netdev - Re: [RFC net-next PATCH V3 0/2] qdisc bulk dequeuing and utilizing delayed tailptr updates

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Date:	Fri, 19 Sep 2014 13:59:20 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Daniel Borkmann <dborkman@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Dave Taht <dave.taht@...il.com>, toke@...e.dk
Subject: Re: [RFC net-next PATCH V3 0/2] qdisc bulk dequeuing and utilizing
 delayed tailptr updates

On Fri, 2014-09-19 at 22:49 +0200, Jesper Dangaard Brouer wrote:
> This patchset uses DaveM's recent API changes to dev_hard_start_xmit(),
> from the qdisc layer, to implement dequeue bulking.
> 
> RFC V3: Keeping the ball rolling.
> 
> This patchset should now use BQL correctly.  I've done lots of testing
> for Head-of-Line blocking issues that can occur due to requeue of a
> SKB bulk list.  I've not been able to provoke any HoL blocking
> situation, simply because BQL is doing such a good job, thus I'm
> unable to "overshoot" HW/BQL limits with more than a single packet.
> 
> This patch chooses a very conservative approach, as by default only
> allowing dequeue of one extra packet, besides the normal dequeue.
> 
> Open questions:
> 
> - How do we expose tuning to userspace?
>   Patch adds /proc/sys/net/core/qdisc_bulk_dequeue_limit but I don't like it...
>   Per device tunable?
> 

bql is using /sys, of course ;)

# grep . /sys/class/net/eth1/queues/tx-0/byte_queue_limits/*
/sys/class/net/eth1/queues/tx-0/byte_queue_limits/hold_time:1000
/sys/class/net/eth1/queues/tx-0/byte_queue_limits/inflight:0
/sys/class/net/eth1/queues/tx-0/byte_queue_limits/limit:113314
/sys/class/net/eth1/queues/tx-0/byte_queue_limits/limit_max:1879048192
/sys/class/net/eth1/queues/tx-0/byte_queue_limits/limit_min:0

Maybe you could simply reuse byte_queue_limits/limit, I am not sure we
need a specific tunable.


> - Can/should we limit dequeue bulking to devices supporting BQL?
> 

Yes please. This will be an incentive to get BQL on drivers.



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