Re: [dpdk-dev] [PATCH] Avoid possible memory cpoy when sort hugepages

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Dec 10, 2014 at 10:25:41AM +0800, Michael Qiu wrote:
> When the first address is the compared address in the loop,
> it will also do memory copy, which is meaningless,
> worse more, when hugepg_tbl is mostly in order. This should
> be a big deal in large hugepage memory systerm(like hunderd
> or thousand GB).

I actually doubt the time taken by this sorting is a significant part of the
initialization time taken, even for hundreds of GBs of memory. Do you have
any measurements to confirm this?
[However, that's not to say that we can't merge in this patch]


> 
> Meanwhile smallest_idx never be a value of -1,so remove this check.
> 
> This patch also includes some coding style fix.
> 
> Signed-off-by: Michael Qiu <michael.qiu@intel.com>
> ---
>  lib/librte_eal/linuxapp/eal/eal_memory.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/lib/librte_eal/linuxapp/eal/eal_memory.c b/lib/librte_eal/linuxapp/eal/eal_memory.c
> index e6cb919..700aba2 100644
> --- a/lib/librte_eal/linuxapp/eal/eal_memory.c
> +++ b/lib/librte_eal/linuxapp/eal/eal_memory.c
> @@ -678,14 +678,13 @@ error:
>  static int
>  sort_by_physaddr(struct hugepage_file *hugepg_tbl, struct hugepage_info *hpi)
>  {
> -	unsigned i, j;
> -	int compare_idx;
> +	unsigned i, j, 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;
> +		compare_idx = i;
>  
>  		/*
>  		 * browse all entries starting at 'i', and find the
> @@ -704,11 +703,9 @@ sort_by_physaddr(struct hugepage_file *hugepg_tbl, struct hugepage_info *hpi)
>  			}
>  		}
>  
> -		/* should not happen */
> -		if (compare_idx == -1) {
> -			RTE_LOG(ERR, EAL, "%s(): error in physaddr sorting\n", __func__);
> -			return -1;
> -		}
> +		/* avoid memory copy when the first entry is the compared */
> +		if (compare_idx == i)
> +			continue;
>  
>  		/* swap the 2 entries in the table */
>  		memcpy(&tmp, &hugepg_tbl[compare_idx],
> -- 
> 1.9.3
>