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

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Message-ID: <20190614013449-mutt-send-email-mst@kernel.org>
Date:   Fri, 14 Jun 2019 01:35:00 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jasowang@...hat.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] virtio_net: enable napi_tx by default

On Thu, Jun 13, 2019 at 12:24:57PM -0400, 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>


Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
> 
> now that we have ethtool support and real production deployment,
> it seemed like a good time to revisit this discussion.
> 
> ---
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 0d4115c9e20b..4f3de0ac8b0b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -26,7 +26,7 @@
>  static int napi_weight = NAPI_POLL_WEIGHT;
>  module_param(napi_weight, int, 0444);
>  
> -static bool csum = true, gso = true, napi_tx;
> +static bool csum = true, gso = true, napi_tx = true;
>  module_param(csum, bool, 0444);
>  module_param(gso, bool, 0444);
>  module_param(napi_tx, bool, 0644);
> -- 
> 2.22.0.rc2.383.gf4fbbf30c2-goog