Comment 31 for bug 177646

p4-clockmod won't reduce clock, the CPU just stays idle during N out of M clock cycles. The power consumption during those cycles is just as if the CPU was normally idle, the clock modulation will just "force" it to be idle.

p4-clockmod is unlikely to reduce power consumption unless the CPU load is normally above the throttle (i.e. CPU load at 50%, throttle at 25%), that kind of cases are the ones where p4-clockmod would help the battery last longer, but it is unlikely in a current desktop system. The more common situation will be CPU load under 10%, while the minimum throttle is 12% (or higher, in case of some bugged CPUs which can't use the lowest throttles), so it wouldn't really help.

Furthermore, I think it is possible that using a clock governor other than "performance" (ie. ondemand) with p4-clockmod will still worsen the performance, if it makes the clock flutuate too much, making the CPU waste time switching the throttling state.