq/stats/total_trans
10237
This is the basic of all kind of governors today exists. One of the most common governor used in Linux is OnDemand Governor.
But, this does not stop one to write their own. Infact, there are as many as 28 governors I am aware of in android. This is explained in one of my previous blogs.
More to come on the governors. leave comment to improve the knowledge base.
happy blogging !
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