Re: [PATCH] procfs: provide slub's /proc/slabinfo

Christoph Lameter
2008-01-05T20:18:15Z

On Fri, 4 Jan 2008, Matt Mackall wrote:

> > SLUB 32 (all memory of the 4k page is used for 128 byte objects)
> > SLAB 29/30 (management structure occupies first two/three objects)
> > SLOB 30(?) (Alignment results in object being 136 byte of effective size,
> > we have 16 bytes leftover that could be used for a
> > very small allocation. Right?)
>
> Don't know how you got to 136, the minimum alignment is 4 on x86. But I

Right I am thinking about 64 bit systems where the alignment is 8 bytes.

> already said in my last email that SLUB would win for the special case
> of power of two allocations. But as long as we're looking at worst
> cases, let's consider an alloc of 257 bytes..

Yup that hits it by forcing a rounding up to a size of 512 bytes because
there is no intermediate cache size before 1024. The rounding up is
a pretty weak spot in terms of memory use.

> SLUB 8 (1016 bytes wasted)
> SLOB 15 (105 bytes wasted, with 136 bytes still usable)

Well we can actually turn this around. What I gave was not actually the
worst case for SLOB. The worst case is an 8 byte allocation where SLOB
needs double the memory of SLUB.

SLUB 512 (Nothing wasted)
SLOB 256 (Half of the page wasted for metadata)
SLAB 119 (32 byte mininum alloc size + management struct needs)

But these are all extreme cases. Depends on the mix of allocs who wins and
from what I can tell the avoiding of rounding up to a power of two gives
SLOB a key advantage. If we would find the worst offenders there and use
kmem_cache_alloc instead then we may be able to offset that advantage.