From owner-freebsd-current  Tue Apr  2 06:45:14 1996
Return-Path: owner-current
Received: (from root@localhost)
          by freefall.freebsd.org (8.7.3/8.7.3) id GAA29042
          for current-outgoing; Tue, 2 Apr 1996 06:45:14 -0800 (PST)
Received: from dyson.iquest.net (dyson.iquest.net [198.70.144.127])
          by freefall.freebsd.org (8.7.3/8.7.3) with ESMTP id GAA29037
          for <freebsd-current@freefall.freebsd.org>; Tue, 2 Apr 1996 06:45:11 -0800 (PST)
Received: (from root@localhost) by dyson.iquest.net (8.7.5/8.6.9) id JAA00430; Tue, 2 Apr 1996 09:41:15 -0500 (EST)
From: "John S. Dyson" <toor@dyson.iquest.net>
Message-Id: <199604021441.JAA00430@dyson.iquest.net>
Subject: Re: calcru: negative time:
To: bde@zeta.org.au (Bruce Evans)
Date: Tue, 2 Apr 1996 09:41:15 -0500 (EST)
Cc: bde@zeta.org.au, phk@critter.tfs.com, freebsd-current@freefall.freebsd.org,
        kuku@gilberto.physik.rwth-aachen.de
In-Reply-To: <199604021118.VAA10642@godzilla.zeta.org.au> from "Bruce Evans" at Apr 2, 96 09:18:01 pm
X-Mailer: ELM [version 2.4 PL24 ME8]
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: owner-current@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

> 
> >> >My i386 has really high numbers here.
> >> 
> >> Highly negative numbers?  They can't go more than 10000 usec negative
> >> on i386's due to the causes that I know about.
> 
> >Generally in the ~5000 range (+/-3000) and occasionally >>10000.
> 
> I think 5000 means that you missed a whole clock interrupt.  E.g.:
> 
> actual time	event		reported time	comments
> 0		Xintr0()
> 10		hardclock()
> 9000		splhigh()
> 10000		Xintr0()			sets ipending bit for intr0
> 19000		microtime()	19000		no problem yet
> 20000		Xintr0()			oops, missed a whole intr0
> 24000		microtime()	14000		oops
> 25000		splx()
> 25005		hardclock()
> 26000		microtime()	16000		oops
> 
> Another possibility is that TIMER0_LATCH_COUNT = 20 is too small.  Now
> I'm worried that it is too small if there are bus-hogging DMA
> controllers.  My U34F seems to cause a maximum latency of > 160 usec.
> siointr() can also cause a latency of a few hundred usec if there
> multiple active lines.  TIMER0_LATCH_COUNT should be smaller than 62.5
> for operation at 16Khz (used by pcaudio).  Perhaps the correct fix is
> to drop support for pcaudio.
> 
> >It seems to be connected to fork/exec on my mach.
> 
> It (actually failing of a more complete test in mi_switch()) seems
> to be connected with doing a bunch of disk accesses after the system
> has been idle for some time.  I suspect vm.  I didn't like changing
> splimp() in vm to splhigh().
> 
> I turned off my i586 clock fixup code in hardclock() and got the
> expected reports from test code in hardclock() about negative
> microtime deltas.  The worst case over a period of 8 hours was
> -37 usec.  This normally isn't a problem because microtime() calls
> are normally separated by more than 37 usec.
> 
> >How about ESDI disks, could they monopolize the cpu on slow systems ?
> 
> The could only monopolize the cpu, not the clock.  The worst case is if
> wdstrategy() is called at splhigh() (it shouldn't be) and writes 16 (?)
> 512-byte sectors in 2 ms to 4ms.  After doing the initial transfer it
> can only be called at splbio() when it can't hurt the clock.
> 
> Bruce
> 
>