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SubjectRe: is irq smp affinity good for anything?
On Tue, 11 Mar 2003 wrote:

> if IRQ affinity cannot help in interrupt latency and
> interrupt auto/balancing get's the same throughtput
> as irq binding... what is irq smp affinity for???
> from this mail from intel
> I understand that intel's interrupt auto/balancing works equal that manually
> tunning the interrupts....
> so?
> Ulisses
> On Mon, Mar 10, 2003 at 11:22:22PM +0100, wrote:
> >
> > Hi all
> >
> > I have measured interrupt latency of a bi-processor system with akpm's intlat/timepeg
> > utilities and kernel 2.4.20. The system uses is a two-way PIII@800Mhz
> > -- Intel motherboard, ISP 2100 if I remember ok, with a SCSI disk on a aic-7896
> >
> > I tried to know if I could reduce the latency of an interrupt handler by
> > binding this handler to a particular cpu and not allowing any other
> > interrupt to execute in that cpu.
> >
> > I found that overall latency increases, but also the latency on both cpus,
> > that is, I could not reduce the latency on the interrupt I was interested
> >
> > I also tried to disallowing just scsi & ethernet handlers to execute on the
> > cpu in wich I'm binding the interrupt handler I'm insterested with, I get
> > similar results
> >
> > the interrupt handler I'm interested with is an ATM card receiving
> > around 6000 interrupts/second
> >

33 MHz machines easily handle 6,000 interrupts per second --
unless you are trying to execute code within that interrupt
that requires 1/6000th of a second to execute! Perhaps it's
not a "latency" problem, but an interrupt code-bloat problem
where most of the stuff should be executed out of the interrupt

There are timer queues and kernel threads available that might
help you. Also a tightly-coupled user-mode daemon that uses
your driver only to interface with the hardware and not do
any "logic" is probably the way to go.

Dick Johnson
Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.

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