Date: Thu, 20 Oct 2005 09:59:19 -0400 From: John Baldwin <jhb@freebsd.org> To: Bruce Evans <bde@zeta.org.au> 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: <200510200959.21234.jhb@freebsd.org> In-Reply-To: <20051020154709.U99720@delplex.bde.org> References: <200510172310.j9HNAVPL013057@repoman.freebsd.org> <200510181101.03956.jhb@freebsd.org> <20051020154709.U99720@delplex.bde.org>
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On Thursday 20 October 2005 01:53 am, Bruce Evans wrote: > 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. Ah, I didn't realize the sysctl let you use negative quality timecounters. The tunable does serve to automate it for remote machines I guess since it doesn't pessimize the quality on SMP. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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