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

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Date:	Sun, 28 Sep 2014 13:49:21 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Eric Dumazet <eric.dumazet@...il.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, Sep 28, 2014 at 11:52 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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

I see. So xmit_more=true overrides blueflame=on settings.
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 :)

> Is it making sense now ? ;)

It did, but only after studying this BlueFlame thingy ;)

Thanks!
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