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Date: Thu, 8 May 2008 12:59:33 +0200 (CEST) From: Thomas Gleixner <tglx@...utronix.de> To: Philipp Kohlbecher <xt28@....de> cc: linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com> Subject: Re: clocksources: order of preference 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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