netdev - Re: [PATCH net-next 3/9] sfc: Increase size of RX SKB header area

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Date:	Tue, 16 Jul 2013 10:33:55 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-net-drivers@...arflare.com
Subject: Re: [PATCH net-next 3/9] sfc: Increase size of RX SKB header area

On Mon, 2013-06-24 at 21:13 +0100, Ben Hutchings wrote:
> From: Jon Cooper <jcooper@...arflare.com>
> 
> This allows the SKB to hold the headers without reallocation more often.
> 
> Signed-off-by: Ben Hutchings <bhutchings@...arflare.com>
> ---
>  drivers/net/ethernet/sfc/rx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
> index b915e09..6efff3d 100644
> --- a/drivers/net/ethernet/sfc/rx.c
> +++ b/drivers/net/ethernet/sfc/rx.c
> @@ -36,7 +36,7 @@
>  #define EFX_RECYCLE_RING_SIZE_NOIOMMU (2 * EFX_RX_PREFERRED_BATCH)
>  
>  /* Size of buffer allocated for skb header area. */
> -#define EFX_SKB_HEADERS  64u
> +#define EFX_SKB_HEADERS  128u
>  
>  /* This is the percentage fill level below which new RX descriptors
>   * will be added to the RX descriptor ring.
> 

This patch brings performance decrease for tunnels, because it pulls
into skb->head 128 bytes worth of data.

This includes TCP payload, so GRO or TCP coalescing code is less
effective. Each MSS spans 2 memory areas (small part on skb->head,
remaining on the fragment) : GRO packets have only 8 MSS worth of data,
instead of 16.

The fix would be to allocate 128 bytes in skb->head to prevent future
reallocations of skb->head, but pull 64 bytes only.



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