Re: clocksources: order of preference

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 8 May 2008, Philipp Kohlbecher wrote:
> Why is the TSC preferred to the HPET as a clocksource for the x86
> architecture?

Performance. TSC access is extremly fast as it is a per CPU
register. HPET is a chipset device and scales bad when multiple CPUs
try to access it simultanously as the access is serialized in
hardware. Even on a UP system the access overhead is somewhere in the
range of factor 100.

> "Understanding the Linux Kernel" states that the HPET is preferable to the TSC
> due to its richer architecture. Up to version 2.6.17.14,
> arch/i386/kernel/timers/timer.c also contained a comment to that effect and
> accordingly ranked the HPET before the TSC.
> 
> This was changed when the new clocksource infrastructure was introduced with
> version 2.6.18. (The HPET clocksource received a rating of 250; the TSC, 300.)

We always tried to use TSC as the first choice.
 
> Preferring the TSC leads to problems when it is unstable. While this can be
> prevented by setting CONFIG_X86_TSC, certain distribution kernels (striving
> for compatibility) don't, resulting in soft lockups.

No, we only use the TSC, when:

- the TSC is known to be stable (not affected by CPU frequency changes)
- the TSC is sychronized accross CPUs

We also check the TSC with a watchdog mechanism, which verifies that
is is keeping accurate time. When we detect that TSC does not, we
replace it by the next available clock source.

> Are there better reasons to prefer the TSC or may I submit a patch that swaps
> the respective ratings?

You may submit one, but it's very unlikely that is gets applied. :)

Thanks,
	tglx