The objects are packed tightly in SLUB and SLUB can allocate smaller
objects (minimum is 8 SLAB mininum is 32).
On free a SLUB object goes directly back to the slab where it came from.
We have no queues in SLUB so we use the first word of the object as a
freepointer. In SLAB the objects first go onto queues and then are drained
later into the slab. On free in SLAB there is usually no need to touch the
object itself. The object pointer is simply moved onto the queue (works
well in SMP, in NUMA we have overhead identifying the queue and
overhead due to the number of queues needed).
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