netdev - Re: [PATCH net-next v2 2/5] virtio-net: transmit napi

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Date:   Fri, 21 Apr 2017 10:50:12 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v2 2/5] virtio-net: transmit napi

>>> Maybe I was wrong, but according to Michael's comment it looks like he
>>> want
>>> check affinity_hint_set just for speculative tx polling on rx napi
>>> instead
>>> of disabling it at all.
>>>
>>> And I'm not convinced this is really needed, driver only provide affinity
>>> hint instead of affinity, so it's not guaranteed that tx and rx interrupt
>>> are in the same vcpus.
>>
>> You're right. I made the restriction broader than the request, to really
>> err
>> on the side of caution for the initial merge of napi tx. And enabling
>> the optimization is always a win over keeping it off, even without irq
>> affinity.
>>
>> The cycle cost is significant without affinity regardless of whether the
>> optimization is used.
>
>
> Yes, I noticed this in the past too.
>
>> Though this is not limited to napi-tx, it is more
>> pronounced in that mode than without napi.
>>
>> 1x TCP_RR for affinity configuration {process, rx_irq, tx_irq}:
>>
>> upstream:
>>
>> 1,1,1: 28985 Mbps, 278 Gcyc
>> 1,0,2: 30067 Mbps, 402 Gcyc
>>
>> napi tx:
>>
>> 1,1,1: 34492 Mbps, 269 Gcyc
>> 1,0,2: 36527 Mbps, 537 Gcyc (!)
>> 1,0,1: 36269 Mbps, 394 Gcyc
>> 1,0,0: 34674 Mbps, 402 Gcyc
>>
>> This is a particularly strong example. It is also representative
>> of most RR tests. It is less pronounced in other streaming tests.
>> 10x TCP_RR, for instance:
>>
>> upstream:
>>
>> 1,1,1: 42267 Mbps, 301 Gcyc
>> 1,0,2: 40663 Mbps, 445 Gcyc
>>
>> napi tx:
>>
>> 1,1,1: 42420 Mbps, 303 Gcyc
>> 1,0,2:  42267 Mbps, 431 Gcyc
>>
>> These numbers were obtained with the virtqueue_enable_cb_delayed
>> optimization after xmit_skb, btw. It turns out that moving that before
>> increases 1x TCP_RR further to ~39 Gbps, at the cost of reducing
>> 100x TCP_RR a bit.
>
>
> I see, so I think we can leave the affinity hint optimization/check for
> future investigation:
>
> - to avoid endless optimization (e.g we may want to share a single
> vector/napi for tx/rx queue pairs in the future) for this series.
> - tx napi is disabled by default which means we can do optimization on top.

Okay. I'll drop the vi->affinity_hint_set from the patch set for now.