[PATCH v3 3/5] malloc: Arena is not needed for tcache path in free()

[H.J. Lu]

On Thu, Aug 29, 2024 at 2:31 PM Wangyang Guo <wangyang.guo@intel.com> wrote:
>
> Arena is not needed for _int_free_check() in non-DEBUG mode. This commit
> defers arena deference to _int_free_chunk() thus accelerate tcache path.
> When DEBUG enabled, arena can be obtained from p in do_check_inuse_chunk().
>
> Result of bench-malloc-thread benchmark
>
> Test Platform: Xeon-8380
> Ratio: New / Original time_per_iteration (Lower is Better)
>
> Threads#   | Ratio
> -----------|------
> 1 thread   | 0.994
> 4 threads  | 0.968
>
> The data shows it can brings 3% performance gain in multi-thread scenario.
>
> ---
> Changes in v2:
> - _int_free_check() should be put outside of USE_TCACHE.
> - Link to v1: https://sourceware.org/pipermail/libc-alpha/2024-August/159360.html
> ---
>  malloc/malloc.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 264f35e1a3..efb5292e9f 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -2143,6 +2143,9 @@ do_check_inuse_chunk (mstate av, mchunkptr p)
>  {
>    mchunkptr next;
>
> +  if (av == NULL)
> +    av = arena_for_chunk (p);
> +
>    do_check_chunk (av, p);
>
>    if (chunk_is_mmapped (p))
> @@ -3447,9 +3450,10 @@ __libc_free (void *mem)
>        /* Mark the chunk as belonging to the library again.  */
>        (void)tag_region (chunk2mem (p), memsize (p));
>
> -      ar_ptr = arena_for_chunk (p);
>        INTERNAL_SIZE_T size = chunksize (p);
> -      _int_free_check (ar_ptr, p, size);
> +      /* av is not needed for _int_free_check in non-DEBUG mode,
> +        in DEBUG mode, av will fetch from p in do_check_inuse_chunk.  */
> +      _int_free_check (NULL, p, size);

If _int_free_check doesn't need av, can you push it further down?

>
>  #if USE_TCACHE
>        if (tcache_free (p, size))
> @@ -3458,6 +3462,8 @@ __libc_free (void *mem)
>           return;
>         }
>  #endif
> +
> +      ar_ptr = arena_for_chunk (p);
>        _int_free_chunk (ar_ptr, p, size, 0);
>      }
>
> --
> 2.43.0
>


-- 
H.J.