Messages in this thread | | | Date | Thu, 8 May 2008 12:59:33 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: clocksources: order of preference |
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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
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