Re: Memory mapped IO vs Port IO

[jbarnes@sgi.com (Jesse Barnes)]

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

Using a wrapper to point to one of the routines:
  wrapper->write(val, reg) time: 215 cycles
  wrapper->out(val, reg) time: 3931 cycles

Thanks,
Jesse