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SubjectRe: interrupt latency
On Tue, Aug 27, 2002 at 04:44:34PM -0400, Richard B. Johnson wrote:
> On Tue, 27 Aug 2002 wrote:
> > On Tue, Aug 27, 2002 at 02:01:43PM -0400, Richard B. Johnson wrote:
> > > This cannot be. A stock kernel-2.4.18, running a 133 MHz AMD-SC520,
> > > (like a i586) with a 33 MHz bus, handles interrupts off IRQ7 (the lowest
> > > priority), from the 'printer port' at well over 75,000 per second without
> > > skipping a beat or missing an edge. This means that latency is at least
> > > as good as 1/57,000 sec = 0.013 microseconds.
> >
> > Assuming you mean 75,000 then ...
> > Thats 0.013 MILLISECONDS which is 13 microseconds and its not likely.
> Yes 13 microseconds.
> > I bet that your data source drops data or looks at some handshake
> > pins on the parallel connect.
> >
> No. You can easily read into memory 75,000 bytes per second from the
> parallel port, hell RS-232C will do 22,400++ bytes per second (224,000
> baud) on a Windows machine, done all the while to feed a PROM burner. I

You can do it in a tight loop. But you cannot do it otherwise. RS232 works
because most UARTs have fifo buffers. Old Windows did pretty well, because
you could grab the machine and let nothing else happen.

What makes me dubious about your claim is that it is easy to test
and see that a single ISA operation can take 18 microseconds
on most PC hardware.

read tsc
read tsc
compute difference
print worst case every 1000000 times.


run for an hour on a busy machine.

> On a faster machine, i.e., an ordinary 400 MHz Pentium, we have a
> complete Tomographic Imaging machine that gets triggers from the
> parallel port.
> Off the parallel port, hardware writes a byte and sets the interrupt
> line. There is no hand-shake with incoming data. It comes off a
> trigger board that will generate between 50 and 80 thousand
> triggers per second, depending upon some wheel speed. FYI, these
> triggers mark the position at which an X-Ray beam generated image
> data. If we missed a trigger, we end up losing a whole ray of
> image data, which would be a mess.
> Software reads then writes the byte into memory and executes
> wake_up_interruptible() for somebody sleeping in poll(). There is a
> fixed-length circular buffer with no dynamic allocation. This is
> the only thing that could possibly make it fast.
> At the same time, a high-speed data-link, interfacing to the PCI/Bus
> gets 2k of data per trigger so the machine is not exactly idle
> when the triggers are coming into the parallel port. Software
> correlates the trigger data with the image data as part of a
> tomographic reconstruction.
> That's 2048 * 80,000 = ~163 MB/s over the PCI with 80,000 b/s over
> the parallel port at the same time. We originally had a hardware
> "funnel" to combine 4 bytes before generating an interrupt. This
> turned out to be a synchronization nightmare and was scrapped once
> it was found that Linux interrupted fast enough.
> Make a simple module, create an ISR off the printer port, enable
> the printer port (hardware) interrupt line, then use a function
> generator to toggle the printer port interrupt line. You can then
> do all kinds of diagnostics to find out the max rate you can
> interrupt --and the maximum amount of code you can use in that
> ISR before you get overruns. This is what I did before I ever
> signed up to use a ^$@^$!(-@ printer-port for something important.
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
> The US military has given us many words, FUBAR, SNAFU, now ENRON.
> Yes, top management were graduates of West Point and Annapolis.

Victor Yodaiken
Finite State Machine Labs: The RTLinux Company.

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