Date: Sun, 11 Feb 1996 14:20:42 +0100 From: Poul-Henning Kamp <phk@critter.tfs.com> To: Bruce Evans <bde@zeta.org.au> Cc: current@freebsd.org Subject: Re: calibrating clocks Message-ID: <6362.824044842@critter.tfs.com> In-Reply-To: Your message of "Sun, 11 Feb 1996 21:04:46 %2B1100." <199602111004.VAA04514@godzilla.zeta.org.au>
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> >Did you see the email about the "almost correct sub-tic" stunt ? > > I don't think it's OK. It would be too inaccurate to use in mi_switch(), > which seems to be the only place where its overhead is noticeable. OK. > The i586 clock is good enough for the sub-tick count. Even if it varies, > it can be rescaled often. The i8254 clock probably needs to be read on > i586's every clock tick to determine the latency of hardclock(). The > cost of this is < 5usec * 100 Hz = 500 usec/sec = 0.05%. You know about the counter in the APIC too ? It runs at the "bus frequency" but that may vary as well :-( -- Poul-Henning Kamp | phk@FreeBSD.ORG FreeBSD Core-team. http://www.freebsd.org/~phk | phk@login.dknet.dk Private mailbox. whois: [PHK] | phk@ref.tfs.com TRW Financial Systems, Inc. Future will arrive by its own means, progress not so.
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