RE: [PATCH 4/4] IA64: SPARSE_VIRTUAL 16M page size support

["Luck, Tony" <tony.luck@intel.com>]

> This implements granule page sized vmemmap support for IA64.

Christoph,

Your calculations here are all based on a granule size of 16M, but
it is possible to configure 64M granules.

With current sizeof(struct page) == 56, a 16M page will hold enough
page structures for about 4.5G of physical space (assuming 16K pages),
so a 64M page would cover 18G.

4.5G is possibly a bit wasteful (for a system with only a handful
of GBytes per node, and nodes that are not physically contiguous).
18G is definitely going to result in lots of wasted page structs
(that refer to non-existant physical memory around the edges of
each node).

Maybe a granule is not the right unit of allocation ... perhaps 4M
would work better (4M/56 ~= 75000 pages ~= 1.1G)?  But if this is
too small, then a hard-coded 16M would be better than a granule,
because 64M is (IMHO) too big.

-Tony

P.S. This patch breaks the build for tiger_defconfig, zx1_defconfig
etc.  But you may have fit on the "grand-unified theory" of mem_map
management ... so if the benchmarks come in favourably we could
drop all the other CONFIG options.

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