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Date: Sat, 21 Dec 2002 12:24:50 +0100 (CET)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: Brian Gerst <bgerst@didntduck.org>, Andi Kleen <ak@suse.de>,
       <linux-kernel@vger.kernel.org>, <davej@suse.de>
Subject: Re: Intel P6 vs P7 system call performance
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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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