Re: Corrected gettimeofday() test code

From: Poul-Henning Kamp (phk_at_phk.freebsd.dk)
Date: 11/30/03

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    To: "Marc G. Fournier" <scrappy@hub.org>
Date: Sun, 30 Nov 2003 00:57:27 +0100

    In message <20031129191941.X99096@ganymede.hub.org>, "Marc G. Fournier" writes:

    Hmm, I'm not saying this makes sense, but at least there is
    an emergent pattern...

    Col#1 is duration of the shift, sorted in increasing order.
    Col#2 is the delta to the line above.
    Notice stratification on 50msec boundaries:

            695.426189 -
            695.426190 0.000001
            695.426191 0.000001
            695.426191 0.000000
            695.426192 0.000001

            695.476192 0.050000
            695.476193 0.000001
            695.476194 0.000001

            695.526193 0.049999
            695.526194 0.000001
            695.526196 0.000002
            695.526196 0.000000
            695.526198 0.000002
            695.526201 0.000003

            695.576193 0.049992
            695.576195 0.000002

            695.626195 0.050000

            695.676192 0.049997
            695.676195 0.000003

    The only place I can think of 50msec in relation to the timecounter
    is NTIMECOUNTER * 1/HZ.

    Try to modify some of that, and see if you can affect the results.

    In particular, try to yank NTIMECOUNTER _way_ up, like a thousand
    and see if the problem goes away.

    Another (uglier) option is that ACPI/SMI/APM or similar BIOS magic
    screws up the i8254 on a regular basis, or even that the latch
    functionality of the i8254 doesn't work properly. This is not
    unheard off. 

    -- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.
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