debugging sluggish CPU frequency transitions

[Timothy Redmond]

I have a Thinkpad W520 computer (4270CTO) and I am running Fedora 20 and 
keeping its packages up to date.  Often when I am working the cpu 
frequency transitions beautifully and the computer is very responsive. 
I have run some programs that take a long time (hours to days) and have 
watched the frequency go up when there is only one thread running and go 
down when there are several threads running. So it seems to be working 
very nicely.

However sometimes the computer takes a very long time (20 seconds to -- 
I think -- 15 minutes or more) to react to a change in the load. I have 
run commands like "stress --cpu 1" or "stress --cpu 8" and watched i7z. 
  When the governor is powersave it may stay at 800MHz and when the 
governor is performance it may stay between 1000MHz to 1400MHz.  If I 
leave the stress running long enough it seems like eventually the 
frequency will go up to the expected values.  Once the CPU frequency 
goes up I can keep it up by leaving the stress running and my 
applications are responsive.  If I kill the stress process, the system 
may or may not respond quickly to changes in load.

There is some googleable discussion about the BIOS limiting the 
frequency for Thinkpads. However, the value in

      /sys/module/processor/parameters/ignore_ppc


does not seem to matter though and the file

      /sys/devices/system/cpu/cpu0/cpufreq/bios_limit


is never present.  Also the system will respond when I change the 
governor from performance to powersave and back which does not seem 
consistent with a limit provided by the BIOS.

Does anyone know what could be wrong or how I can debug this?

-Timothy



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