netdev - Re: [PATCH net-next-2.6] net: introduce build

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Date:	Tue, 12 Jul 2011 17:40:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] net: introduce build_skb()

Le lundi 11 juillet 2011 à 07:46 +0200, Eric Dumazet a écrit :

> [PATCH] net: introduce build_skb()
> 
> One of the thing we discussed during netdev 2011 conference was the idea
> to change network drivers to allocate/populate their skb at RX
> completion time, right before feeding the skb to network stack.
> 
> Right now, we allocate skbs when populating the RX ring, and thats a
> waste of CPU cache, since allocating skb means a full memset() to clear
> the skb and its skb_shared_info portion. By the time NIC fills a frame
> in data buffer and host can get it, cpu probably threw away the cache
> lines from its caches, because of huge RX ring sizes.
> 
> So the deal would be to allocate only the data buffer for the NIC to
> populate its RX ring buffer. And use build_skb() at RX completion to
> attach a data buffer (now filled with an ethernet frame) to a new skb,
> initialize the skb_shared_info portion, and give the hot skb to network
> stack.

Update :

First results are impressive : About 15% of throughput increase with igb
driver on my small desktop machine, and I am limited by the wire
speed :)

(AMD Athlon(tm) II X2 B24 Processor, 3GHz, cache size : 1024K)

setup : One dual port Intel card : Ethernet controller: Intel
Corporation 82576 Gigabit Network Connection (rev 01)

eth1 direct attach on eth2, Gigabit speed.
eth2 RX ring set to 4096 slots (default is 256)

CPU0 : pktgen sending on eth1, line rate (1488137pps)
CPU1 : receive eth2 interrupts, packets dropped into raw netfilter table
to bypass upper stacks.

Before patch : 15% packet losses, ksoftirqd/1 using 100% of cpu
After patch : residual losses (less than 0.1 %), ksoftirqd not used, 80%
cpu used 

I'll do more tests with a 10Gb card (ixgbe driver) to not be wire
limited.



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