On Wed, 7 Sep 2016 20:21:24 -0700 Tom Herbert <t...@herbertland.com> wrote:
> On Wed, Sep 7, 2016 at 7:58 PM, John Fastabend <john.fastab...@gmail.com> > wrote: > > On 16-09-07 11:22 AM, Jesper Dangaard Brouer wrote: > >> > >> On Wed, 7 Sep 2016 19:57:19 +0300 Saeed Mahameed > >> <sae...@dev.mellanox.co.il> wrote: > >>> On Wed, Sep 7, 2016 at 6:32 PM, Eric Dumazet <eric.duma...@gmail.com> > >>> wrote: > >>>> On Wed, 2016-09-07 at 18:08 +0300, Saeed Mahameed wrote: > >>>>> On Wed, Sep 7, 2016 at 5:41 PM, Eric Dumazet <eric.duma...@gmail.com> > >>>>> wrote: > >>>>>> On Wed, 2016-09-07 at 15:42 +0300, Saeed Mahameed wrote: > >> [...] > >>>> > >>>> Only if a qdisc is present and pressure is high enough. > >>>> > >>>> But in a forwarding setup, we likely receive at a lower rate than the > >>>> NIC can transmit. > >> > >> Yes, I can confirm this happens in my experiments. > >> > >>>> > >>> > >>> Jesper has a similar Idea to make the qdisc think it is under > >>> pressure, when the device TX ring is idle most of the time, i think > >>> his idea can come in handy here. I am not fully involved in the > >>> details, maybe he can elaborate more. > >>> > >>> But if it works, it will be transparent to napi, and xmit more will > >>> happen by design. > >> > >> Yes. I have some ideas around getting more bulking going from the qdisc > >> layer, by having the drivers provide some feedback to the qdisc layer > >> indicating xmit_more should be possible. This will be a topic at the > >> Network Performance Workshop[1] at NetDev 1.2, I have will hopefully > >> challenge people to come up with a good solution ;-) > >> > > > > One thing I've noticed but haven't yet actually analyzed much is if > > I shrink the nic descriptor ring size to only be slightly larger than > > the qdisc layer bulking size I get more bulking and better perf numbers. > > At least on microbenchmarks. The reason being the nic pushes back more > > on the qdisc. So maybe a case for making the ring size in the NIC some > > factor of the expected number of queues feeding the descriptor ring. > > I've also played with shrink the NIC descriptor ring size, it works, but it is an ugly hack to get NIC pushes backs, and I foresee it will hurt normal use-cases. (There are other reasons for shrinking the ring size like cache usage, but that is unrelated to this). > BQL is not helping with that? Exactly. But the BQL _byte_ limit is not what is needed, what we need to know is the _packets_ currently "in-flight". Which Tom already have a patch for :-) Once we have that the algorithm is simple. Qdisc dequeue look at BQL pkts-in-flight, if driver have "enough" packets in-flight, the qdisc start it's bulk dequeue building phase, before calling the driver. The allowed max qdisc bulk size should likely be related to pkts-in-flight. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer
