Re: RFC: Non Power of 2 memory allocator
From: Nick Piggin
Date: Sat Mar 25 2006 - 19:31:07 EST
Hugh Dickins wrote:
On Sat, 25 Mar 2006, Andi Kleen wrote:
Robin Getz <rgetz@xxxxxxxxxxxxxxxxxxxx> writes:
The buddy system allocates things in power of 2 pages sizes (4k, 8k,
16k, 32k, 64k, 128k, 256k, 512k, 1024k), which works fine on most
systems, but an embedded system, which is running without a MMU (
Memory Management Unit) - RAM is precious, and when you only need
129k for an application, you don't want to allocate a power of 2,
which gives you 256k - an extra 127k, which can't be used by
anything else.
In 2.4 I solved this problem at some point by just returning
the excess pages to the buddy allocator. There was even
a nice function to do this (alloc_exact)
That won't work for slab, but does for __get_free_pages() which
is better for large allocations anyways. slab imho doesn't make
sense for allocation anywhere bigger PAGE_SIZE/2. At some
point in 2.6 there was trouble with "compound pages" but I think
that has been resolved.
Just implementing alloc_exact again would be the simplest solution
for your problem.
Nick has put a split_page function into the 2.6.16-git mm/page_alloc.c,
which I believe is supposed to be a helper in this kind of operation.
You'd best take a look at where and how it's used. Perhaps Andi's
alloc_exact should be reimplemented in terms of it.
Indeed. nommu already uses the slab allocator for user allocations.
I guess the problem is that slab probably doesn't do this exact
allocation thing either.
I'd like to see nommu move over to using something like Andi's
alloc_exact (away from slab), because that's how the normal
kernel does it.
--
SUSE Labs, Novell Inc.
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