netdev - Re: [PATCH net-next] virtio_net: enable napi

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Message-ID: <8c0317a3-16e2-8246-a25d-3a3f9337edec@redhat.com>
Date:   Fri, 14 Jun 2019 15:01:22 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev@...r.kernel.org, davem@...emloft.net,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] virtio_net: enable napi_tx by default


On 2019/6/14 下午2:00, Michael S. Tsirkin wrote:
> On Fri, Jun 14, 2019 at 11:28:59AM +0800, Jason Wang wrote:
>> On 2019/6/14 上午12:24, Willem de Bruijn wrote:
>>> From: Willem de Bruijn <willemb@...gle.com>
>>>
>>> NAPI tx mode improves TCP behavior by enabling TCP small queues (TSQ).
>>> TSQ reduces queuing ("bufferbloat") and burstiness.
>>>
>>> Previous measurements have shown significant improvement for
>>> TCP_STREAM style workloads. Such as those in commit 86a5df1495cc
>>> ("Merge branch 'virtio-net-tx-napi'").
>>>
>>> There has been uncertainty about smaller possible regressions in
>>> latency due to increased reliance on tx interrupts.
>>>
>>> The above results did not show that, nor did I observe this when
>>> rerunning TCP_RR on Linux 5.1 this week on a pair of guests in the
>>> same rack. This may be subject to other settings, notably interrupt
>>> coalescing.
>>>
>>> In the unlikely case of regression, we have landed a credible runtime
>>> solution. Ethtool can configure it with -C tx-frames [0|1] as of
>>> commit 0c465be183c7 ("virtio_net: ethtool tx napi configuration").
>>>
>>> NAPI tx mode has been the default in Google Container-Optimized OS
>>> (COS) for over half a year, as of release M70 in October 2018,
>>> without any negative reports.
>>>
>>> Link: https://marc.info/?l=linux-netdev&m=149305618416472
>>> Link: https://lwn.net/Articles/507065/
>>> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>>>
>>> ---
>>>
>>> now that we have ethtool support and real production deployment,
>>> it seemed like a good time to revisit this discussion.
>>
>> I agree to enable it by default. Need inputs from Michael. One possible
>> issue is we may get some regression on the old machine without APICV, but
>> consider most modern CPU has this feature, it probably doesn't matter.
>>
>> Thanks
>>
> Right. If the issue does arise we can always add e.g. a feature flag
> to control the default from the host.
>

Yes.

So

Acked-by: Jason Wang <jasowang@...hat.com>