netdev - Re: Quota in __qdisc

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Message-ID: <20141007200329.5d20a27e@redhat.com>
Date:	Tue, 7 Oct 2014 20:03:29 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, hannes@...essinduktion.org,
	netdev@...r.kernel.org, therbert@...gle.com, fw@...len.de,
	dborkman@...hat.com, jhs@...atatu.com, alexander.duyck@...il.com,
	john.r.fastabend@...el.com, dave.taht@...il.com, toke@...e.dk,
	brouer@...hat.com
Subject: Re: Quota in __qdisc_run()

On Tue, 07 Oct 2014 13:19:38 -0400 (EDT)
David Miller <davem@...emloft.net> wrote:

> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Tue, 07 Oct 2014 08:01:20 -0700
> 
> > On Tue, 2014-10-07 at 16:43 +0200, Hannes Frederic Sowa wrote:
> > 
> >> This needs to be:
> >> 
> >> do
> >>    ...
> >> while ((iskb = iskb->next))
> > 
> > I do not feel needed to break the bulk dequeue at precise quota
> > boundary. These quotas are advisory, and bql prefers to get its full
> > budget for appropriate feedback from TX completion.
> > 
> > Quota was a packet quota, which was quite irrelevant if segmentation had
> > to be done, so I would just let the dequeue be done so that we benefit
> > from optimal xmit_more.
> 
> Yes, this makes sense, do a full qdisc_restart() cycle without boundaries,
> then check how much quota was used afterwards to guard the outermost loop.

According to my measurements, at 10Gbit/s TCP_STREAM test the BQL limit
is 381528 bytes / 1514 = 252 packets, that will (potentially) be bulk
dequeued at once (with your version of the patch).

It seems to have the potential to exceed the weight_p(64) quite a lot.
And with e.g. TX ring size 512, we also also challenge the drivers at
this early adoption phase of tailptr writes.  Just saying...

-- 
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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