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

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Date:   Thu, 13 Jun 2019 12:24:57 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, jasowang@...hat.com, mst@...hat.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net-next] virtio_net: enable napi_tx by default

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.

---
 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