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