RE: [suse-amd64] effective memory in Opteron system

For information , we found that one particular BIOS for Quartets would 'ignore' one or more memory slots. Any DIMMS there were not discoveered by the BIOS. The actual slots affected seemed arbitrarty - the same node would always have teh same slot(s) affected - but different identical nodes had different problem DIMM slots. When you added that 4th DIMM you could try using a different slot to see if it is any better. (yes I know about paired memory and 128-bit access but this is a test) A BIOS rev fixed this for us- you do not say what you BIOS revision is. /usr/sbin/hwinfo is your frined here - it not only tell you the BIOS version , but also the detected DIMMS by slot number and also the A20 physical memory map (hence spot the memory holes) Finally IMHO, the best way forward if Linux is not seeing all the memory you paid for (but the BIOS is seeing it) is to have say a 2GB hole from 2-4GB and push the rest of the memory up. AMD/Celestica have a BIOS that does this that we use. Yours, Daniel. -------------------------------------------------------------- Dr. Dan Kidger, Quadrics Ltd. daniel.kidger@quadrics.com One Bridewell St., Bristol, BS1 2AA, UK 0117 915 5505 ----------------------- www.quadrics.com --------------------
-----Original Message----- From: Andi Kleen [mailto:ak@suse.de] Sent: 25 March 2004 01:39 To: ascotti@email.unc.edu Cc: suse-amd64@suse.com Subject: Re: [suse-amd64] effective memory in Opteron system
On Wed, 24 Mar 2004 20:21:58 -0500 ascotti@email.unc.edu wrote:
Hi,
I have recently purchased a quad-opteron. The MB is a Quartet manufactured by Celestica. The system has 8 GB of RAM (8 dimms, 2 GB per node). The puzzling thing is that only 7.5 GB show up, both during the BIOS startup, and by the OS (SLES 8). Playing around with the dimms, I have found that up to 3 GB everything looks normal. Adding a fourth dimm to the system results in an effective memory of 3.5 GB. Adding extra modules increase the count by 1GB as expected. The dimms are good, and nothing changes by switching dimms or populating the nodes differently. The vendor tells me that this is a known "feature" of the AMD chipset, even for Athlons. I looked up the thing, and found nothing. I am curious to see if this is just a problem with the Opteron chipset, or a more general problem. 0.5 GB seems a lot to disappear...
It depends on the BIOS. The hardware can only map a single DIMM to an continuous address. Your BIOS decides to map the first three DIMMs at 0,1GB,2GB. Then there is the PCI memory hole directly below 4GB (PCI memory mappings, AGP aperture, IOMMU etc., can be several hundred MB). Your BIOS could chose to leave 3GB-4GB free and only put the IO mappings there and put the next DIMM at 4GB, but it chose instead to map the DIMM at 3GB and so part of its memory is lost to the IO hole.
There may be subtle reasons why it does that.
Complain to your BIOS vendor about this, there is nothing Linux has to do with it.
-Andi
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daniel.kidger@quadrics.com