Re: [PATCH RFC 11/11] net/mlx5e: XDP TX xmit more

On Wed, Sep 7, 2016 at 7:58 PM, John Fastabend [off-list ref] wrote:

On 16-09-07 11:22 AM, Jesper Dangaard Brouer wrote:

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On Wed, 7 Sep 2016 19:57:19 +0300 Saeed Mahameed [off-list ref] wrote:

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On Wed, Sep 7, 2016 at 6:32 PM, Eric Dumazet [off-list ref] wrote:

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On Wed, 2016-09-07 at 18:08 +0300, Saeed Mahameed wrote:

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On Wed, Sep 7, 2016 at 5:41 PM, Eric Dumazet [off-list ref] wrote:

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On Wed, 2016-09-07 at 15:42 +0300, Saeed Mahameed wrote:

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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.

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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.

BQL is not helping with that?

Tom

.John