Re: a question about p4_clockmod module on Xeon quad core processors

[Wes Felter <wesley@felter.org>]

This will give you an effective speed of 250 MHz:

for cpu in 0 1 2 3; do
	cpufreq-set -c $cpu -g userspace
	cpufreq-set -c $cpu -u 2.0GHz
	wrmsr -p $cpu 0x19A 0x12
done

0x19A is the IA32_CLOCK_MODULATION MSR and 0x12 is calculated from the 
Intel 64 and IA-32 Architectures Software Developer’s Manual section 
13.5.3. Legal MSR values are 0x12 (12.5% duty cycle) - 0x1E (87.5%) or 
0x00 for 100%. I tested it on a quad-core Xeon and it works.

This is somewhat reckless, so use it with caution.

Wes Felter - wesley@felter.org