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

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Date:	Sun, 28 Sep 2014 11:52:44 -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 11:07 -0700, Alexei Starovoitov wrote:

> this is great improvement!
> Thank you for leading this effort.
> 10G line rate is definitely nice :)
> Hopefully Or can demo similar numbers with 40G nic as well :)
> 
> > +       if (ring->bf_enabled && desc_size <= MAX_BF && !bounce &&
> > +           !vlan_tx_tag_present(skb) && send_doorbell) {
> 
> feels something wrong here, since it checks for
> send_doorbell, but iowrite() happens in the 'else' part
> of this branch with another 'if (send_doorbell)'
> 

If we do not plan to send a doorbell, we should not use blueframe.

Blueframe is always sending a doorbell by design, as it uses a single
flip buffer.

So if you want to see any improvement thanks to skb->xmit_more, we have
to use the iowrite32(), not the blueframe.

Therefore , if send_doorbell == false, we do not want the doorbell

Is it making sense now ? ;)


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