netdev - Re: [RFC] tcp: use order-3 pages in tcp

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Date:	Thu, 20 Sep 2012 07:37:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vijay Subramanian <subramanian.vijay@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] tcp: use order-3 pages in tcp_sendmsg()

On Wed, 2012-09-19 at 15:20 -0700, Vijay Subramanian wrote:
> > I did some tests and got no problem so far, even using splice() [ this
> > one was tricky because it only deals with order-0 pages at this moment ]
> >
> > NIC tested : ixgbe, igb, bnx2x, tg3, mellanox mlx4
> 
> 
> I applied this patch to net-next and tested with e1000e driver.
> With iperf I got around 8 % improvement on loopback.
> 
> Tested-by: Vijay Subramanian <subramanian.vijay@...il.com>
> 
> 
> Vijay

If you keep the producer and consumer on separate cpus, and use large
enough send() (64KB or 128KB), gain is more like 15 or 20%

iperf uses 8KB writes, while netperf uses a 16KB default.

TCP stack has a problem because /proc/sys/net/ipv4/tcp_reordering
default value (3) is too small for loopback, since a packet contains 4
MSS

A single reorder and some packets are retransmitted.

Following setting is better

echo 16 >/proc/sys/net/ipv4/tcp_reordering

loopback is lossless, so its always surprising we can have TCP
retransmits on this medium ;)

Thanks


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