Re: Thermal scheduling
Re: Thermal scheduling
- Subject: Re: Thermal scheduling
- From: John Siracusa <email@hidden>
- Date: Wed, 29 Sep 2004 11:00:52 -0400
On Wed, 29 Sep 2004 16:27:38 +0200, Markus Hitter <email@hidden> wrote:
> Am 29.09.2004 um 15:54 schrieb John Siracusa:
>
> > One theory presented to explain the results was that the kernel is
> > taking CPU temperature into account when doling out CPU cycles.
>
> Well, I'm not really a kernel expert, but the scheduler's task is to
> decide which process/thread gets the next CPU quantum, where a CPU
> quantum is about 10 ms. If there's no process in need of processing
> time, it sends the CPU into doze mode.
The observed result of the test program is that it runs at X iterations/sec
for some time, then drops to X - N iterations/sec for the remainder. The
drop seems to coincide with a CPU temp that passes some threshold.
This drop is not seen on any Macs other than the dual 2.5GHz Power Mac. The
concern is that people buying a 2.5GHz Mac (at great expense) cannot use the
full capacity of the CPUs for more than a short period of time.
> > I thought it might be the CPU hog "punisher" in the kernel
>
> Surely, there are more mechanisms going on in the kernel, independent
> from the scheduler. If the CPU supports running at different speeds,
> you should be able to switch between them without telling the
> scheduler.
>
> This is how I understand things but again, I'm no expert :-)
The goal is not to throttle the CPU, but to make it run a particular task
using all available CPU resources. Obviously there is some overhead for the
OS, etc., but the one-time drop-off in speed on the 2.5GHz Power Mac (and
nowhere else when running the same code, same OS, etc.) has led some people
to believe that the issue is CPU heat, and that the OS (or maybe even the
hardware itself) is compensating for it by slowing down slightly.
-John
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