memory problem

[Matthew Seaman]

RJ45 wrote:

> THe system has exactly 4GB of memory but the memory is not complitely
> seen by the system.
> 
> At boot thime I Get this warning
> 
> 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 total
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 
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 running
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 access to
more than 4GB RAM by giving each process it's own separate 4GB address space,
rather than sharing the space between all processes.  Any one process cannot
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
> 
> real memory  = 3757965312 (3583 MB)
> avail memory = 3678597120 (3508 MB)
> 
> I do not know why this happens.

That number is the amount of memory less what is wired down for the kernel.
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 mappings
etc. etc. Approximately 500MB consumed by the kernel is not unreasonable for
such a machine.

	Cheers,

	Matthew

-- 
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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