Re: Memory mapped IO vs Port IO

[Andi Kleen]

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() ?

-Andi

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