netdev - Re: Netperf UDP issue with connected sockets

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Message-ID: <20161117144248.23500001@redhat.com>
Date:   Thu, 17 Nov 2016 14:42:48 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Rick Jones <rick.jones2@....com>, netdev@...r.kernel.org,
        brouer@...hat.com
Subject: Re: Netperf UDP issue with connected sockets

On Thu, 17 Nov 2016 05:20:50 -0800
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Thu, 2016-11-17 at 09:16 +0100, Jesper Dangaard Brouer wrote:
> 
> > 
> > I noticed there is a Send-Q, and the perf-top2 is _raw_spin_lock, which
> > looks like it comes from __dev_queue_xmit(), but we know from
> > experience that this stall is actually caused by writing the
> > tailptr/doorbell in the HW.  Thus, this could benefit a lot from
> > bulk/xmit_more into the qdisc layer.  
> 
> The Send-Q is there because of TX-completions being delayed a bit,
> because of IRQ mitigation.
> 
> (ethtool -c eth0)
> 
> It happens even if you do not have a qdisc in the first place.
> 
> And we do have xmit_more in the qdisc layer already.

I can see that qdisc layer does not activate xmit_more in this case.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer


$ ethtool -c mlx5p4
Coalesce parameters for mlx5p4:
Adaptive RX: on  TX: off
stats-block-usecs: 0
sample-interval: 0
pkt-rate-low: 0
pkt-rate-high: 0

rx-usecs: 3
rx-frames: 32
rx-usecs-irq: 0
rx-frames-irq: 0

tx-usecs: 16
tx-frames: 32
tx-usecs-irq: 0
tx-frames-irq: 0

rx-usecs-low: 0
rx-frame-low: 0
tx-usecs-low: 0
tx-frame-low: 0

rx-usecs-high: 0
rx-frame-high: 0
tx-usecs-high: 0
tx-frame-high: 0