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Date: Tue, 17 Dec 2002 11:11:19 -0800
From: "H. Peter Anvin" <hpa@transmeta.com>
Organization: Transmeta Corporation
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To: dean gaudet <dean-list-linux-kernel@arctic.org>
CC: Linus Torvalds <torvalds@transmeta.com>,
       Dave Jones <davej@codemonkey.org.uk>, Ingo Molnar <mingo@elte.hu>,
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Subject: Re: Intel P6 vs P7 system call performance
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dean gaudet wrote:
> On Mon, 16 Dec 2002, Linus Torvalds wrote:
> 
>>It's not as good as a pure user-mode solution using tsc could be, but
>>we've seen the kinds of complexities that has with multi-CPU systems, and
>>they are so painful that I suspect the sysenter approach is a lot more
>>palatable even if it doesn't allow for the absolute best theoretical
>>numbers.
> 
> don't many of the multi-CPU problems with tsc go away because you've got a
> per-cpu physical page for the vsyscall?
> 
> i.e. per-cpu tsc epoch and scaling can be set on that page.
> 
> the only trouble i know of is what happens when an interrupt occurs and
> the task is rescheduled on another cpu... in theory you could test %eip
> against 0xfffffxxx and "rollback" (or complete) any incomplete
> gettimeofday call prior to saving a task's state.  but i bet that test is
> undesirable on all interrupt paths.
> 

Exactly.  This is a real problem.

	-hpa

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