From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Tue, 19 Apr 2005 11:35:42 -0700
From: Max Krasnyansky <maxk@domain.hid>
MIME-Version: 1.0
Subject: Re: [Adeos-main] Re: Interrupt Latency Question
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To: Paolo Mantegazza <mantegazza@domain.hid>
Cc: Fillod Stephane <stephane.fillod@domain.hid>, Wolfgang Grandegger <wolfgang.grandegger@domain.hid>, rtai@domain.hid, adeos-main@gna.org

Hi Paolo,

> If you have a multicpus machine and can reserve CPUs to real time only,
> than the picture will change a lot. No Linux activity on them, just your
> real time programs and irq handlers, likely stuck and fully cached to 
> those CPUs.
> 
> This is the solution you'll see native in Linux soon. With true lowcost 
> multicpus on a single chip massively available within a short time at 
> the kids' game and mama's word processors store it will change the whole 
> picture.

Actually this is kind of available right now with vanilla 2.6 kernel.
I'm talking about CPU reservation. Here is an example.
Let's say we have dual CPU box and we want to dedicate CPU 1 to
our application:
- Configure the kernel with following boot options:
	isolcpus=1 acpi_irq_nobalance noirqbalance

   This excludes CPU 1 from the scheduler balancing logic. And disables
   ACPI and SW irq balancing.
   Make sure that you don't run user-space IRQ balancer.

- Redirect all interrupts to CPU 0
   for i in /proc/irq/*; do
	echo 1 > $i/smp_affinity;
   done

- Your app call now migrate to CPU 1
   int cpu = 1;
   uint32_t mask = (1 << cpu);
   sched_setaffinity(0, sizeof(mask), (cpu_set_t *) &mask);

That's it. CPU 1 is yours. There will be almost zero activity on it, besides
your task of course.

Max