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SubjectRe: CPU frequency shifting "problems"

On Thu, 27 Sep 2001, Padraig Brady wrote:
>
> >
> >For example, on a transmeta CPU, the TSC will run at a constant
> >"nominal" speed (the highest the CPU can go), although the real CPU
> >speed will depend on the load of the machine and temperature etc.
>
> As does the P4 from what I understand.

That might explain why the P4 "rdtsc" is so slow.

> So a question..
> What are the software dependencies on this auto/manual frequency shifting?

None. At least not as long as the CPU _does_ do it automatically, and the
TSC appears to run at a constant speed even if the CPU does not.

For example, the Intel "SpeedStep" CPU's are completely broken under
Linux, and real-time will advance at different speeds in DC and AC modes,
because Intel actually changes the frequency of the TSC _and_ they don't
document how to figure out that it changed.

With a CPU that does makes TSC appear constant-frequency, the fact that
the CPU itself can go faster/slower doesn't matter - from a kernel
perspective that's pretty much equivalent to the different speeds you get
from cache miss behaviour etc.

Linus

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