On Wed, Sep 08, 2004 at 08:09:57PM +1000, Con Kolivas wrote:
> Andreas Mohr wrote:
> >Hi,
> >
> >On Wed, Sep 08, 2004 at 07:21:06PM +1000, Con Kolivas wrote:
> >
> >>Andreas Mohr wrote:
> >>
> >>>Some more explanation:
> >>>AFAIK unlikely() should only be used when you're damn sure that this will
> >>>NEVER occur (e.g. failure cases), since it will artificially aggravate
> >>>penalty of this check in case it does occur.
> >>>IRQ sharing is *much* too common to justify using unlikely() here.
> >>>*ESPECIALLY* since IRQ sharing is very latency critical!
> >>
> >>The magnitude of it's effect is overstated somewhat. Firstly we're
> >>talking about a delay in the nanosecond range, and second the cpu will
> >>optimise that out if it finds it's a frequently used path. Despite all
> >>that, however, I agree it's probably inappropriate. {un}likely() should
> >>be used for a path that is always/never traversed except in fault
> >>handling, jiffy wrap etc.
> >
> >True, it shouldn't make too much of a difference.
> >
> >BTW, I just checked linux-2.6.8.1/sound/pci/intel8x0.c ,
> >and to me it seems it doesn't take IRQ sharing into account at all.
> >
> >Would it be useful to carefully optimize many IRQ handlers for IRQ sharing
> >and send you a patch for inclusion in -ck?
>
> If you could demonstrate some objective benefit I think akpm would even
> be interested in putting such a thing in -mm.
hmm, maybe the fact that some laptop manufacturers think
that a single IRQ line is more than enough for those
few devices (nic, audio, usb, modem, 1394, and ACPI)
might be a considerable reason ...
(I might be completely wrong here, though ...)
best,
Herbert
> Cheers,
> Con
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