Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I am running CentOS 5.3 as a VirtualBox 3.0.0 guest running on Windows XP.

The Windows host CPU usage is constantly at 50% although the CentOS guest is completely idle (i.e. 0.00 load average).

I know this is a common problem related to the 1000Hz frequency that the CentOS kernel runs at, and previously a special kernel-vm packages were released to resolve the issue. However, these packages are out of date and the README says that they are not not necessary as of CentOS 5.3. I found out that there is supposedly a kernel parameter divider=10 that reduces the frequency to 100Hz with the standard kernel but it doesn't seem to have any effect when running inside VirtualBox.

Is there any way to resolve the issue without resorting to a custom kernel?

share|improve this question

The proper syntax for the divider parameter is:

RHEL-5 32 bits: "divider=10 clocksource=acpi_pm"

RHEL-5 64 bits: "notsc divider=10"

Had you already tried these with this syntax or simply divider=10?

share|improve this answer
I am running CentOS 32bit and I used the "divider=10 clocksource=acpi_pm" line. Here is my GRUB configuration: title CentOS (2.6.18-128.1.16.el5) root (hd0,0) kernel /boot/vmlinuz-2.6.18-128.1.16.el5 ro root=LABEL=/ divider=10 clocksource=acpi_pm initrd /boot/initrd-2.6.18-128.1.16.el5.img But still, when I boot with this configuration the host CPU usage is always at 50%. – sagi Jul 5 '09 at 14:28

Have you tried to install the guest extensions? Although not directly related to the clock frequency issue you describe, it helped a lot for me to reduce CPU load due to paravirtualized drivers.

share|improve this answer
That definently helps a lot ! – Antoine Benkemoun Jul 6 '09 at 6:33
I installed the guest extensions; it made no difference. The guest extensions mainly improve GUI performance and I don't run X on this VM. – sagi Jul 6 '09 at 15:19

it is sufficient to modify the startup parameters in the bootloader (grub) by adding "divider=10" at the end of the menu entry.

It will therefore read:

title CentOS (2.6.18-164.6.1.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-164.6.1.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet divider=10
        initrd /initrd-2.6.18-164.6.1.el5.img

This solves the issue


share|improve this answer

I applied Stephano's recipe on CentOS 5.0 - 32 bits, CPU use fall from 80% to less than 20% after menu.lst modification (i just added "divider=10", nothing else...). Thanks...

share|improve this answer

Have you tried to start a second virtual box instance, and run it at the same time, as described here:

share|improve this answer
Good link but boy does that make me think VirtualBox is rubbish! – Chopper3 Mar 17 '11 at 14:38

Check out:

Hopefully it will help (please let us know if you try).

share|improve this answer

Maybe this will help others; it's rather simple and avoids having to manually recompile the kernel:


share|improve this answer

I am using guest CentOS 5.4 on Windows XP host and I was having the same problems untill I have found a solution here :

It works for me.

regards Olek olekg (at)

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.