Comments on: Linux CPU Speed Reporting

By: EdLesMann

EdLesMann — Wed, 16 Apr 2008 18:45:39 +0000

Thanks!
I had noticed a few days ago that my X2 4200+ was shifting between 1.8Ghz and 2.2Ghz in /proc/cpuinfo. I was wondering why it did that.. now I know!
Thanks again!

By: Scott Alan Miller

Scott Alan Miller — Thu, 20 Mar 2008 12:19:10 +0000

To actually test you CPU speed you will need a dedicated utility for that. I don’t believe that there is any native Linux utility (at least not in Red Hat and SUSE that I use on my servers) that will definitely get you the resultant speed on an overclocked CPU.

If your PowerNow is turned on, then the CPU will get clocked down and won’t run at the overclocked speed – at least not all of the time. You can force your CPU higher by changing the Front Side Bus (FSB) frequency but voltage and stepping will get overridden by the PowerNow controls. So once they kick in it all goes haywire.

You can disable the CPU speed changes to guarantee that you get the fastest possible speed but this will result in a lot of power consumption and heat generation. You will need to disable Cool’n’Quiet in the BIOS and stop running processes like “cpuspeed”.

By: Tony

Tony — Wed, 19 Mar 2008 12:13:41 +0000

Excellent and timely article (at least for me, I was just searching for this type of info). One issue I’m seeing, though, is getting a proper reading of current cpu speed in an overclocked scenario. I have an AMD Athlon 64 X2 4200+, which is a dual core 2.2GHz. I’ve pushed it to 2.4GHz (2398.33MHz, to be exact). Here is the output of the above commands for my system:

# dmidecode | grep “Current Speed” | head -n 1
Current Speed: 2400 MHz
# dmidecode | grep “Max Speed” | head -n 1
Max Speed: 3000 MHz
# grep MHz /proc/cpuinfo | head -n 1
cpu MHz : 1000.000
(pegged the cpu, then ran again:)
# grep MHz /proc/cpuinfo | head -n 1
cpu MHz : 2200.000
# dmesg | grep “MHz processor”
time.c: Detected 2398.330 MHz processor.
# dmesg | grep “powernow” | head -n 6
powernow-k8: Found 1 AMD Athlon(tm) 64 X2 Dual Core Processor 4200+ processors (2 cpu cores) (version 2.00.00)
powernow-k8: 0 : fid 0xe (2200 MHz), vid 0x8
powernow-k8: 1 : fid 0xc (2000 MHz), vid 0xa
powernow-k8: 2 : fid 0xa (1800 MHz), vid 0xc
powernow-k8: 3 : fid 0x2 (1000 MHz), vid 0x12

So I’m wondering, how can I tell my actual current cpu speed, given that it’s overclocked. dmidecode reports it, but only shows the static top value, not current speed as shifted by the powernow code. Any ideas? Thanks!