I am working on a Dell Latitude E4300 notebook with an Intel Core 2 Duo P9400 processor. I am using rdtsc for time measurement quite a long time and aside from the beginning of the "speed step" technology this worked fine. Unfortunately this is no longer the case with my new notebook, where the incrementation of the TSC decreases as soon as speed stepping is enabled. It shows the full 2,4 GHz when under load but goes down to about 250 Mhz when in idle. I bound my test-tool to one CPU and the value is not jumping around - so it is not a multiprocessor problem.
I already searched about 2 days about any sign that Intel (or the motherboard vendors) changed the strategy of the invariant TSC, but had no success. I have no explanation for this behavior.
According CPU specifications, there should not be any throttling mechanism, that affects TSC, besides changing the bus speed - which does not seem to happen...
Does anybody experience the same thing? Does anybody have a clue what could cause this?