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	Sun, 8 Nov 1998 21:05:41 +1100
Date: Sun, 8 Nov 1998 21:05:41 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199811081005.VAA24481@godzilla.zeta.org.au>
To: bde@zeta.org.au, phk@critter.freebsd.dk
Subject: Re: Grrr... calcru: negative time blah blah blah
Cc: current@FreeBSD.ORG, wpaul@skynet.ctr.columbia.edu
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>>Timecounter strangeness seems to be only a symptom here.  I've seen
>>similar strangeness caused by ddb masking interrupts.  However,
>>{micro|nano}[run]time() only have obvious problems when tco_delta()
>>overflows a u_int.   Overflow occurs after only about 10 seconds for
>>a 400MHz tsc timecounter, but not for one hour for an i8254 timecounter.
>
>Uhm, it happens earlier for a i8254, in fact it happens whenever more
>than one interrupt is lost.  The majority of the bits are software
>bits.

That's neither (C) overflow nor in {micro|nano}[run]time().  When the
low-level i8254 timecounter is not called often enough, the timecounter
just drops some multiple of timer0_max_count (about 11932) timecounter
ticks.  This doesn't necessarily happen when more than one interrupt
is lost -- it happens when the low-level timecounter is not called for
more than (2 - epsilon) interrupt periods.  There must have been some
low-level timecounter calls for the observed timecounter deltas to be
more than (2 * timer0_max_count).

Bruce

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