From: John Fastabend <john.fastabend@gmail•com>
To: Sharat Masetty <sharat04@gmail•com>
Cc: Linux Netdev List <netdev@vger•kernel.org>,
David Miller <davem@davemloft•net>
Subject: Re: Multiqueue TX
Date: Mon, 28 Jul 2014 16:03:37 -0700 [thread overview]
Message-ID: <53D6D6C9.2070607@gmail.com> (raw)
In-Reply-To: <CAJzFV36cL9=qNcF-cb0OSx1A39FP2Gaa98HBqr9D35fQ3BfU=Q@mail.gmail.com>
On 07/28/2014 03:37 PM, Sharat Masetty wrote:
> Hi David,
>
> Can you please answer my question when you get a chance?
>
> Regards,
> Sharat
Little patience ;) Monday's are always a bit busy.
>
> On Sat, Jul 26, 2014 at 1:31 AM, Sharat Masetty <sharat04@gmail•com> wrote:
>> I have a question from this David's nice presentation
>> http://vger.kernel.org/~davem/davem_nyc09.pdf on multiqueue TX. My
>> questions are on the slide named "PICTURE WITH NON-TRIVIAL QDISC"
>>
>> 1) What is the rationale behind such an arrangement, where one would
>> have multiple TX queues pointing to the on single Qdisc?
The simplest example is to take a look at sch_tbf and try to work out
how we could rate limit a device. Here you have a single qdisc being
used for all TX queues. HTB as you note below is another example.
How would you propose creating rate limits across entire interfaces
without a single Qdisc.
Its ongoing work to create queuing disciplines that don't bottleneck
on the qdisc lock and distribute the state.
>> 2) If the Qdisc in this illustration was a non trivial Qdisc such as
>> htb, which has multiple leaf Qdisc's, how would the traffic shaping be
>> preserved?
Not sure I follow. Using HTB as a root qdisc on an interface works the
same if the interface has 1 queue or 128 queues.
Maybe you need to review netdev_pick_tx and follow skb->queue_mapping
through the device notice that it is not really used by the qdisc.
>> 3) How would the back pressure/flow control of these individual TX
>> queues work in this case?
To sort this out look into netif_stop_subqueue() and how the ethernet
drivers use this call. Then look at how it is used in ./net/sch_generic
by the dequeue_skb() call. I think it should be clear by reviewing this
usage how the back pressure works from the driver to the qdisc. Its
fairly straight forward and uses a bit to indicate to the qdisc stack
the driver can not consume more skbs.
>>
>> Thanks
>> Sharat
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--
John Fastabend Intel Corporation
prev parent reply other threads:[~2014-07-28 23:03 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-26 7:31 Multiqueue TX Sharat Masetty
2014-07-28 22:37 ` Sharat Masetty
2014-07-28 22:43 ` David Miller
2014-07-28 23:03 ` John Fastabend [this message]
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