Re: interrupt latency ?

From: Richard B. Johnson (
Date: Mon Feb 10 2003 - 13:30:49 EST

On Mon, 10 Feb 2003, [iso-8859-1] Jörn Engel wrote:

> On Mon, 10 February 2003 11:20:54 -0500, Richard B. Johnson wrote:
> > On Mon, 10 Feb 2003, [iso-8859-1] J?rn Engel wrote:
> >
> > > Write a simple handler for parport or so, that is called when line #1
> > > toggles from low to high and responds by pulling line #2 from low to
> > > high.
> > > Now hook up a signal generator and an oszilloscope and measure the
> > > time from signal generation to the physical reaction.
> >
> > Yes, and you will find that you can replicate a square-wave, through
> > the hardware and software up to about 50 kHz with a 400 MHz Pentium
> > if you disconnect your network during the tests.
> >
> > My tests, several years ago, in the ISR simply XOR-ed a saved
> > copy of bit zero with `1` to toggle it and wrote it out the
> > data port. This would occur at every IRQ7, generated by hitting
> > bit 2 of the control port with a function generator. This should
> > produce a symmetrical /2 when you look at bit 0. You can line up
> > the starting 'high' of the function generator, with either a high or
> > low of bit one (because it's /2) and measure the time, which from
> > my notebook looks like 1.2 to 1.4 microseconds on a 400MHz machine.
> >
> > You can increase the interrupt rate until the machine is no longer
> > able to keep up. This usually occurs around 110 kHz or higher.
> 1.2us translates to 800kHz or 500 clock cycles. That is a good
> response time.
> 110kHz max rate translates to 3500 clock cycles for the complete
> interrupt path. This means that the return path takes six times longer
> to complete than the initialisation path. Odd.
> Somehow, I get this feeling that linux could still do better. 500
> cycles is already better than the 2000 we observed, but most of that
> should come from the different architecture.
> An assembler interrupt handler that saves registers, tweaks a couple
> of bits, restores registers and gets the hell out of here should be in
> the order of 100 cycles, maybe less. Why is linux wasting all this
> time?
> Jörn

With any complete system, you need more than to save a few
registers. If you are to be able to use 'C' for the ISR,
you need to have a stack with the access registers (on ix86,
the segment registers) need to be initialized, the appropriate
interrupt context needs to occur, i.e., kernel mode. Further,
you are going to talk to the legacy interrupt controller(s) through
I/O ports that take about 300 ns per access. Also, to prevent
unexpected reentry some kernel code has to be executed and then
the module (driver) ISR called from kernel code. It is not something
as simple, or as broken as "void interrupt far ISR()" as DOS
thunkers are used to programming.

As previously stated, if you use IOAPIC and have a fast CPU, your
speed increases. All my work on characterizing the interrupt system
on Linux was based upon legacy I/O and a 400 MHz CPU. From that
work, I was able to find the major problems with so-called latency
and I was able to get a SC520 embedded system (33 MHz clock, 133 MHz
CPU core) to work with an average interrupt rate of over 15 kHz and
a peak exceeding 30 kHz. I did not have a disk-drive to worry about,
but I had Network I/O so I needed to modify the PCnet 32 driver
(AMD chip) so it didn't loop in the ISR. That fixed the only latency
problem I had.

FYI execution speed and interrupt latency doesn't scale well. You
are most always I/O bound somewhere.

Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.

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