Re: [PATCH] dynamic tick patch

[john stultz <johnstul@us.ibm.com>]

On Thu, 2005-01-20 at 00:26 +0100, Thomas Gleixner wrote:
> On Wed, 2005-01-19 at 14:59 -0800, George Anzinger wrote:
> > I don't think you will ever get good time if you EVER reprogramm the PIT.
> 
> Why not ? If you have a continous time source, which keeps track of
> "ticks" regardless the CPU state, why should PIT reprogramming be evil ?

That's a big if.  The problem is that while the PIT has its problems
(such as lost ticks), it runs at a known frequency and is reasonably
accurate. Time sources like the TSC have the problem that it doesn't run
at a known frequency, and thus we have to calibrate it (usually using
the PIT). Unfortunately this calibration is not extremely accurate
(George can go on to the reasons why), which causes the TSC to be a poor
stand alone time source.

That said, the PIT is a poor time source as well, as it does loose ticks
and is very slow to access. ACPI PM and HPET are better as they don't
have the lost tick problem, but they are still off chip and slower to
access then the TSC.

For an example of your ideal continuous timesource, check out the
timebase on PPC/PPC64. Other arches also have similar well behaved time
hardware. 

thanks
-john