virtio_net: enable napi_tx by default
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 86a5df14 ("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 0c465be1 ("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: NWillem de Bruijn <willemb@google.com> Acked-by: NMichael S. Tsirkin <mst@redhat.com> Acked-by: NJason Wang <jasowang@redhat.com> Signed-off-by: NDavid S. Miller <davem@davemloft.net>
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