IOMMU: This costs you 64 MB of RAM
30th September 2009
If you have happened to observe similar messages in your dmesg:
[ 0.004000] Checking aperture…
[ 0.004000] No AGP bridge found
[ 0.004000] Node 0: aperture @ 20000000 size 32 MB
[ 0.004000] Aperture pointing to e820 RAM. Ignoring.
[ 0.004000] Your BIOS doesn’t leave a aperture memory hole
[ 0.004000] Please enable the IOMMU option in the BIOS setup
[ 0.004000] This costs you 64 MB of RAM
[ 0.004000] Mapping aperture over 65536 KB of RAM @ 20000000
and you are using AMD-based system w/o AGP video, then my advice is: just leave that as is, do not bother “improving”! Any tinkering with kernel boot options won’t do you any good, as the kernel has already done the best it could.
Just a note: all those messages at the top of the post should only happen if you have 4 or more GiBs of RAM. If you have less than that, and do have those messages – my experience might be inappropriate for your case.
Another note: my BIOS does not have any IOMMU settings (or “Memory hole remapping†settings), so I didn’t try that. You should check if your BIOS has IOMMU-related options first, just as kernel message suggests.
Read on for details.
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