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From: John Baldwin <jhb@freebsd.org>
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Date: Fri, 4 Mar 2011 14:17:44 -0500
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	(bigwig.baldwin.cx); Fri, 04 Mar 2011 14:17:46 -0500 (EST)
Cc: Dmitry Krivenok <krivenok.dmitry@gmail.com>, Andriy Gapon <avg@freebsd.org>
Subject: Re: hw.physmem (loader.conf and sysctl)
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On Friday, March 04, 2011 12:48:55 pm Andriy Gapon wrote:
> on 04/03/2011 16:36 Dmitry Krivenok said the following:
> > Hello Hackers,
> > I've limited the amount of physical memory visible for my FreeBSD-8.2 by 
adding
> > the following in loader.conf:
> > 
> > $ cat /boot/loader.conf | grep hw.physmem
> > hw.physmem="500M"
> > $
> > 
> > However, according to sysctl, the system sees
> > 
> > $ sysctl hw.physmem
> > hw.physmem: 507445248
> > $
> > 
> > The difference is (500 * 2**20 - 507445248) / 2**20 == 16.0625 Mb.
> > How does the system use this "hidden" memory?
> 
> Some memory is taken by structures that describe usable pages.
> There is one vm_page_t structure per each 4KB page.
> I believe that that memory is excluded from physmem.

Also, the message buffer for dmesg, and the kernel binary itself.

-- 
John Baldwin