Date: Tue, 2 Apr 1996 09:41:15 -0500 (EST) From: "John S. Dyson" <toor@dyson.iquest.net> To: bde@zeta.org.au (Bruce Evans) Cc: bde@zeta.org.au, phk@critter.tfs.com, freebsd-current@freefall.freebsd.org, kuku@gilberto.physik.rwth-aachen.de Subject: Re: calcru: negative time: Message-ID: <199604021441.JAA00430@dyson.iquest.net> In-Reply-To: <199604021118.VAA10642@godzilla.zeta.org.au> from "Bruce Evans" at Apr 2, 96 09:18:01 pm
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> > >> >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 > >
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