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CPU frequency and voltage scaling statistics in the Linux(TM) kernel
L i n u x c p u f r e q - s t a t s d r i v e r
- information for users -
Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Contents
1. Introduction
2. Statistics Provided (with example)
3. Configuring cpufreq-stats
1. Introduction
cpufreq-stats is a driver that provides CPU frequency statistics for each CPU.
These statistics are provided in /sysfs as a bunch of read_only interfaces. This
interface (when configured) will appear in a separate directory under cpufreq
in /sysfs (<sysfs root>/devices/system/cpu/cpuX/cpufreq/stats/) for each CPU.
Various statistics will form read_only files under this directory.
This driver is designed to be independent of any particular cpufreq_driver
that may be running on your CPU. So, it will work with any cpufreq_driver.
2. Statistics Provided (with example)
cpufreq stats provides following statistics (explained in detail below).
- time_in_state
- total_trans
- trans_table
All the statistics will be from the time the stats driver has been inserted
to the time when a read of a particular statistic is done. Obviously, stats
driver will not have any information about the frequency transitions before
the stats driver insertion.
--------------------------------------------------------------------------------
<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # ls -l
total 0
drwxr-xr-x 2 root root 0 May 14 16:06 .
drwxr-xr-x 3 root root 0 May 14 15:58 ..
--w------- 1 root root 4096 May 14 16:06 reset
-r--r--r-- 1 root root 4096 May 14 16:06 time_in_state
-r--r--r-- 1 root root 4096 May 14 16:06 total_trans
-r--r--r-- 1 root root 4096 May 14 16:06 trans_table
--------------------------------------------------------------------------------
- reset
Write-only attribute that can be used to reset the stat counters. This can be
useful for evaluating system behaviour under different governors without the
need for a reboot.
- time_in_state
This gives the amount of time spent in each of the frequencies supported by
this CPU. The cat output will have "<frequency> <time>" pair in each line, which
will mean this CPU spent <time> usertime units of time at <frequency>. Output
will have one line for each of the supported frequencies. usertime units here
is 10mS (similar to other time exported in /proc).
--------------------------------------------------------------------------------
<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat time_in_state
3600000 2089
3400000 136
3200000 34
3000000 67
2800000 172488
--------------------------------------------------------------------------------
- total_trans
This gives the total number of frequency transitions on this CPU. The cat
output will have a single count which is the total number of frequency
transitions.
--------------------------------------------------------------------------------
<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat total_trans
20
--------------------------------------------------------------------------------
- trans_table
This will give a fine grained information about all the CPU frequency
transitions. The cat output here is a two dimensional matrix, where an entry
<i,j> (row i, column j) represents the count of number of transitions from
Freq_i to Freq_j. Freq_i is in descending order with increasing rows and
Freq_j is in descending order with increasing columns. The output here also
contains the actual freq values for each row and column for better readability.
--------------------------------------------------------------------------------
<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat trans_table
From : To
: 3600000 3400000 3200000 3000000 2800000
3600000: 0 5 0 0 0
3400000: 4 0 2 0 0
3200000: 0 1 0 2 0
3000000: 0 0 1 0 3
2800000: 0 0 0 2 0
--------------------------------------------------------------------------------
3. Configuring cpufreq-stats
To configure cpufreq-stats in your kernel
Config Main Menu
Power management options (ACPI, APM) --->
CPU Frequency scaling --->
[*] CPU Frequency scaling
[*] CPU frequency translation statistics
[*] CPU frequency translation statistics details
"CPU Frequency scaling" (CONFIG_CPU_FREQ) should be enabled to configure
cpufreq-stats.
"CPU frequency translation statistics" (CONFIG_CPU_FREQ_STAT) provides the
basic statistics which includes time_in_state and total_trans.
"CPU frequency translation statistics details" (CONFIG_CPU_FREQ_STAT_DETAILS)
provides fine grained cpufreq stats by trans_table. The reason for having a
separate config option for trans_table is:
- trans_table goes against the traditional /sysfs rule of one value per
interface. It provides a whole bunch of value in a 2 dimensional matrix
form.
Once these two options are enabled and your CPU supports cpufrequency, you
will be able to see the CPU frequency statistics in /sysfs.
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