4GB memory and more

Heiko Wundram (Beenic) wundram at beenic.net
Mon Dec 24 15:06:24 PST 2007

Am Montag, 24. Dezember 2007 23:21:32 schrieb Robert Fitzpatrick:
> I have a couple of Supermicro servers and upgraded both with more
> memory. I upgraded our ESMTP server from 1GB to 4GB and our MX server
> from 2GB to 5GB. Below are the dmesg memory findings and, yes, I get the
> memory over 4GB ignored when booting up. The ESMTP even says that about
> 130MB is ignored. I was reading about building into the kernel PAE
> options for using above 4GB of memory, but in the dmesg I see PAE in the
> Features. Does this mean the support is there and I just need some BIOS
> tweaking?

Nope. This means that your CPU supports PAE, but says nothing about the 
operating system itself (whether it uses PAE or not).

When you want a PAE-enabled kernel (i.e., one that uses PAE to see the "extra" 
memory), see the PAE kernel configuration in the sys/i386/conf directory of 
your sources tree, and build your own kernel with (for example, if you don't 
want to customize the kernel configuration):

make kernconf=PAE buildworld buildkernel installkernel

This builds a kernel which actually uses that interface to see the extra 
memory and allows you to access it in standard i386 mode.

Generally, it's considered better practise to use the AMD64 architecture to 
access the high memory (because of performance considerations and driver 
compatability), but I don't know whether your CPUs actually support the x64 
operating mode, so you may be stuck with using PAE. YMMV.

Heiko Wundram
Product & Application Development

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