Re: [patch] context-switching overhead in X, ioport(), 2.6.8.1

[Andi Kleen <ak@muc.de>]

Ingo Molnar <mingo@elte.hu> writes:

> while debugging/improving scheduling latencies i got the following
> strange latency report from Lee Revell:
>
>   http://krustophenia.net/testresults.php?dataset=2.6.8.1-P6#/var/www/2.6.8.1-P6
>
> this trace shows a 120 usec latency caused by XFree86, on a 600 MHz x86
> system. Looking closer reveals:
>
>   00000002 0.006ms (+0.003ms): __switch_to (schedule)
>   00000002 0.088ms (+0.082ms): finish_task_switch (schedule)
>
> it took more than 80 usecs for XFree86 to do a context-switch!
>
> it turns out that the reason for this (massive) context-switching
> overhead is the following change in 2.6.8:
>
>       [PATCH] larger IO bitmaps
[...]

At least older XFree86 (4.2/3 time frame) used to only use iopl(). I
know it because at some point ioperm() was completely broken on
x86-64, but the X server never hit it. I wonder why they changed
that. Anyways, perhaps it would be better to just change the X server
back to use iopl(), because it will be always faster than using
ioperm.

-Andi