Re: Memory mapped IO vs Port IO

[Jesse Barnes]

On Fri, Sep 12, 2003 at 08:00:03PM +0200, Andi Kleen wrote:
> jbarnes@xxxxxxx (Jesse Barnes) writes:
> 
> > Ok, Andi asked for benchmarks, so I ran some.  Let this should be a
> > lesson on why you shouldn't use port I/O :)  I ran these on an SGI Altix
> > w/900 MHz McKinley processors.
> > 
> > Just straight calls to the routines (all of these are based on the
> > average of 100 iterations):
> >   writeq(val, reg) time: 64 cycles
> >   outl(val, reg) time: 2126 cycles
>                          ^^^^^
> > 
> > A simple branch:
> >   if (use_mmio)
> > 	writeq(val, reg) time: 132 cycles
> >   else
> > 	outl(val, reg) time: 1990 cycles
>                              ^^^^^
> Something seems to be wrong in your numbers.
> 
> Surely the outl in the if () cannot be faster than the pure outl() ?

Well, they're approximate at best...  For outl the branch doesn't seem
to matter since it takes forever regardless.

Jesse
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