If you are going to use CPU frequency scaling on P4 you will probably use p4-clockmod and get in trouble with ondemand governor like me. If you try to switch to the ondemand governor you can see following message in dmesg:
ondemand governor failed, too long transition latency of HW, fallback to performance governorSolution
Just change the following string in kernel sources arch/x86/kernel/cpu/cpufreq/p4-clockmod.c like here
old string
policy->cpuinfo.transition_latency = 1000000;new string
policy->cpuinfo.transition_latency = 10000001;