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Date: Thu, 19 Dec 2002 14:23:26 -0800
From: "H. Peter Anvin" <hpa@transmeta.com>
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To: Pavel Machek <pavel@ucw.cz>
CC: dean gaudet <dean-list-linux-kernel@arctic.org>,
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       Dave Jones <davej@codemonkey.org.uk>, Ingo Molnar <mingo@elte.hu>,
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Subject: Re: Intel P6 vs P7 system call performance
References: <Pine.LNX.4.44.0212162204300.1800-100000@home.transmeta.com> <Pine.LNX.4.50.0212162241150.26163-100000@twinlark.arctic.org> <20021218235327.GC705@elf.ucw.cz> <3E0245C1.5060902@transmeta.com> <20021219222136.GC17941@atrey.karlin.mff.cuni.cz>
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Pavel Machek wrote:
> Hi!
> 
> 
>>>>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.
>>>
>>>Problem is that cpu's can randomly drift +/- 100 clocks or so... Not
>>>nice at all.
>>>
>>
>>???100 clocks is what... ?50 ns these days?  You can't get that kind of
>>accuracy for anything outside the CPU core anyway...
> 
> 50ns is bad enough when it makes your time go backwards.
> 

Backwards??  Clock spreading should make the rate change, but it should
never decrement.

	-hpa


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