Re: RFC: Non Power of 2 memory allocator

[Nick Piggin <nickpiggin@yahoo.com.au>]

Hugh Dickins wrote:
> On Sat, 25 Mar 2006, Andi Kleen wrote:
>
> > Robin Getz <rgetz@blackfin.uclinux.org> 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.
Send instant messages to your online friends http://au.messenger.yahoo.com 
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/