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. .John
