On Wed, Jan 28, 2015 at 09:30:56AM -0600, Christoph Lameter wrote: > On Wed, 28 Jan 2015, Joonsoo Kim wrote: > > > > GFP_SLAB_ARRAY new is best for large quantities in either allocator since > > > SLAB also has to construct local metadata structures. > > > > In case of SLAB, there is just a little more work to construct local > > metadata so > > GFP_SLAB_ARRAY_NEW would not show better performance > > than GFP_SLAB_ARRAY_LOCAL, because it would cause more overhead due to > > more page allocations. Because of this characteristic, I said that > > which option is > > the best is implementation specific and therefore we should not expose it. > > For large amounts of objects (hundreds or higher) GFP_SLAB_ARRAY_LOCAL > will never have enough objects. GFP_SLAB_ARRAY_NEW will go to the page > allocator and bypass free table creation and all the queuing that objects > go through normally in SLAB. AFAICT its going to be a significant win. > > A similar situation is true for the freeing operation. If the freeing > operation results in all objects in a page being freed then we can also > bypass that and put the page directly back into the page allocator (to be > implemented once we agree on an approach). > > > Even if we narrow down the problem to the SLUB, choosing correct option is > > difficult enough. User should know how many objects are cached in this > > kmem_cache > > in order to choose best option since relative quantity would make > > performance difference. > > Ok we can add a function call to calculate the number of objects cached > per cpu and per node? But then that is rather fluid and could change any > moment. > > > And, how many objects are cached in this kmem_cache could be changed > > whenever implementation changed. > > The default when no options are specified is to first exhaust the node > partial objects, then allocate new slabs as long as we have more than > objects per page left and only then satisfy from cpu local object. I think > that is satisfactory for the majority of the cases. > > The detailed control options were requested at the meeting in Auckland at > the LCA. I am fine with dropping those if they do not make sense. Makes > the API and implementation simpler. Jesper, are you ok with this?
IMHO, it'd be better to choose a proper way of allocation by slab itself and not to expose options to API user. We could decide the best option according to current status of kmem_cache and requested object number and internal implementation. Is there any obvious example these option are needed for user? Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
