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

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 -
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