Re: [PATCH] virtio-net: mergeable buffer size should include virtio-net header

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Thu, 14 Nov 2013 10:59:12 -0800

> On Thu, 2013-11-14 at 10:41 -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@google.com>
>> Signed-off-by: Michael Dalton <mwdalton@google.com>
>> ---
> 
> Acked-by: Eric Dumazet <edumazet@google.com>

Applied, thanks.