netdev - RE: [PATCH net-next] net: introduce netdevice gso_min

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Date:	Mon, 6 Oct 2014 10:20:08 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Amir Vadai <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	Eric Dumazet <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	"Or Gerlitz" <ogerlitz@...lanox.com>,
	Ido Shamay <idos@...lanox.com>
Subject: RE: [PATCH net-next] net: introduce netdevice gso_min_segs attribute

From: Eric Dumazet <edumazet@...gle.com>
> Some TSO engines might have a too heavy setup cost, that impacts
> performance on hosts sending small bursts (2 MSS per packet).
> 
> This patch adds a device gso_min_segs, allowing drivers to set
> a minimum segment size for TSO packets, according to the NIC
> performance.
> 
> Tested on a mlx4 NIC, this allows to get a ~110% increase of
> throughput when sending 2 MSS per packet.

Doesn't this all depend on what you need to optimise for.
I can think of three^Wseveral main cases:
1) minimising cpu use while saturating the local network.
2) minimising latency for single packets.
3) maximising throughput for a single connection.
4) minimising cpu use when handling a large number of connections.
plus all the variations in packet size.

I'm not sure what you are trading for what here.

Maybe gso = tx_bursting is almost always faster on some hardware?
(Especially if an idle mac engine is 'kicked' for the first packet
of a burst.)

	David