Matthew Seaman m.seaman at infracaninophile.co.uk
Fri Apr 10 16:04:09 UTC 2009

gahn wrote:
> Hi all:
> I have a machine with 2GB memory. in the file loader.conf, there is a line:
> #hw.physmem="1G"                # Limit physical memory. See loader(8)
> Could I remove the "#" and change that to "2G"?
> Interestingly, the sysctl indicates the parameter of "hw.physmem" is not changeable.
> Any guru here give me some enlightenment?

hw.physmem is a loader tunable: ie. you can only set it from the boot
loader before the kernel is fully operational.  Once the kernel is running
it can't be altered.

hw.physmem is designed to let you test running a kernel with less RAM
than is physically installed in a machine.  Not having to pop the case
and physically pull memory sticks out can be pretty useful.  It's not of
general interest -- only for kernel and various other software developers
in the main -- as the usual thing is to make use of all the RAM you have
available or (in the case of 32bit machines) that the system is capable of

If hw.physmem is unset in loader.conf the kernel will automatically use all
the memory available to it: this is the correct and desirable behaviour for
the vast majority of systems.  For a 2GB machine, that means the system will
use all the RAM that's installed.



Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                  Kent, CT11 9PW

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