On Thu, Jan 07, 2016 at 03:04:23PM +0100, Jesper Dangaard Brouer wrote: > This patch introduce a new API call kfree_bulk() for bulk freeing > memory objects not bound to a single kmem_cache. > > Christoph pointed out that it is possible to implement freeing of > objects, without knowing the kmem_cache pointer as that information is > available from the object's page->slab_cache. Proposing to remove the > kmem_cache argument from the bulk free API. > > Jesper demonstrated that these extra steps per object comes at a > performance cost. It is only in the case CONFIG_MEMCG_KMEM is > compiled in and activated runtime that these steps are done anyhow. > The extra cost is most visible for SLAB allocator, because the SLUB > allocator does the page lookup (virt_to_head_page()) anyhow. > > Thus, the conclusion was to keep the kmem_cache free bulk API with a > kmem_cache pointer, but we can still implement a kfree_bulk() API > fairly easily. Simply by handling if kmem_cache_free_bulk() gets > called with a kmem_cache NULL pointer. > > This does increase the code size a bit, but implementing a separate > kfree_bulk() call would likely increase code size even more. > > Below benchmarks cost of alloc+free (obj size 256 bytes) on > CPU i7-4790K @ 4.00GHz, no PREEMPT and CONFIG_MEMCG_KMEM=y. > > Code size increase for SLAB: > > add/remove: 0/0 grow/shrink: 1/0 up/down: 74/0 (74) > function old new delta > kmem_cache_free_bulk 660 734 +74 > > SLAB fastpath: 85 cycles(tsc) 21.468 ns (step:0) > sz - fallback - kmem_cache_free_bulk - kfree_bulk > 1 - 101 cycles 25.291 ns - 41 cycles 10.499 ns - 130 cycles 32.522 ns This looks experimental error. Why does kfree_bulk() takes more time than fallback? Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>