On Tue, May 08, 2007 at 07:24:07PM -0700, Christoph Lameter wrote: > On Tue, 8 May 2007, Matt Mackall wrote: > > > > > Yes. It can in fact put 512 8-byte objects in a 4k page. More > > > > > > So can SLUB. > > > > Not without at least a bit per-object of overhead. So you can either > > fit 512 objects in 4160 bytes or 504 objects in 4k. > > Slub uses a linked list pointer in the page struct which is NULL if all > objects are allocated. There is no bit per object overhead.
Ahh, I'd forgotten about that feature. > > For the kmalloc case, we do have an 8-byte header, which works out to > > be about 1/8th of the slop that mainline kmalloc over SLAB has on > > 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: +/* + * 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; As I pointed out in our private thread, according to these measurements: http://lwn.net/Articles/124374/ total bytes allocated: 47118848 slack bytes allocated: 8717262 number of allocs: 132796 ...the average kmalloc allocation with SLAB is 355 bytes with an average slack of 66 bytes. As SLUB uses the same kmalloc cache size strategy, I expect the same there. SLOB's kmalloc overhead is 8 bytes, always. That's 1/8th the average SLAB kmalloc overhead. -- Mathematics is the supreme nostalgia of our time. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
