2

I'm having the following problem with Kubuntu 15.10. Sometimes when I put my laptop(Dell e6420) to sleep and later wake it up, I see that CPU is running at really low frequency (~600Mhz)

This is normal behaviour, just after fresh boot.
(Can't post images, yet) https://i.stack.imgur.com/h7MWg.png

This is wrong behaviour, after waking up from sleep.
https://i.stack.imgur.com/xui51.png

The commands I execute to check CPU details are:

$ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver<br>
$ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor<br>
$ grep MHz /proc/cpuinfo<br>

This certainly is a bug. Do you know any possible solution to resolve this problem. I heard that it has something to do with intel_pstate driver.

TellMeWhy
  • 17,484
mattdrzazga
  • 21
  • 1
  • Are you sure that you ran the script with high CPU load? – meskobalazs Dec 15 '15 at 12:43
  • I forgot to add that I tried opening a few instances of a file manager to check how high frequency can go. After that I ran this script and took screenshots. – mattdrzazga Dec 15 '15 at 13:29
  • 1
    You really should try the application stress to hog one of your cores, then check the values. If it is still wrong, then you have a problem :) – meskobalazs Dec 15 '15 at 13:54

2 Answers2

1

The CPU does that by itself to save power. It's normal behaviour for any laptop running any OS. Take a look in BIOS and see if you can find something like "CPU Power saving" or something like that. It should do the trick.

TheUnkn0wn
  • 11
  • 1
  • This is not a normal behaviour because I am not able to use full power of my CPU after resuming from sleep. Cpu is stuck and I must reboot to restore system to normal. – mattdrzazga Dec 15 '15 at 13:32
1

Your problem is a combination of: A known BIOS problem with that Dell LapTop, resulting in clock modulation being enabled when you resume from sleep; A fundamental incompatibility between the current version of the intel_pstate CPU frequency driver and clock modulation. The result is that the requested CPU pstate will always be the minimum, regardless of CPU load. The actual resulting CPU frequency will be a little variable within that pstate * the clock modulation value.

To fix the problem you can turn off clock modulation. To more or less workaround the problem you can disable the intel_pstate driver and use the acpi-cpufreq CPU frequency driver instead.

Please see here and here for more details.

Community
  • 1
Doug Smythies
  • 15,448
  • 5
  • 44
  • 61