Re: [PATCH 4/4] kern: optimize the layout of struct kmem_cache

[Samuel Thibault]

Justus Winter, le Mon 06 Jan 2014 00:34:55 +0100, a écrit :
> * kern/slab.h (struct kmem_cache): Reorder the fields so that all hot
> fields are within the first cache line.

Did you try to benchmark this a bit?

I'm unsure about all movements: nr_free_slabs, for instance, should
probably really go along nr_slabs, as well as nr_objs, etc.

> ---
>  kern/slab.h | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/kern/slab.h b/kern/slab.h
> index fd65893..457dee4 100644
> --- a/kern/slab.h
> +++ b/kern/slab.h
> @@ -157,6 +157,10 @@ typedef void (*kmem_slab_free_fn_t)(vm_offset_t, 
> vm_size_t);
>   * Cache of objects.
>   *
>   * Locking order : cpu_pool -> cache. CPU pools locking is ordered by CPU ID.
> + *
> + * Currently, SLAB_USE_CPU_POOLS is not defined.  KMEM_CACHE_NAME_SIZE
> + * is chosen so that the struct fits into two cache lines.  The first
> + * cache line contains all hot fields.
>   */
>  struct kmem_cache {
>  #if SLAB_USE_CPU_POOLS
> @@ -172,19 +176,21 @@ struct kmem_cache {
>      struct list free_slabs;
>      struct rbtree active_slabs;
>      int flags;
> +    size_t bufctl_dist; /* Distance from buffer to bufctl   */
> +    size_t slab_size;
> +    unsigned long bufs_per_slab;
> +    unsigned long nr_objs;  /* Number of allocated objects */
> +    unsigned long nr_free_slabs;
> +    kmem_cache_ctor_t ctor;
> +    /* All fields below are cold  */
>      size_t obj_size;    /* User-provided size */
> +    /* Assuming ! SLAB_USE_CPU_POOLS, here is the cacheline boundary */
>      size_t align;
>      size_t buf_size;    /* Aligned object size  */
> -    size_t bufctl_dist; /* Distance from buffer to bufctl   */
> -    size_t slab_size;
>      size_t color;
>      size_t color_max;
> -    unsigned long bufs_per_slab;
> -    unsigned long nr_objs;  /* Number of allocated objects */
>      unsigned long nr_bufs;  /* Total number of buffers */
>      unsigned long nr_slabs;
> -    unsigned long nr_free_slabs;
> -    kmem_cache_ctor_t ctor;
>      kmem_slab_alloc_fn_t slab_alloc_fn;
>      kmem_slab_free_fn_t slab_free_fn;
>      char name[KMEM_CACHE_NAME_SIZE];
> -- 
> 1.8.5.2
> 

-- 
Samuel
<N> (* If you have a precise idea of the intended use of the following code, 
please
<N>    write to Eduardo.Gimenez@inria.fr and ask for the prize :-)
<N>    -- Eduardo (11/8/97) *)
 -+- N sur #ens-mim - et c'était un des développeurs -+-