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From: Robert Watson <rwatson@FreeBSD.org>
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To: Poul-Henning Kamp <phk@phk.freebsd.dk>
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Cc: cvs-src@FreeBSD.org, src-committers@FreeBSD.org, cvs-all@FreeBSD.org
Subject: Re: cvs commit: src/sys/sys time.h src/sys/kern kern_time.c 
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On Sun, 27 Nov 2005, Poul-Henning Kamp wrote:

> In message <200511270055.jAR0tIkF032480@repoman.freebsd.org>, Robert Watson writes:
>
>>  This offers a minimum update rate of 1/HZ,
>>  but in practice will often be more frequent due to the frequency of
>>  time stamping in the kernel:
>
> Not quite...
>
> The precision is guaranteed to be no worse than 1msec, and is unlikely 
> to be significantly better.

Sadly, for some workloads it will be -- we update the cached time stamp 
for every kernel context switch, so workloads that trigger lots of context 
switches will also trigger time stamp updates.  I.e., loopback network 
traffic due to the netisr and user space context switches, high in-bound 
network triffic due to ithread and netisr context switches, etc.

Robert N M Watson