Re: Intel P6 vs P7 system call performance

Ingo Molnar (mingo@elte.hu)
Sat, 21 Dec 2002 12:24:50 +0100 (CET)


On Tue, 17 Dec 2002, Linus Torvalds wrote:

> > How about this patch? Instead of making a per-cpu trampoline, write to
> > the msr during each context switch.
>
> I wanted to avoid slowing down the context switch, but I didn't actually
> time how much the MSR write hurts you (it needs to be conditional,
> though, I think).

this is the solution i took in the original vsyscall patches. The syscall
entry cost is at least one factor more important than the context-switch
cost. The MSR write was not all that slow when i measured it (it was on
the range of 20 cycles), and it's definitely something the chip makers
should keep fast.

Ingo

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