netdev - Re: [PATCH net-next] net: allocate skbs on local node

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Message-Id: <20101016.115413.104038999.davem@davemloft.net>
Date:	Sat, 16 Oct 2010 11:54:13 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, mchan@...adcom.com, eilong@...adcom.com,
	akpm@...ux-foundation.org, hch@....de, cl@...ux-foundation.org
Subject: Re: [PATCH net-next] net: allocate skbs on local node

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 12 Oct 2010 07:05:25 +0200

> [PATCH net-next] net: allocate skbs on local node
> 
> commit b30973f877 (node-aware skb allocation) spread a wrong habit of
> allocating net drivers skbs on a given memory node : The one closest to
> the NIC hardware. This is wrong because as soon as we try to scale
> network stack, we need to use many cpus to handle traffic and hit
> slub/slab management on cross-node allocations/frees when these cpus
> have to alloc/free skbs bound to a central node.
> 
> skb allocated in RX path are ephemeral, they have a very short
> lifetime : Extra cost to maintain NUMA affinity is too expensive. What
> appeared as a nice idea four years ago is in fact a bad one.
> 
> In 2010, NIC hardwares are multiqueue, or we use RPS to spread the load,
> and two 10Gb NIC might deliver more than 28 million packets per second,
> needing all the available cpus.
> 
> Cost of cross-node handling in network and vm stacks outperforms the
> small benefit hardware had when doing its DMA transfert in its 'local'
> memory node at RX time. Even trying to differentiate the two allocations
> done for one skb (the sk_buff on local node, the data part on NIC
> hardware node) is not enough to bring good performance.
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>

Applied.
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