Date: Thu, 20 Oct 2005 15:53:15 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: John Baldwin <jhb@FreeBSD.org> Cc: cvs-src@FreeBSD.org, src-committers@FreeBSD.org, Andrew Gallatin <gallatin@cs.duke.edu>, cvs-all@FreeBSD.org, David Xu <davidxu@FreeBSD.org> Subject: Re: cvs commit: src/sys/amd64/amd64 cpu_switch.S machdep.c Message-ID: <20051020154709.U99720@delplex.bde.org> In-Reply-To: <200510181101.03956.jhb@freebsd.org> References: <200510172310.j9HNAVPL013057@repoman.freebsd.org> <20051018094402.A29138@grasshopper.cs.duke.edu> <200510181101.03956.jhb@freebsd.org>
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On Tue, 18 Oct 2005, John Baldwin wrote: > On Tuesday 18 October 2005 09:44 am, Andrew Gallatin wrote: >> It is a shame we can't find a way to use the TSC as a timecounter on >> SMP systems. It seems that about 40% of the context switch time is >> spent just waiting for the PIO read of the ACPI-fast or i8254 to >> return. > > You can try it by just setting the kern.timecounter.smp_tsc=1 tunable on boot. There is no need for this. Just set the timecounter using sysctl after booting (and quickly switch it back if it doesn't work). This tuneable, like most, shouldn't exist. It may be a relic from when the TSC wasn't put in the list of available timecounters in the SMP case. It is now put in the list with a negative "quaility", but the sysctl to set the timecounter correctly not restricted by the quality. Bruce
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