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

[Christoph Lameter <clameter@sgi.com>]

On Tue, 8 May 2007, Matt Mackall wrote:

> > Exactly. That overhead does not exist in SLUB. Thus SLOB is less efficient 
> > than SLUB.
> 
> What size object does kmalloc(80) return? In SLAB, the answer is 128
> bytes with 48 bytes of slack space. In SLOB, the answer is 88 for 8
> bytes of slack space. Looks like SLUB is in the same camp as SLAB
> here:

There is a 96 sized general slab. So it would go up to that size. But you 
can create a 80 byte slab of course. And that may cost minimal overhead 
since 80 byte slabs may be merged. If one already exist then you get it 
for free.

> +/*
> + * We keep the general caches in an array of slab caches that are used for
> + * 2^x bytes of allocations.
> + */
> +extern struct kmem_cache kmalloc_caches[KMALLOC_SHIFT_HIGH + 1];
> ..
> +	if (size <=        128) return 7;

96 please. You skipped the first part.

> SLOB's kmalloc overhead is 8 bytes, always. That's 1/8th the average
> SLAB kmalloc overhead.

SLUB can generate an 80 byte slab with minimal overhead if you wanted. But 
yes I agree the flexbility there is an advantage if you have objects of 
various sizes.