netdev - Re: [WIP] net+mlx4: auto doorbell

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Date:   Wed, 30 Nov 2016 14:40:17 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Rick Jones <rick.jones2@....com>, netdev@...r.kernel.org,
        Saeed Mahameed <saeedm@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Achiad Shochat <achiad@...lanox.com>
Subject: Re: [WIP] net+mlx4: auto doorbell

On Wed, 2016-11-30 at 23:30 +0100, Jesper Dangaard Brouer wrote:
> On Wed, 30 Nov 2016 11:30:00 -0800
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > On Wed, 2016-11-30 at 20:17 +0100, Jesper Dangaard Brouer wrote:
> > 
> > > Don't take is as critique Eric.  I was hoping your patch would have
> > > solved this issue of being sensitive to TX completion adjustments.  You
> > > usually have good solutions for difficult issues. I basically rejected
> > > Achiad's approach/patch because it was too sensitive to these kind of
> > > adjustments.  
> > 
> > Well, this patch can hurt latencies, because a doorbell can be delayed,
> > and softirqs can be delayed by many hundred of usec in some cases.
> > 
> > I would not enable this behavior by default.
> 
> What about another scheme, where dev_hard_start_xmit() can return an
> indication that driver choose not to flush (based on TX queue depth),
> and there by requesting stack to call flush at a later point.
> 
> Would that introduce less latency issues?


Again, how is it going to change anything when your netperf UDP sends
one packet at a time ?

qdisc gets one packet , dequeue it immediately.

No pressure. -> doorbell will be sent as before.

Some TX producers do not even use a qdisc to begin with.

Please think of a solution that does not involve qdisc layer at all.