Date: Mon, 07 Sep 1998 08:37:07 +0200 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: Mike Smith <mike@smith.net.au> Cc: Thomas Dean <tomdean@ix.netcom.com>, current@FreeBSD.ORG Subject: Re: Dmesg Timecounter Varies Message-ID: <21872.905150227@critter.freebsd.dk> In-Reply-To: Your message of "Sun, 06 Sep 1998 14:37:47 PDT." <199809062137.OAA10133@word.smith.net.au>
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In message <199809062137.OAA10133@word.smith.net.au>, Mike Smith writes: >> I have noticed that the Timecounter value varies greatly in DMESG. >> >> I am running an SMP kernel. I extracted the values from >> /var/log/messages and passed them thru a stat process: >> >> Timecounter Cost Statistics >> Processing /var/log/messages and /var/log/messages.?.gz >> >> Frequency: 1193182 Hz - Always this value, unless stated. >> 1/freq Time: 838 nsec >> No. Cycles: 3 - floor( mean/(1/freqTime) ). >> Cycle Time: 948 nsec - mean/(no. cycles). >> Overhead: 332 nsec - Total overhead in Timecounter cost. >> >> Entries: 125 - Number of Timecounter entries in messages. >> Minimum: 2517 nsec >> Maximum: 41603 nsec - Wow! Flyer! >> Mean: 2846 nsec >> Std. Dev.: 3494.46 >> >> Notice the maximum flyer. What causes this variability? > >Unexpected execution discontinuities; most often SMI activity. We are >going to have to discontinue our (ab)use of the TSC simply because it's >not reliable enough. PC timekeeping *sucks*. Ahh, but mind you mike, this is >not< the TSC, this is the i8254 :-( -- Poul-Henning Kamp FreeBSD coreteam member phk@FreeBSD.ORG "Real hackers run -current on their laptop." "ttyv0" -- What UNIX calls a $20K state-of-the-art, 3D, hi-res color terminal To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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