[CentOS] 4gb ram shows as 3.3gb

John R Pierce pierce at hogranch.com
Thu Jul 26 21:08:27 UTC 2007

Martin Marques wrote:
> Nick wrote:
>> Martin Marques wrote:
>>> The thing is, and I'm still trying to find a solution for one of our 
>>> servers, the the memory addresses from 4Gb down are used for PCI, 
>>> VGA, onboard stuff, and even though the memory isn't used, the 
>>> addresses are, and so they are not passed to the OS.
>>> Can you send some info on the motherboard you have?
>> It's a Dell Poweredge 1950 straight out of the box...
> What chipset does the motherboard have? Asus, Intel, MSI, etc?

Asus and MSI don't make chipsets, nor do they make Dell server boards.

google leads me to dell.com where the 'tech specs' for this dual Xeon 
5000/5100/5300 series 1U server... which says its a Intel 5000X chipset.

but thats neither here nor there, PAE is a universal issue for any x86 
32bit system with 4GB+ ram, with PAE disabled, the BIOS, PCI, AGP or 
PCI-express, etc IO spaces consume anywheres from .5 to 1GB of the 32bit 
address space.   PAE is a hardware workaround implemented in pretty much 
all Intel and AMD CPUs made in the last 5+ years, and allows the OS to 
access more than 4GB of physical address space.  PAE introduces some 
hardware overhead because it involves larger page tables and another 
level of indirection in the TLB lookups.

CentOS 5 installs defaults to PAE off because there are some systems 
where PAE is crash-happy.    Someone here has already explained how to 
enable PAE and in fact the original poster tried it and is happy with 
his full 4GB now.

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