Re: TSC unstable on Intel Pentium M processor 750

[Robert Hancock]

Mao Yilu wrote:
> Hi,
>
> I masked all the interrupts except keyboard interrupt before the instruction âhltâ, So the CPU ran nothing until I click the keyboard button. And I used ârdtscllâ and âdo_gettimeofdayâ to get the halt time. But the result of âdo_gettimeofdayâ was longer than the one of ârdtscllâ about 3s per minute. I donât know why.
> I timed some math job using the same way in contrary to the âhltâ instruction. The TSC was not correct either. In 3 minutes, the result of âdo_gettimeofdayâ was longer than the one of ârdtscllâ about 5s.
> The processor is Intel Pentium M processor 750. From the software developerâs manual of Intel, the processor clock of Pentium M processors is impacted by Intel SpeedStep technology, while some other processors is not. Maybe this is another feature Pentium M processors own.
> Pentium M processors support 5 C-states. I am sure the TSC doesnât stop in C0 and C1 states. But I am not sure about other states. Maybe other states will stop the clock, and âhltâ instruction will make the CPU into deeper state.
> These are my guess. I wanna the truth. Thank you.
>
> Mao Yilu

On many processors the TSC will stop in various C-states, and also the 
TSC frequency changes when CPU frequency changes. This is why using 
rdtsc in userspace is not reliable, since the code can't know whether or 
not TSC can be used on that CPU reliably or how to scale the results.
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