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Date:      Thu, 15 Jan 2009 13:31:49 -0600
From:      Dan Nelson <dnelson@allantgroup.com>
To:        Michael Powell <nightrecon@verizon.net>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: kernel configuration
Message-ID:  <20090115193147.GA61100@dan.emsphone.com>
In-Reply-To: <gknt72$39i$1@ger.gmane.org>
References:  <496E06D1.2070706@gmail.com> <20090114181522.GB4487@aurora.oekb.co.at> <B4DD918A-6026-4206-9C14-70DCD0028CFB@mac.com> <gknt72$39i$1@ger.gmane.org>

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In the last episode (Jan 15), Michael Powell said:
> Chuck Swiger wrote:
> > While it is true that you can comment out all but i686 and get a
> > working kernel, you will experience reduced performance.  There are
> > a number of low-level assembly routines (cf sys/i386/i386/support.s
> > such as i586_bcopy) that are conditionalized off of I586_CPU only,
> > even though they provide an advantage on i686 platforms also.
> 
> Thank you very kindly for this info tidbit. In my ignorance I had
> routinely built my kernels with only I686_CPU. Since the few boxen I
> have are all downlevel every niche I can squeak out some performance
> is sought after. Thanks indeed!

Actually, those functions are only enabled if the CPU is truly a
586-class processor.  See /sys/i386/isa/npx.c , the npx_attach()
function.  There is a test for cpu_class==CPUCLASS_586, while most
modern CPUs are CPUCLASS_686.

-- 
	Dan Nelson
	dnelson@allantgroup.com



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