netdev - Re: [PATCH net-next 1/4] virtio-net: mergeable buffer size should include virtio-net header

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Date:	Tue, 12 Nov 2013 14:41:30 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michael Dalton <mwdalton@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Eric Northup <digitaleric@...gle.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next 1/4] virtio-net: mergeable buffer size should
 include virtio-net header

On Tue, 2013-11-12 at 14:21 -0800, Michael Dalton wrote:
> Commit 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page
> frag allocators") changed the mergeable receive buffer size from PAGE_SIZE
> to MTU-size. However, the merge buffer size does not take into account the
> size of the virtio-net header. Consequently, packets that are MTU-size
> will take two buffers intead of one (to store the virtio-net header),
> substantially decreasing the throughput of MTU-size traffic due to TCP
> window / SKB truesize effects.
> 
> This commit changes the mergeable buffer size to include the virtio-net
> header. The buffer size is cacheline-aligned because skb_page_frag_refill
> will not automatically align the requested size.
> 
> Benchmarks taken from an average of 5 netperf 30-second TCP_STREAM runs
> between two QEMU VMs on a single physical machine. Each VM has two VCPUs and
> vhost enabled. All VMs and vhost threads run in a single 4 CPU cgroup
> cpuset, using cgroups to ensure that other processes in the system will not
> be scheduled on the benchmark CPUs. Transmit offloads and mergeable receive
> buffers are enabled, but guest_tso4 / guest_csum are explicitly disabled to
> force MTU-sized packets on the receiver.
> 
> next-net trunk before 2613af0ed18a (PAGE_SIZE buf): 3861.08Gb/s
> net-next trunk (MTU 1500- packet uses two buf due to size bug): 4076.62Gb/s
> net-next trunk (MTU 1480- packet fits in one buf): 6301.34Gb/s
> net-next trunk w/ size fix (MTU 1500 - packet fits in one buf): 6445.44Gb/s
> 
> Suggested-by: Eric Northup <digitaleric@...gle.com>
> Signed-off-by: Michael Dalton <mwdalton@...gle.com>
> ---

Acked-by: Eric Dumazet <edumazet@...gle.com>


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