Re: [RFC 1/3] Slab infrastructure for array operations

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Jan 23, 2015 at 03:37:28PM -0600, Christoph Lameter wrote:
> This patch adds the basic infrastructure for alloc / free operations
> on pointer arrays. It includes a fallback function that can perform
> the array operations using the single alloc and free that every
> slab allocator performs.
> 
> Allocators must define _HAVE_SLAB_ALLOCATOR_OPERATIONS in their
> header files in order to implement their own fast version for
> these array operations.
> 
> Array operations allow a reduction of the processing overhead
> during allocation and therefore speed up acquisition of larger
> amounts of objects.
> 
> Signed-off-by: Christoph Lameter <cl@linux.com>
> 
> Index: linux/include/linux/slab.h
> ===================================================================
> --- linux.orig/include/linux/slab.h
> +++ linux/include/linux/slab.h
> @@ -123,6 +123,7 @@ struct kmem_cache *memcg_create_kmem_cac
>  void kmem_cache_destroy(struct kmem_cache *);
>  int kmem_cache_shrink(struct kmem_cache *);
>  void kmem_cache_free(struct kmem_cache *, void *);
> +void kmem_cache_free_array(struct kmem_cache *, size_t, void **);
>  
>  /*
>   * Please use this macro to create slab caches. Simply specify the
> @@ -290,6 +291,39 @@ static __always_inline int kmalloc_index
>  void *__kmalloc(size_t size, gfp_t flags);
>  void *kmem_cache_alloc(struct kmem_cache *, gfp_t flags);
>  
> +/*
> + * Additional flags that may be specified in kmem_cache_alloc_array()'s
> + * gfp flags.
> + *
> + * If no flags are specified then kmem_cache_alloc_array() will first exhaust
> + * the partial slab page lists of the local node, then allocate new pages from
> + * the page allocator as long as more than objects per page objects are wanted
> + * and fill up the rest from local cached objects. If that is not enough then
> + * the remaining objects will be allocated via kmem_cache_alloc()
> + */
> +
> +/* Use objects cached for the processor */
> +#define GFP_SLAB_ARRAY_LOCAL		((__force gfp_t)0x40000000)
> +
> +/* Use slabs from this node that have objects available */
> +#define GFP_SLAB_ARRAY_PARTIAL		((__force gfp_t)0x20000000)
> +
> +/* Allocate new slab pages from page allocator */
> +#define GFP_SLAB_ARRAY_NEW		((__force gfp_t)0x10000000)

Hello, Christoph.

Please correct my e-mail address next time. :)
iamjoonsoo.kim@lge.com or js1304@gmail.com

IMHO, exposing these options is not a good idea. It's really
implementation specific. And, this flag won't show consistent performance
according to specific slab implementation. For example, to get best
performance, if SLAB is used, GFP_SLAB_ARRAY_LOCAL would be the best option,
but, for the same purpose, if SLUB is used, GFP_SLAB_ARRAY_NEW would
be the best option. And, performance could also depend on number of objects
and size.

And, overriding gfp flag isn't a good idea. Someday gfp could use
these values and they can't notice that these are used in slab
subsystem with different meaning.

Thanks.