Messages in this thread | | | Date | Sun, 01 Dec 2002 22:48:14 -0800 | From | "Martin J. Bligh" <> | Subject | Re: Maximum Physical Memory on 2.4 and ia32 |
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> It's a practical limit. The mem_map array alone would consume 720 megabytes, > so you have no normal-zone memory left. > > At 16G, mem_map[] consumes 180 megabytes and there's 540ish megabytes > of normal zone left for general use. > > Even at this 20:1 highmem:lowmem ratio, the system will be struggling. > Any time you have normal-zone data structures which are pinned by > pages, the maths gets you in the end. > > buffer_heads, pagetable pages, radix-tree nodes, pte_chains and inodes > are normal-zone data structures which, depending on the kernel version, > may be pinned into the normal zone by highmem pages. > > In 2.5, with ext2's no-buffer-head option, shared pagetables, highpte, > with your fingers crossed and the wind in the south east, 32G might > be practical.
32Gb was indeed what we've been working towards for 2.6, and we've been running that on some workloads.
However, if you're willing to run with a 2:2 or even a 1:3 user:kernel split instead of the default 3:1, you can get away with all sorts of things, probably including 64Gb. I've got the hardware to build such a beast, but haven't bothered yet (we have enough problems already ;-)). Big databases won't like it, but other workloads without huge individual processes (or shared mappings) will be fine.
M.
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