The following reply was made to PR i386/135447; it has been noted by GNATS.

From: Alexander Motin <m...@freebsd.org>
To: Andriy Gapon <a...@freebsd.org>
Cc: Dmitry Kubov <d...@garant.ru>, j...@freebsd.org, 
 bug-follo...@freebsd.org
Subject: Re: i386/135447: [i386] [request] Intel Core i7 and Nehalem-EP new
 features not supported
Date: Mon, 20 Sep 2010 18:42:57 +0300

 Andriy Gapon wrote:
 > on 20/09/2010 17:54 Dmitry Kubov said the following:
 >> dev.cpu.7.cx_supported: C1/3 C2/205 C3/245
 > Note these------------------------^^^----^^^
 >> dev.cpu.7.cx_lowest: C3
 >> dev.cpu.7.cx_usage: 100.00% 0.00% 0.00% last 500us
 > And this --------------------------------------^^^^^
 >> C2/C3 not used at all
 > 
 > 205 * 3 and 245 * 3 are both greater than 500, so this is the reason why 
 > they are
 > never entered.
 
 The only way to enter C-states with so high latency is significantly
 increase CPUs' continuous sleep time. Sleep time of 500ms there is
 artificial and calculated as 1000000/(2*hz). 8.1 was unable yet to
 measure real sleep time in C1. But 2*hz is quite realistic estimation
 for idle system.
 
 Recently I have committed to 9-CURRENT large set of patches, making idle
 CPUs to not wake up on timer interrupts when it is not needed. It allows
 idle CPUs sleep up to as much as 100000us, making any C-states available
 now effectively usable. I can acknowledge that TurboBoost on my Core i7
 870 gives about 10% benefit when only one physical core is used:
 http://docs.freebsd.org/cgi/mid.cgi?4C959830.3060808
 
 I have requests and wish to merge these changes into 8-STABLE, but most
 likely it won't happen in nearest few months, as code is very new and
 requires more testing.
 
 Until that time I recommend you to follow this guide:
 http://wiki.freebsd.org/TuningPowerConsumption
 It was actually oriented on laptops, but effective usage of C2/C3 states
  was one of it's goals. Also on my Core i7 870 LAPIC dies in C2/C3
 states, so consider migration to i8254 timer, as also described in this
 guide.
 
 -- 
 Alexander Motin
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