Re: [PATCH] malloc: fix malloc performance may becomes worse as the number of malloc increases

[Thomas Monjalon <thomas@monjalon.net>]

Looking for reviewers please.

10/02/2023 07:30, Fengnan Chang:
> Here is a simple test case:
> "
> uint64_t entry_time, time;
> size_t size = 4096;
> unsigned align = 4096;
> for (int j = 0; j < 10; j++) {
> 	entry_time = rte_get_timer_cycles();
> 	for (int i = 0; i < 2000; i++) {
> 		rte_malloc(NULL, size, align);
> 	}
> 	time = (rte_get_timer_cycles()-entry_time) * 1000000 /
> 		rte_get_timer_hz();
> 	printf("total open time %lu avg time %lu\n", time, time/2000);
> }
> "
> 
> Single rte_malloc cost time may becomes wrose as the number of malloc
> increases, In my env, first round avg time is 15us, second is 44us,
> third is 77us, fourth is 168us...
> 
> The reason is,in the malloc process, malloc_elem_alloc may split new_elem
> if there have too much free space after new_elem, and insert the trailer
> into freelist. When alloc 4k with align 4k, the trailer very likely insert
> to free_head[2] again, it makes free_head[2] longer. when alloc 4k again,
> it will search free_head[2] from begin, with the number of malloc increases,
> search free_head[2] need more time, so the performance will become worse.
> Same problem will also occurs in alloc 64k with align 64k, but if alloc
> 4k with align 64, doesn't have this problem.
> 
> Fix this by adjust free_head list size range, make free_head[3] hold
> elements which bigger or equal 4k, free_head[4] hold elements which bigger
> or equal 16k.
> In terms of probabilities, when alloc 4k or 16k, the probability of finding
> a suitable elem from a larger size list is greater than from a smaller
> size list.
> 
> Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
> ---
>  lib/eal/common/malloc_elem.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/eal/common/malloc_elem.c b/lib/eal/common/malloc_elem.c
> index 83f05497cc..35a2313d04 100644
> --- a/lib/eal/common/malloc_elem.c
> +++ b/lib/eal/common/malloc_elem.c
> @@ -367,11 +367,11 @@ prev_elem_is_adjacent(struct malloc_elem *elem)
>   * containing larger elements.
>   *
>   * Example element size ranges for a heap with five free lists:
> - *   heap->free_head[0] - (0   , 2^8]
> - *   heap->free_head[1] - (2^8 , 2^10]
> - *   heap->free_head[2] - (2^10 ,2^12]
> - *   heap->free_head[3] - (2^12, 2^14]
> - *   heap->free_head[4] - (2^14, MAX_SIZE]
> + *   heap->free_head[0] - (0   , 2^8)
> + *   heap->free_head[1] - [2^8 , 2^10)
> + *   heap->free_head[2] - [2^10 ,2^12)
> + *   heap->free_head[3] - [2^12, 2^14)
> + *   heap->free_head[4] - [2^14, MAX_SIZE]
>   */
>  size_t
>  malloc_elem_free_list_index(size_t size)
> @@ -385,8 +385,8 @@ malloc_elem_free_list_index(size_t size)
>  	if (size <= (1UL << MALLOC_MINSIZE_LOG2))
>  		return 0;
>  
> -	/* Find next power of 2 >= size. */
> -	log2 = sizeof(size) * 8 - __builtin_clzl(size - 1);
> +	/* Find next power of 2 > size. */
> +	log2 = sizeof(size) * 8 - __builtin_clzl(size);
>  
>  	/* Compute freelist index, based on log2(size). */
>  	index = (log2 - MALLOC_MINSIZE_LOG2 + MALLOC_LOG2_INCREMENT - 1) /
>