hw.physmem (loader.conf and sysctl)

[John Baldwin]

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