Re: [PATCH 1/3] Separate IRQ-stacks from 4K-stacks option

[Bill Davidsen]

Lee Revell wrote:
> On Fri, 2004-09-10 at 11:34, Arjan van de Ven wrote:
>
> > On Fri, Sep 10, 2004 at 05:28:52PM +0200, Andrea Arcangeli wrote:
> >
> > > On Fri, Sep 10, 2004 at 05:15:38PM +0200, Arjan van de Ven wrote: 
> > >
> > > > What we should consider regardless is disable the nesting of irqs for
> > > > performance reasons but that's an independent matter
> > >
> > > disabling nesting completely sounds a bit too aggressive, but limiting
> > > the nesting is probably a good idea.
> >
> > disabling is actually not a bad idea; hard irq handlers run for a very short
> > time
>
>
> The glaring exception is the IDE io completion, which can run for 2000+
> usec even with a modern chipset and drive.  Here's a 600 usec trace:
>
> http://krustophenia.net/testresults.php?dataset=2.6.8-rc4-bk3-O7#/var/www/2.6.8-rc4-bk3-O7/ide_irq_latency_trace.txt
>
> The timer, RTC, and soundcard interrupts (among others) will not like
> being delayed this long.  Ingo mentioned that this was not always done
> in hardirq context; presumaby the I/O completion was done in a softirq
> like SCSI.  What was the motivation for moving such a long code path
> into the hard irq handler?

Certainly if you run ppp the serial port won't like being ignored that 
long, and if you pull down data on a parallel port that really won't 
like it. The soundcard is probably only a problem if you're recording 
input, in spite of some posts here about skipping, the world doesn't end 
if you get a skip, although 2ms shouldn't cause that anyway.

--
bill davidsen <davidsen@xxxxxxx>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979
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