lkml.org 
[lkml]   [2001]   [Sep]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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

-
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/

\
 
 \ /
  Last update: 2005-03-22 13:03    [W:0.100 / U:2.560 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site