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Date: Mon, 28 Jul 2014 16:03:37 -0700 From: John Fastabend <john.fastabend@...il.com> To: Sharat Masetty <sharat04@...il.com> CC: Linux Netdev List <netdev@...r.kernel.org>, David Miller <davem@...emloft.net> Subject: Re: Multiqueue TX 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@...il.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 > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- John Fastabend Intel Corporation -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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