[PATCH] malloc: Improve arena_for_chunk()

[Adhemerval Zanella Netto]

On 18/03/25 09:58, Wilco Dijkstra wrote:
> 
> Change heap_max_size() to improve performance of arena_for_chunk().
> Instead of a complex calculation, using a simple mask operation to get the
> arena base pointer. HEAP_MAX_SIZE should be larger than the huge page size,
> otherwise heaps will use not huge pages.
> 
> On AArch64 this removes 6 instructions from arena_for_chunk(), and 
> bench-malloc-thread improves by 1.1-1.8%.
> 
> Passes regress, OK for commit?


LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> 
> ---
> 
> diff --git a/malloc/arena.c b/malloc/arena.c
> index 353b63488d3c9963276dde35c063aab9dd67f663..405ae829c01a89ec9042d9eb8a6c98435a6ecea6 100644
> --- a/malloc/arena.c
> +++ b/malloc/arena.c
> @@ -40,19 +40,20 @@
>     mmap threshold, so that requests with a size just below that
>     threshold can be fulfilled without creating too many heaps.  */
>  
> -/* When huge pages are used to create new arenas, the maximum and minimum
> -   size are based on the runtime defined huge page size.  */
> +/* HEAP_MAX_SIZE should be larger than the huge page size, otherwise heaps will
> +   use not huge pages.  It is a constant so arena_for_chunk() is efficient.  */
>  
>  static inline size_t
>  heap_min_size (void)
>  {
> -  return mp_.hp_pagesize == 0 ? HEAP_MIN_SIZE : mp_.hp_pagesize;
> +  return mp_.hp_pagesize == 0 || mp_.hp_pagesize > HEAP_MAX_SIZE
> +	  ? HEAP_MIN_SIZE : mp_.hp_pagesize;
>  }
>  
>  static inline size_t
>  heap_max_size (void)
>  {
> -  return mp_.hp_pagesize == 0 ? HEAP_MAX_SIZE : mp_.hp_pagesize * 4;
> +  return HEAP_MAX_SIZE;
>  }
>  
>  /***************************************************************************/
> @@ -313,7 +314,7 @@ ptmalloc_init (void)
>    TUNABLE_GET (mxfast, size_t, TUNABLE_CALLBACK (set_mxfast));
>    TUNABLE_GET (hugetlb, size_t, TUNABLE_CALLBACK (set_hugetlb));
>  
> -  if (mp_.hp_pagesize > 0)
> +  if (mp_.hp_pagesize > 0 && mp_.hp_pagesize <= heap_max_size ())
>      {
>        /* Force mmap for main arena instead of sbrk, so MAP_HUGETLB is always
>           tried.  Also tune the mmap threshold, so allocation smaller than the
> @@ -460,7 +461,7 @@ alloc_new_heap  (size_t size, size_t top_pad, size_t pagesize,
>  static heap_info *
>  new_heap (size_t size, size_t top_pad)
>  {
> -  if (__glibc_unlikely (mp_.hp_pagesize != 0))
> +  if (mp_.hp_pagesize != 0 && mp_.hp_pagesize <= heap_max_size ())
>      {
>        heap_info *h = alloc_new_heap (size, top_pad, mp_.hp_pagesize,
>  				     mp_.hp_flags);
>