Re: [PATCH 2/2] net: Update alloc frag to reduce get/put page usage and recycle pages

On Wed, 2012-07-11 at 19:02 -0700, Alexander Duyck wrote:

The gain will be minimal if any with the 1500 byte allocations, however 
there shouldn't be a performance degradation.

I was thinking more of the ixgbe case where we are working with only 256 
byte allocations and can recycle pages in the case of GRO or TCP.  For 
ixgbe the advantages are significant since we drop a number of the 
get_page calls and get the advantage of the page recycling.  So for 
example with GRO enabled we should only have to allocate 1 page for 
headers every 16 buffers, and the 6 slots we use in that page have a 
good likelihood of being warm in the cache since we just keep looping on 
the same page.

Its not possible to get 16 buffers per 4096 bytes page.

sizeof(struct skb_shared_info)=0x140 320

Add 192 bytes (NET_SKB_PAD + 128)

Thats a minimum of 512 bytes (but ixgbe uses more) per skb.

In practice for ixgbe, its :

#define IXGBE_RXBUFFER_512   512    /* Used for packet split */
#define IXGBE_RX_HDR_SIZE IXGBE_RXBUFFER_512 

skb = netdev_alloc_skb_ip_align(rx_ring->netdev, IXGBE_RX_HDR_SIZE)

So 4 buffers per PAGE

Maybe you plan to use IXGBE_RXBUFFER_256 or IXGBE_RXBUFFER_128 ?