TSC instead of ACPI: powerd doesn't work anymore
(to be expected?)
David O'Brien
obrien at freebsd.org
Sun Oct 30 23:58:52 PST 2005
On Sun, Oct 30, 2005 at 07:30:39AM -0700, Scott Long wrote:
> David O'Brien wrote:
> >On Sun, Oct 30, 2005 at 02:39:40PM +0800, David Xu wrote:
> >>David O'Brien wrote:
> >>>On Fri, Oct 28, 2005 at 03:34:57PM +0200, Alexander Leidinger wrote:
> >>>
> >>>>I don't have the message at hand. I just had time to write the mail,
> >>>>but I
> >>>>don't have my laptop with me to reproduce the message. But it's easy to
> >>>>reproduce, just take a PC which is able to make use of powerd and
> >>>>switch to
> >>>>using TSC as the timecounter.
> >>>
> >>>What is the motivation to use the TSC as a timecounter?
> >>
> >>TSC is faster than any others, on many systems, so-called ACPI-fast
> >>timer is really a slow chip,
> >
> >Correct, but why is it felt the latency of the ACPI timer is an issue?
> >Of course we all want things to as fast as possible, but is that just an
> >abstract desire, or a real issue was run into?
>
> ACPI-fast requires an ioport read which takes about 1us (according to
> Google). Do that 1000 times a second and you have each CPU spending
> 1% of its time doing nothing but reading the clock. Yikes.
But we've lived with using the ACPI timercounter (vs. TSC) for quite a
while now. Why all of a sudden are the authors of this thread having an
issue with it now. I know about the recent MySQL thread - but with the
TSC being untrustable on MP and power managed systems, why is there such
a desire to use the TSC?
--
-- David (obrien at FreeBSD.org)
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