Re: + fix-spellings-of-slab-allocator-section-in-init-kconfig.patch added to -mm tree

[Matt Mackall <mpm@selenic.com>]

On Tue, May 08, 2007 at 07:57:37PM -0700, Christoph Lameter wrote:
> On Wed, 9 May 2007, Nick Piggin wrote:
> 
> > > Exactly. That overhead does not exist in SLUB. Thus SLOB is less efficient
> > > than SLUB.
> > 
> > What you trade for that is that one page page can only serve one slab.
> 
> Right.
> 
> > For small systems, I would not be surprised if that was less space
> > efficient, even just looking at kmalloc caches in isolation. Or do you
> > have numbers to support your conclusion?
> 
> No I do not have any number beyond the efficiency calculations based on 
> whole slabs. We would have to do some experiments to figure out how much 
> space is actually wasted through partial slabs.

The expectation would be (PAGE_SIZE + (PAGE_SIZE % size))/2 on average
per cache.
 
> The situation becomes different with allocation and frees. Then we may 
> have lots of partial slabs that we allocate from. But the SLOB approach 
> also will have holes to manage. So I do not see how this could be a 
> benefit unless you only have a few precious pages and you need to put 
> multiple object sizes into it. A 4M system still has 1000 pages.

A 4M system has approximately zero pages free once you've actually got
stuff running in userspace. The marginal utility of each page is very
high.

-- 
Mathematics is the supreme nostalgia of our time.