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

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Message-ID: <CALx6S34vqa+13c_4m0CKqL_6U+37DQ15T=WRAdUZNL3iYo1J0g@mail.gmail.com>
Date:   Wed, 7 Sep 2016 20:21:24 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Saeed Mahameed <saeedm@....mellanox.co.il>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        iovisor-dev <iovisor-dev@...ts.iovisor.org>,
        Linux Netdev List <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH RFC 11/11] net/mlx5e: XDP TX xmit more

On Wed, Sep 7, 2016 at 7:58 PM, John Fastabend <john.fastabend@...il.com> wrote:
> On 16-09-07 11:22 AM, Jesper Dangaard Brouer wrote:
>>
>> On Wed, 7 Sep 2016 19:57:19 +0300 Saeed Mahameed <saeedm@....mellanox.co.il> wrote:
>>> On Wed, Sep 7, 2016 at 6:32 PM, Eric Dumazet <eric.dumazet@...il.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.dumazet@...il.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.
>

BQL is not helping with that?

Tom

> .John