Date: Thu, 14 Sep 2006 09:55:09 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: RJ45 <rj45@slacknet.com> Cc: freebsd-questions@freebsd.org Subject: Re: memory problem Message-ID: <450918ED.6030206@infracaninophile.co.uk> In-Reply-To: <Pine.LNX.4.61.0609140220420.18820@slacknet.com> References: <Pine.LNX.4.61.0609140220420.18820@slacknet.com>
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RJ45 wrote:
> THe system has exactly 4GB of memory but the memory is not complitely
> seen by the system.
>=20
> At boot thime I Get this warning
>=20
> 524288Kb of memory above 4GB ignored
A normal 32bit OS can only address 4GB RAM -- but your system has various=
L2 and other caches built into the CPUs, etc., which count towards the to=
tal
RAM count. So the excess is trimmed from the main memory.
If you need to use more than 4GB RAM then either switch to a 64Bit OS, or=
=20
investigate 'PAE'.
You can run the 64bit version of FreeBSD on Intel Xeons (so long as they
support EMT64) or the various AMD 64 bit processors. Xeons don't have as=
complete 64-bitness as Opterons so performance may not be as good as runn=
ing
32bit. Mind you that sort of thing depends heavily on the particular
workload and you should benchmark against your expected workloads.
PAE 'Page Alternate Extensions' is frankly a bit of a haque to allow acce=
ss to
more than 4GB RAM by giving each process it's own separate 4GB address sp=
ace,
rather than sharing the space between all processes. Any one process can=
not
grow beyond 4GB, but the total over all processes can be more than 4GB. =
There
is support in FreeBSD but with some severe limitations. Many drivers are=
not
compatible with a PAE system.
> and then if I check
>=20
> real memory =3D 3757965312 (3583 MB)
> avail memory =3D 3678597120 (3508 MB)
>=20
> I do not know why this happens.
That number is the amount of memory less what is wired down for the kerne=
l.
If you're on a 'big' system -- with lots of RAM -- then the kernel itself=
has to be larger because it needs to allocate memory to contain page mapp=
ings
etc. etc. Approximately 500MB consumed by the kernel is not unreasonable =
for
such a machine.
Cheers,
Matthew
--=20
Dr Matthew J Seaman MA, D.Phil. Flat 3
7 Priory Courtyard
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
Kent, CT11 9PW, UK
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