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Date: Tue, 17 Dec 2002 11:26:50 -0800
From: "Martin J. Bligh" <mbligh@aracnet.com>
To: "H. Peter Anvin" <hpa@transmeta.com>,
       Linus Torvalds <torvalds@transmeta.com>
cc: Dave Jones <davej@codemonkey.org.uk>, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org
Subject: Re: Intel P6 vs P7 system call performance
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>> 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.
> 
> The complexity only applies to nonsynchronized TSCs though, I would
> assume.  I believe x86-64 uses a vsyscall using the TSC when it can
> provide synchronized TSCs, and if it can't it puts a normal system call
> inside the vsyscall in question.

You can't use the TSC to do gettimeofday on boxes where they aren't 
syncronised anyway though. That's nothing to do with vsyscalls, you just
need a different time source (eg the legacy stuff or HPET/cyclone).

M.

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