On Mon, Aug 08, 2011 at 02:29:36PM +0300, Jukka Ruohonen wrote: > We should not really trust ACPI/FADT here. See acpicpu(4) how this is > derived from the actual CPU information. Additionally, I suggested decreasing > the quality of tsc(9) based on this information a long time ago, but joerg@ > had concerns about this.
The basic concern here is that TSC and LAPIC timer are an order of magnitude faster than the alternatives. IIRC TSC and LAPIC on my old Core 2 laptop needed around 100ns +- 20% and the HPET around 1200ns for gettimeofday (e.g. including system call overhead). I forgot whether the phase shift correction for LAPIC was ever implemented, but with it, there is a whole range of machines where LAPIC is stable and TSC isn't. More importantly, if the LAPIC is stopped, you are hosed anyway since it also drives hardclock. The performance difference is quite small though, so dropping TSC's quality below LAPIC wouldn't be too bad. Joerg
