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

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Date:	Thu, 12 Jul 2012 07:06:41 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	netdev@...r.kernel.org, davem@...emloft.net,
	jeffrey.t.kirsher@...el.com, Eric Dumazet <edumazet@...gle.com>
Subject: 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 ?



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