Re: [RFC PATCH] greatly reduce SLOB external fragmentation

[Christoph Lameter]

On Thu, 10 Jan 2008, Linus Torvalds wrote:

> It's not even clear that a buddy allocator even for the high-order pages 
> is at all the right choice. Almost nobody actually wants >64kB blocks, and 
> the ones that *do* want bigger allocations tend to want *much* bigger 
> ones, so it's quite possible that it could be worth it to have something 
> like a three-level allocator:

Excellent! I am definitely on board with this.

>  - huge pages (superpages for those crazy db people)
> 
>    Just a simple linked list of these things is fine, we'd never care 
>    about coalescing large pages together anyway.
> 
>  - "large pages" (on the order of ~64kB) - with *perhaps* a buddy bitmap 
>    setup to try to coalesce back into huge-pages, but more likely just 
>    admitting that you'd need something like migration to ever get back a 
>    hugepage that got split into large-pages.
> 
>    So maybe a simple bitmap allocator per huge-page for large pages. Say 
>    you have a 4MB huge-page, and just a 64-bit free-bitmap per huge-page 
>    when you split it into large pages.
> 
>  - slab/slub/slob for anything else, and "get_free_page()" ends up being 
>    just a shorthand for saying "naturally aligned kmalloc of size 
>    "PAGE_SIZE<<order"
> 
> and maybe it would all work out ok. 

Hmmm... a 3 level allocator? Basically we would have BASE_PAGE 
STANDARD_PAGE and HUGE_PAGE? We could simply extend the page allocator to 
have 3 pcp lists for these sizes and go from there?

Thinking about the arches this would mean

	BASE_PAGE	STANDARD_PAGE	HUGE_PAGE
x86_64	4k		64k		2M
i386	4k		16k		4M
ia64	16k		256k		1G

?

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/