netdev - Re: [PATCH net-next] mlx4: optimize xmit path

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Date:	Sun, 28 Sep 2014 19:22:11 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Or Gerlitz <gerlitz.or@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Amir Vadai <amirv@...lanox.com>,
	Or Gerlitz <or.gerlitz@...il.com>
Subject: Re: [PATCH net-next] mlx4: optimize xmit path

On Sun, 2014-09-28 at 13:49 -0700, Alexei Starovoitov wrote:

> I see. So xmit_more=true overrides blueflame=on settings.

Yes, unless Mellanox folks have another way.

> I wonder what is the performance difference bf=on vs bf=off,
> also whether a burst of N packets via bf is slower than
> burst via queue+doorbell.
> Some fun exploration for driver experts :)

Prior situation : bf=on : ~4.5 Mpps

queue + doorbell every 8 packets : ~8 Mpps, up to 10Mpps if tuned
properly.

Rewritten mlx4 tx path and no burst (bf=on doorbell at every packet) :
5.3 Mpps

With the full mlx4 patch and burst = 8 -> 14.9 Mpps

This is on a 40Gb NIC, with 108 bytes packets.

(Using <= 104 bytes packets actually gives lower pps because of
'inlining' done by the driver :  8 Mpps for PKTSIZE=40 )

# cat /sys/module/mlx4_en/parameters/inline_thold
104



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