netdev - Re: [net-next PATCH V5] qdisc: bulk dequeue support for qdiscs with TCQ_F

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Date:	Wed, 1 Oct 2014 19:28:40 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Jamal Hadi Salim <jhs@...atatu.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Daniel Borkmann <dborkman@...hat.com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Dave Taht <dave.taht@...il.com>,
	Toke Høiland-Jørgensen 
	<toke@...e.dk>, brouer@...hat.com
Subject: Re: [net-next PATCH V5] qdisc: bulk dequeue support for qdiscs with
 TCQ_F_ONETXQUEUE

On Wed, 01 Oct 2014 11:34:55 -0400 Jamal Hadi Salim <jhs@...atatu.com> wrote:

> The issue is: if i am going to attempt to do a bulk transfer
> every single time (with new code) and for the common use case the
> result is "no need to do bulking" then you just added extra code that
> is unnecessary for that common case.
> Even a single extra if statement at high packet rate is still
> costly and would be easy to observe.
[...]
>
> That is if the added code ends up being hit
> meaningfully. Jesper said (and it was my experience as well)
> that it was _hard_ to achieve bulking in such a case.

Let me close this one...

The bulk dequeue code *is* being hit meaningfully.  It is _only_ going
to be activated when (q->qlen > 0) see __dev_xmit_skb() in dev.c.
When there is no queue pfifo_fast is allowed to directly call
sch_direct_xmit() without a enqueue/dequeue cycle.  Thus, this code
would not be hit for every packet.

Thus, code is activated only when q->qlen is >= 1. And I have already
shown that we see a win with just bulking 2 packets:
 "Subject: qdisc/UDP_STREAM: measuring effect of qdisc bulk dequeue"
 http://thread.gmane.org/gmane.linux.network/331152/focus=331154

I actually believe, that I have already measured and shown that going
though the bulk dequeue is a win, this should show direct_xmit vs.
bulking:
 "Subject: qdisc/trafgen: Measuring effect of qdisc bulk dequeue, with trafgen"
 http://thread.gmane.org/gmane.linux.network/331152

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