Re: Cost of an IRQ

MOLNAR Ingo (mingo@chiara.csoma.elte.hu)
Sat, 6 Feb 1999 20:51:14 +0100 (CET)

the cost is different on SMP and UP. On SMP the full 'user-visible' cost
of handling an interrupt is around 4-5 usecs. (pretty much stable across
CPUs and boards) On UP it's 8-10 usecs. I'd do the IRQ thing because even
if it turns out to be marginally slower, it's a more robust design and IRQ
latency is expected to get better and better with future boards. (and also
we are pretty much close to using the APIC for UP too, as we already do
for the Visual Workstation)

-- mingo

On Sat, 6 Feb 1999, Jan Kasprzak wrote:

>
> Hello,
>
> how expensive is to return from an IRQ handler, obtain another
> IRQ (in my case coming from the ISA bus) and enter the IRQ handler again?
>
> I need this for the COSA driver - COSA can tell me if it is able
> to transfer another packet, so I can rerun another iteration of the
> IRQ handler and save one IRQ (as described above), but I need to wait for
> ~ 5-10 usecs in IRQ handler for it. Is it better to do udelay(10)
> in IRQ handler, or to return from the handler and wait for another IRQ?
>
> The COSA card is primarily dedicated for routers, so it doesn't matter
> that in those 10 usecs CPU can do something other.
>
> -Yenya
>
> --
> \ Jan "Yenya" Kasprzak <kas at fi.muni.cz> http://www.fi.muni.cz/~kas/
> \\ PGP: finger kas at aisa.fi.muni.cz 0D99A7FB206605D7 8B35FCDE05B18A5E //
> \\\ Czech Linux Homepage: http://www.linux.cz/ ///
> /// Can you say "ignored email" three times quickly while chewing \\\
> // on an apple? --Linus Torvalds \\
>
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