Re: [PATCH 2/2] lib/librte_eal: Remove unnecessary hugepage zero-filling

[Stephen Hemminger <stephen@networkplumber.org>]

On Sun, 22 Nov 2015 14:13:35 -0500
Zhihong Wang <zhihong.wang@intel.com> wrote:

> The kernel fills new allocated (huge) pages with zeros.
> DPDK just has to populate page tables to trigger the allocation.
> 
> Signed-off-by: Zhihong Wang <zhihong.wang@intel.com>
> ---
>  lib/librte_eal/linuxapp/eal/eal_memory.c | 20 ++++++--------------
>  1 file changed, 6 insertions(+), 14 deletions(-)
> 
> diff --git a/lib/librte_eal/linuxapp/eal/eal_memory.c b/lib/librte_eal/linuxapp/eal/eal_memory.c
> index 0de75cd..21a5146 100644
> --- a/lib/librte_eal/linuxapp/eal/eal_memory.c
> +++ b/lib/librte_eal/linuxapp/eal/eal_memory.c
> @@ -399,8 +399,10 @@ map_all_hugepages(struct hugepage_file *hugepg_tbl,
>  			return -1;
>  		}
>  
> +		/* map the segment, and populate page tables,
> +		 * the kernel fills this segment with zeros */
>  		virtaddr = mmap(vma_addr, hugepage_sz, PROT_READ | PROT_WRITE,
> -				MAP_SHARED, fd, 0);
> +				MAP_SHARED | MAP_POPULATE, fd, 0);
>  		if (virtaddr == MAP_FAILED) {
>  			RTE_LOG(ERR, EAL, "%s(): mmap failed: %s\n", __func__,
>  					strerror(errno));
> @@ -410,7 +412,6 @@ map_all_hugepages(struct hugepage_file *hugepg_tbl,
>  
>  		if (orig) {
>  			hugepg_tbl[i].orig_va = virtaddr;
> -			memset(virtaddr, 0, hugepage_sz);
>  		}
>  		else {
>  			hugepg_tbl[i].final_va = virtaddr;
> @@ -529,22 +530,16 @@ remap_all_hugepages(struct hugepage_file *hugepg_tbl, struct hugepage_info *hpi)
>  
>  			old_addr = vma_addr;
>  
> -			/* map new, bigger segment */
> +			/* map new, bigger segment, and populate page tables,
> +			 * the kernel fills this segment with zeros */
>  			vma_addr = mmap(vma_addr, total_size,
> -					PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +					PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE, fd, 0);
>  
>  			if (vma_addr == MAP_FAILED || vma_addr != old_addr) {
>  				RTE_LOG(ERR, EAL, "%s(): mmap failed: %s\n", __func__, strerror(errno));
>  				close(fd);
>  				return -1;
>  			}
> -
> -			/* touch the page. this is needed because kernel postpones mapping
> -			 * creation until the first page fault. with this, we pin down
> -			 * the page and it is marked as used and gets into process' pagemap.
> -			 */
> -			for (offset = 0; offset < total_size; offset += hugepage_sz)
> -				*((volatile uint8_t*) RTE_PTR_ADD(vma_addr, offset));
>  		}
>  
>  		/* set shared flock on the file. */
> @@ -592,9 +587,6 @@ remap_all_hugepages(struct hugepage_file *hugepg_tbl, struct hugepage_info *hpi)
>  			}
>  		}
>  
> -		/* zero out the whole segment */
> -		memset(hugepg_tbl[page_idx].final_va, 0, total_size);
> -
>  		page_idx++;
>  	}
>  

Nice, especially on slow machines or with large memory.

Acked-by: Stephen Hemminger <stephen@networkplumber.org>