Messages in this thread |  | | Date | Thu, 27 Sep 2001 16:23:38 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: 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
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |