On Tue, 10 Sep 2013, Joonsoo Kim wrote: > On Mon, Sep 09, 2013 at 02:44:03PM +0000, Christoph Lameter wrote: > > On Mon, 9 Sep 2013, Joonsoo Kim wrote: > > > > > 32 byte is not minimum object size, minimum *kmalloc* object size > > > in default configuration. There are some slabs that their object size is > > > less than 32 byte. If we have a 8 byte sized kmem_cache, it has 512 objects > > > in 4K page. > > > > As far as I can recall only SLUB supports 8 byte objects. SLABs mininum > > has always been 32 bytes. > > No. > There are many slabs that their object size are less than 32 byte. > And I can also create a 8 byte sized slab in my kernel with SLAB. Well the minimum size for the kmalloc array is 32 bytes. These are custom slabs. KMALLOC_SHIFT_LOW is set to 5 in include/linux/slab.h. Ok so there are some slabs like that. Hmmm.. We have sizes 16 and 24 in your list. 16*256 is still 4096. So this would still work fine if we would forbid a size of 8 or increase that by default to 16. > > On x86 f.e. it would add useless branching. The branches are never taken. > > You only need these if you do bad things to the system like requiring > > large contiguous allocs. > > As I said before, since there is a possibility that some runtime loaded modules > use a 8 byte sized slab, we can't determine index size in compile time. Otherwise > we should always use short int sized index and I think that it is worse than > adding a branch. We can enforce a mininum slab size and an order limit so that it fits. And then there would be no additional branching. -- 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>