netdev - Re: [net-next PATCH] dql: add a burst attribute

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Message-ID: <CAA93jw5Xz132yUT83NW3JsO80byH8o00h2pswjarUazqwSErRw@mail.gmail.com>
Date:	Tue, 30 Sep 2014 08:39:13 -0700
From:	Dave Taht <dave.taht@...il.com>
To:	Florian Westphal <fw@...len.de>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>
Subject: Re: [net-next PATCH] dql: add a burst attribute

On Tue, Sep 30, 2014 at 8:26 AM, Florian Westphal <fw@...len.de> wrote:
> Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On Tue, 2014-09-30 at 15:46 +0200, Florian Westphal wrote:
>> > Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > > From: Eric Dumazet <edumazet@...gle.com>
>> > >
>> > > Add a new dql attribute, to control how much packets we are allowed to
>> > > burst from qdisc to device.
>> >
>> > I understand the motivation for this, but I find it a bit out-of-place
>> > to have a 'packet' type counter in bql context?
>> >
>> > Would it perhaps make more sense to restrict bulk dequeues by an upper
>> > (possibly changeable from userspace) byte counter limit?
>>
>> The byte count is already provided : its the BQL limit.
>> We already have ways to tune it (limit_min & limit_max)
>> We do not think we need something else here.
>
> So you're saying that a bulk dequeue should just grab as many skbs
> as possible until no more available or dql_avail exhausted?
>
> The "magic" value was just to be conservative and not induce any
> hol blocking, which is also why Jesper reduced it again in the latest
> submission.
>
> Then, we might later be able to remove the TSO restriction and switch
> to a pure byte-based limit.
>
> (I don't think having a packet-based count makes sense once tso/gso
>  skbs can be bulk dequeued).
>
> Sorry for the confusion.

I still have some hope that we can one day fix wifi packet aggregation,
which is a limit of 42 packets or 64k bytes per aggregate (best case),
with something BQL-like.

As for the size of the tx ring problem vs GSO, there is at least one
driver with a very limited tx ring that I can think of that tears apart
GSO packets in the driver... but I'm not sure if it's mainlined.

I would not mind at all if TSO/GSO were banned on devices running
slower than 100Mbit.

Perhaps exporting the tx-ring size would be saner than a burst
parameter?

-- 
Dave Täht

https://www.bufferbloat.net/projects/make-wifi-fast
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