Re: interrupt latency

[Mark Hounschell <markh@compro.net>]

"Wessler, Siegfried" wrote:
> 
> Hello,
> 
> I am running and will in near future kernel 2.4.18 on an embedded system.
> 
> I have to speed up interrupt latency and need to understand how in what
> timing tasklets are called and arbitraded.
> 
> I have to dig deep, but the kernel tree is quiet huge. As a non kernel
> programmer I ask you, if anyone could give me a hint, where to start reading
> from and which kernel source to pick first.
> 
> Any help highly appreaciated.
> (BTW: I will not bother you personaly with further questions unless you give
> permission.)
> 
> What's behind it: We patched NMI and do some stuff we have to do very
> regularly in there. After NMI we have to quiet fast start a kernel or even a
> user space function with low latency. Also I measured 8 milliseconds after a
> hardware interrupt before the corresponding interrupt function is called. At
> RTI time it is even longer (around 12 microseconds). Need to find a way to
> exactly understand why, and maybe speed up a bit.
> 
> Thank You.
> Siegfried.

I've found that with the combination of process affinity and irq affinity you
can get very good interrupt latency/determinism. We use a pci card that has
some external interrupts and some 250ns resolution timers and have found the
interrupt latency/determinism of the external interrupts to be more than 
exceptable as long as the process and irq of that pci card are forced
to one cpu and ALL other processes/irq's are forced to another cpu. Of coarse
you need an SMP box for best results. We found that with a UMP box you can
get the latency but there is no determinism to that latency.

Mark