Re: [dpdk-dev] [PATCH] replaced O(n^2) sort in sort_by_physaddr() with qsort() from standard library

["Gonzalez Monroy, Sergio" <sergio.gonzalez.monroy@intel.com>]

Following conversation in 
http://dpdk.org/ml/archives/dev/2015-September/023230.html :

On 17/12/2014 13:31, rolette at infiniteio.com (Jay Rolette) wrote:
> Signed-off-by: Jay Rolette <rolette at infiniteio.com>
> ---
Update commit title/description, maybe something like:
   eal/linux: use qsort for sorting hugepages
   Replace O(n^2) sort in sort_by_physaddr() with qsort() from standard 
library
>   lib/librte_eal/linuxapp/eal/eal_memory.c | 59 +++++++++++---------------------
>   1 file changed, 20 insertions(+), 39 deletions(-)
>
> diff --git a/lib/librte_eal/linuxapp/eal/eal_memory.c b/lib/librte_eal/linuxapp/eal/eal_memory.c
> index bae2507..3656515 100644
> --- a/lib/librte_eal/linuxapp/eal/eal_memory.c
> +++ b/lib/librte_eal/linuxapp/eal/eal_memory.c
> @@ -670,6 +670,25 @@ error:
>   	return -1;
>   }
>   
> +static int
> +cmp_physaddr(const void *a, const void *b)
> +{
> +#ifndef RTE_ARCH_PPC_64
> +	const struct hugepage_file *p1 = (const struct hugepage_file *)a;
> +	const struct hugepage_file *p2 = (const struct hugepage_file *)b;
> +#else
> +	// PowerPC needs memory sorted in reverse order from x86
> +	const struct hugepage_file *p1 = (const struct hugepage_file *)b;
> +	const struct hugepage_file *p2 = (const struct hugepage_file *)a;
> +#endif
> +	if (p1->physaddr < p2->physaddr)
> +		return -1;
> +	else if (p1->physaddr > p2->physaddr)
> +		return 1;
> +	else
> +		return 0;
> +}
> +
There were a couple of comments from Thomas about the comments style and 
#ifdef:
- Comment style should be modified as per 
http://dpdk.org/doc/guides/contributing/coding_style.html#c-comment-style
- Regarding the #ifdef, I agree with Jay that it is the current approach 
in the file.
>   /*
>    * Sort the hugepg_tbl by physical address (lower addresses first on x86,
>    * higher address first on powerpc). We use a slow algorithm, but we won't
> @@ -678,45 +697,7 @@ error:
>   static int
>   sort_by_physaddr(struct hugepage_file *hugepg_tbl, struct hugepage_info *hpi)
>   {
> -	unsigned i, j;
> -	int compare_idx;
> -	uint64_t compare_addr;
> -	struct hugepage_file tmp;
> -
> -	for (i = 0; i < hpi->num_pages[0]; i++) {
> -		compare_addr = 0;
> -		compare_idx = -1;
> -
> -		/*
> -		 * browse all entries starting at 'i', and find the
> -		 * entry with the smallest addr
> -		 */
> -		for (j=i; j< hpi->num_pages[0]; j++) {
> -
> -			if (compare_addr == 0 ||
> -#ifdef RTE_ARCH_PPC_64
> -				hugepg_tbl[j].physaddr > compare_addr) {
> -#else
> -				hugepg_tbl[j].physaddr < compare_addr) {
> -#endif
> -				compare_addr = hugepg_tbl[j].physaddr;
> -				compare_idx = j;
> -			}
> -		}
> -
> -		/* should not happen */
> -		if (compare_idx == -1) {
> -			RTE_LOG(ERR, EAL, "%s(): error in physaddr sorting\n", __func__);
> -			return -1;
> -		}
> -
> -		/* swap the 2 entries in the table */
> -		memcpy(&tmp, &hugepg_tbl[compare_idx],
> -			sizeof(struct hugepage_file));
> -		memcpy(&hugepg_tbl[compare_idx], &hugepg_tbl[i],
> -			sizeof(struct hugepage_file));
> -		memcpy(&hugepg_tbl[i], &tmp, sizeof(struct hugepage_file));
> -	}
> +	qsort(hugepg_tbl, hpi->num_pages[0], sizeof(struct hugepage_file), cmp_physaddr);
>   	return 0;
>   }
Why not just remove sort_by_physaddr() and call qsort() directly?

Sergio