Re: Memory mapped IO vs Port IO

[Andi Kleen <ak@suse.de>]

On Fri, 12 Sep 2003 11:24:30 -0700
jbarnes@sgi.com (Jesse Barnes) wrote:

> On Fri, Sep 12, 2003 at 11:11:48AM -0700, Tim Hockin wrote:
> > On Fri, Sep 12, 2003 at 08:00:03PM +0200, Andi Kleen wrote:
> > > jbarnes@sgi.com (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() ?
> > 
> > Also - a perhaps more useful test is a write followed by a read.
> 
> Well, someone else will have to run that test.  On Altix, a read() is
> freakishly expensive, and I'm not really interested in showing everyone
> how bad it is ;)

I guess the read will be very bad everywhere, and the PIO inl even worse.

-Andi