Why do cpuinfo_cur_freq and /proc/cpuinfo report different numbers?

Question

When I do

sudo watch -n1 cat /sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq

I get 1.8 - 2.7 GHz. It never goes above 2.7.

And when I do

watch -n1 "cat /proc/cpuinfo | grep MHz"

I get 768 MHz - 1.8 GHz. It never goes above 1.8.

Anyone know what is going on?

Answer 1

Most CPU's now include the ability to adjust their speed to help in saving on battery/power usage. It's typically called CPU frequency scaling. The realtime speed of the CPU is reported by this:

$ sudo cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq

The absolute (max) CPU speed is being reported by this:

$ cat /proc/cpuinfo

Specifically this line:

model name  : Intel(R) Core(TM) i5 CPU       M 560  @ 2.67GHz

The line that shows cpu MHz doesn't show the maximum speed of your CPU. This value is your current speed. On a multi-core system such as an i7 or i5 you can see this with this command:

$ cat /proc/cpuinfo |grep MHz
cpu MHz     : 1199.000
cpu MHz     : 1199.000
cpu MHz     : 1199.000
cpu MHz     : 2667.000

You can however see the absolute (max) speed with this command:

$ lscpu 
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
CPU(s):                4
Thread(s) per core:    2
Core(s) per socket:    2
CPU socket(s):         1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 37
Stepping:              5
CPU MHz:               2667.000
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              3072K
NUMA node0 CPU(s):     0-3

NOTE: the number of cores that it has, NUMAS node0 CPU(s) is 4, i.e. 0,1,2, and 3.

CPU scaling & governoring?

The mode your system is in is called the scaling governor. Similar to a governor on a car. You can see which ones are available with this command:

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
powersave ondemand userspace performance 

You can also see which one is currently active:

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
ondemand

NOTE: The commands I'm showing only include the 1st cpu, cpu0. You can either substitute in a * in the path to see all the cores or you can selectively see cpu1, etc.

You can see the maximum & minimum CPU speeds available for your governor's profile:

$ sudo cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
2667000
$ sudo cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freq 
1199000

More details are available in this article, titled: CPU frequency scaling in Linux with cpufreq.

So what about cpuinfo_cur_freq?

This parameter has more to do with the specification of the CPU and which profile it's currently in, rather than anything useful with respect to how the CPU is currently operating. For actual operational telemetry I'd use the scaling_* kernel tunables.

Example

I put the following script together to show the CPU Cores column-wise so it would be easier to see what the various Kernel tunables looked like:

#!/bin/bash

nthCore=$(lscpu|grep node0|cut -d"-" -f2)

for i in /sys/devices/system/cpu/cpu0/cpufreq/{cpuinfo,scaling}_*; do
  pname=$(basename $i)
  [[ "$pname" == *available* ]] || [[ "$pname" == *transition* ]] || \
  [[ "$pname" == *driver* ]]    || [[ "$pname" == *setspeed* ]] && continue
  echo "$pname: "
  for j in `seq 0 $nthCore`;do
    kparam=$(echo $i | sed "s/cpu0/cpu$j/")
    sudo cat $kparam
  done
done | paste - - - - - | column -t

When you run it you get the following output:

$ ./cpuinfo.bash
cpuinfo_cur_freq:  2667000   2667000   2667000   2667000
cpuinfo_max_freq:  2667000   2667000   2667000   2667000
cpuinfo_min_freq:  1199000   1199000   1199000   1199000
scaling_cur_freq:  2667000   2266000   1333000   2667000
scaling_governor:  ondemand  ondemand  ondemand  ondemand
scaling_max_freq:  2667000   2667000   2667000   2667000
scaling_min_freq:  1199000   1199000   1199000   1199000

You can see that the scaling_cur_freq tunable is showing a slowdown in core # 1 & 2.