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Sender: Alexander Motin <mavbsd@gmail.com>
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Date: Mon, 05 Nov 2012 18:08:05 +0200
From: Alexander Motin <mav@FreeBSD.org>
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To: Joe Holden <lists@rewt.org.uk>, Davide Italiano <davide@freebsd.org>, 
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Subject: Re: Dynamic Ticks/HZ
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Hi.

Full interrupt rate on some CPU means that your system is not idle, but 
running some process. Another possibility is that you have DUMMYNET 
compiled into your kernel, which tends to schedule callout for every HZ 
tick, effectively blocking skipping interrupts for one of CPUs.

Check 'top -SH' for running processes, or 'top -SH -m io -o vcsw' for 
processes doing a lot of context switches. That most likely should give 
an answer.

-- 
Alexander Motin