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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: New nanosecond stat patch for 2.5.44
Date: 30 Oct 2002 16:34:42 -0800
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References: <20021030004457.GC22170@bjl1.asuk.net> <Pine.LNX.3.96.1021030155404.14229A-100000@gatekeeper.tmr.com> <20021030221724.GA25231@bjl1.asuk.net>
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Followup to:  <20021030221724.GA25231@bjl1.asuk.net>
By author:    Jamie Lokier <lk@tantalophile.demon.co.uk>
In newsgroup: linux.dev.kernel
> 
> That's some of the overhead.  The other overhead is reading the clock,
> which is quite high on x86 when TSC is not available.  On a Pentium
> with no reliable TSC, I think that the time for a read() system call
> is comparable to the time to read the clock.
> 

Typically the way you deal with not having a usably cheap
nanosecond-resolution clock is that you use the best available clock
(say if HZ=1000 you'll increment by 1000000 each timer tick), and then
simply use an atomic counter for the smaller divisions.  This makes
the relation "is A newer than B" correct, while avoiding the overhead
of producing exact timestamps below the available resolution.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>
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