On Wed, 2012-12-05 at 22:36 +1100, paul.szabo@sydney.edu.au wrote: > Dear Ben, > > > Although PAE supports up to 64 GB RAM, everything the kernel accesses > > must be mapped into 1 GB of virtual address space (about 880 MB of > > persistently mapped 'normal memory', plus temporary mappings of the > > remaining 'high memory'). The use of such a large amount of high memory > > is problematic, though I don't know whether it entirely explains this > > behaviour. (The memory stats don't seem to account for much of the > > normal memory, as there is ~40 MB free but the various classes of > > allocations seem to add up to only ~300 MB.) > > > > These machines should all be installed with the amd64 kernel. Is there > > any reason you would prefer not to do that? Perhaps the kernel flavour > > selection in the installer should be changed to favour that based on the > > RAM size, though I'm not sure what the critical value should be. > > Are you suggesting that the kernel lies, that 32-bit cannot handle 64GB? > Would it help to test the issue on a 16GB machine (I have one with > 2*X5460 CPUs and one with single i5-3570), or with 24GB (have several > with 2*E5335 to 2*X5460) Or you can test on the kernel larger machines by restricting what the kernel uses with the 'mem' parameter, e.g. mem=16G. > I have seen recommendations to use 64-bit amd64. I am somewhat reluctant > on "jumping ship": I want continuity (when I upgrade by installing a > little more memory), want similarity between my various machines; and > have observed 32-bit being "faster" in some situations. > > But really: this is a bug in the 32-bit build. Do I know that the same > or similar or worse bugs are not present also in the 64-bit build off > the same sources? A 64-bit kernel doesn't have a split between normal and high memory. Ben. -- Ben Hutchings Computers are not intelligent. They only think they are.
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