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Date:	Thu, 21 Jan 2016 12:27:30 +0100
From:	Jesper Dangaard Brouer <>
To:	Tom Herbert <>
Cc:	Eric Dumazet <>,
	Or Gerlitz <>,
	David Miller <>,
	Eric Dumazet <>,
	Linux Netdev List <>,
	Alexander Duyck <>,
	Alexei Starovoitov <>,
	Daniel Borkmann <>,
	Marek Majkowski <>,
	Hannes Frederic Sowa <>,
	Florian Westphal <>,
	Paolo Abeni <>,
	John Fastabend <>,
	Amir Vadai <>,
Subject: Re: Optimizing instruction-cache, more packets at each stage

On Wed, 20 Jan 2016 15:27:38 -0800 Tom Herbert <> wrote:

> eth_type_trans touches headers

True, the eth_type_trans() call in the driver is a major bottleneck,
because it touch the packet header and happens very early in the driver.

In my experiments, where I extract several packet before calling
napi_gro_receive(), and I also delay calling eth_type_trans().  Most of
my speedup comes from this trick, as the prefetch() now that enough

 while ((skb = __skb_dequeue(&rx_skb_list)) != NULL) {
	skb->protocol = eth_type_trans(skb, rq->netdev);
	napi_gro_receive(cq->napi, skb);

What is the HW could provide the info we need in the descriptor?!?

eth_type_trans() does two things:

1) determine skb->protocol

Could the HW descriptor deliver the "proto", or perhaps just some bits
on the most common proto's?

The skb->pkt_type don't need many bits.  And I bet the HW already have
the information.  The BROADCAST and MULTICAST indication are easy.  The
PACKET_OTHERHOST, can be turned around, by instead set a PACKET_HOST
indication, if the eth->h_dest match the devices dev->dev_addr (else a
SW compare is required).

Is that doable in hardware?

Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of

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