netdev - Re: TCP small packets throughput and multiqueue virtio-net

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Date:	Fri, 08 Mar 2013 09:26:14 -0800
From:	Rick Jones <rick.jones2@...com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>, KVM <kvm@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Stephen Hemminger <stephen@...workplumber.org>,
	jpirko@...hat.com
Subject: Re: TCP small packets throughput and multiqueue virtio-net


>
> Well, the point is : if your app does write(1024) bytes, thats probably
> because it wants small packets from the very beginning. (See the TCP
> PUSH flag ?)

I think that raises the question of whether or not Jason was setting the 
test-specific -D option on his TCP_STREAM tests, to have netperf make a 
setsockopt(TCP_NODELAY) call.

happy benchmarking,

rick jones

> If the transport is slow, TCP stack will automatically collapse several
> write into single skbs (assuming TSO or GSO is on), and you'll see big
> GSO packets with tcpdump [1]. So TCP will help you to get less overhead
> in this case.
>
> But if transport is fast, you'll see small packets, and thats good for
> latency.
>
> So my opinion is : its exactly behaving as expected.
>
> If you want bigger packets either :
> - Make the application doing big write()
> - Slow the vmexit ;)
>
> [1] GSO fools tcpdump : Actual packets sent to the wire are not 'big
> packets', but they hit dev_hard_start_xmit() as GSO packets.
>
>
>
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