Date: Thu, 17 May 2001 22:22:13 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Mike Smith <msmith@FreeBSD.ORG> Cc: freebsd-questions@FreeBSD.ORG, arch@FreeBSD.ORG Subject: Re: Gettimeofday Again... Message-ID: <Pine.BSF.4.21.0105172158350.17368-100000@besplex.bde.org> In-Reply-To: <200105170813.f4H8DhE01424@mass.dis.org>
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On Thu, 17 May 2001, Mike Smith wrote: > > > I don't change the timercounter method defaults, and I sure hope you > > > aren't advocating that people change their timecounter defaults. If > > > the TSC is a reasonable default, the system should figure it out and > > > use it without requiring intervention. > > > > It's only a reasonable default if apm (or possibly acpica) is configured > > (and used). > > The TSC is never a reasonable default; there is no good way to be certain > that the TSC is and/or will remain stable. Even with ACPI, you can't be > entirely sure. This must be why Linux uses it by default ;-). See linux/arch/i386/config.in, option CONFIG_X86_TSC. Linux-2.4.1 still only uses it to give an offset from the last i8254 clock interrupt, like FreeBSD used to do 3+ years ago before timecounters. This may limit the errors from the TSC frequency changing to between -10 and 0 msec (hopefully the frequency is calibrated when it is as large as possible; then if it slows down down you underestimate the offset). Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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