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Message-ID: <20170425023512-mutt-send-email-mst@kernel.org>
Date: Tue, 25 Apr 2017 02:35:18 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, jasowang@...hat.com,
virtualization@...ts.linux-foundation.org, davem@...emloft.net,
Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v3 0/5] virtio-net tx napi
On Mon, Apr 24, 2017 at 01:49:25PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
>
> Add napi for virtio-net transmit completion processing.
Acked-by: Michael S. Tsirkin <mst@...hat.com>
> Changes:
> v2 -> v3:
> - convert __netif_tx_trylock to __netif_tx_lock on tx napi poll
> ensure that the handler always cleans, to avoid deadlock
> - unconditionally clean in start_xmit
> avoid adding an unnecessary "if (use_napi)" branch
> - remove virtqueue_disable_cb in patch 5/5
> a noop in the common event_idx based loop
> - document affinity_hint_set constraint
>
> v1 -> v2:
> - disable by default
> - disable unless affinity_hint_set
> because cache misses add up to a third higher cycle cost,
> e.g., in TCP_RR tests. This is not limited to the patch
> that enables tx completion cleaning in rx napi.
> - use trylock to avoid contention between tx and rx napi
> - keep interrupts masked during xmit_more (new patch 5/5)
> this improves cycles especially for multi UDP_STREAM, which
> does not benefit from cleaning tx completions on rx napi.
> - move free_old_xmit_skbs (new patch 3/5)
> to avoid forward declaration
>
> not changed:
> - deduplicate virnet_poll_tx and virtnet_poll_txclean
> they look similar, but have differ too much to make it
> worthwhile.
> - delay netif_wake_subqueue for more than 2 + MAX_SKB_FRAGS
> evaluated, but made no difference
> - patch 1/5
>
> RFC -> v1:
> - dropped vhost interrupt moderation patch:
> not needed and likely expensive at light load
> - remove tx napi weight
> - always clean all tx completions
> - use boolean to toggle tx-napi, instead
> - only clean tx in rx if tx-napi is enabled
> - then clean tx before rx
> - fix: add missing braces in virtnet_freeze_down
> - testing: add 4KB TCP_RR + UDP test results
>
> Based on previous patchsets by Jason Wang:
>
> [RFC V7 PATCH 0/7] enable tx interrupts for virtio-net
> http://lkml.iu.edu/hypermail/linux/kernel/1505.3/00245.html
>
>
> Before commit b0c39dbdc204 ("virtio_net: don't free buffers in xmit
> ring") the virtio-net driver would free transmitted packets on
> transmission of new packets in ndo_start_xmit and, to catch the edge
> case when no new packet is sent, also in a timer at 10HZ.
>
> A timer can cause long stalls. VIRTIO_F_NOTIFY_ON_EMPTY avoids stalls
> due to low free descriptor count. It does not address a stalls due to
> low socket SO_SNDBUF. Increasing timer frequency decreases that stall
> time, but increases interrupt rate and, thus, cycle count.
>
> Currently, with no timer, packets are freed only at ndo_start_xmit.
> Latency of consume_skb is now unbounded. To avoid a deadlock if a sock
> reaches SO_SNDBUF, packets are orphaned on tx. This breaks TCP small
> queues.
>
> Reenable TCP small queues by removing the orphan. Instead of using a
> timer, convert the driver to regular tx napi. This does not have the
> unresolved stall issue and does not have any frequency to tune.
>
> By keeping interrupts enabled by default, napi increases tx
> interrupt rate. VIRTIO_F_EVENT_IDX avoids sending an interrupt if
> one is already unacknowledged, so makes this more feasible today.
> Combine that with an optimization that brings interrupt rate
> back in line with the existing version for most workloads:
>
> Tx completion cleaning on rx interrupts elides most explicit tx
> interrupts by relying on the fact that many rx interrupts fire.
>
> Tested by running {1, 10, 100} {TCP, UDP} STREAM, RR, 4K_RR benchmarks
> from a guest to a server on the host, on an x86_64 Haswell. The guest
> runs 4 vCPUs pinned to 4 cores. vhost and the test server are
> pinned to a core each.
>
> All results are the median of 5 runs, with variance well < 10%.
> Used neper (github.com/google/neper) as test process.
>
> Napi increases single stream throughput, but increases cycle cost.
> The optimizations bring this down. The previous patchset saw a
> regression with UDP_STREAM, which does not benefit from cleaning tx
> interrupts in rx napi. This regression is now gone for 10x, 100x.
> Remaining difference is higher 1x TCP_STREAM, lower 1x UDP_STREAM.
>
> The latest results are with process, rx napi and tx napi affine to
> the same core. All numbers are lower than the previous patchset.
>
>
> upstream napi
> TCP_STREAM:
> 1x:
> Mbps 27816 39805
> Gcycles 274 285
>
> 10x:
> Mbps 42947 42531
> Gcycles 300 296
>
> 100x:
> Mbps 31830 28042
> Gcycles 279 269
>
> TCP_RR Latency (us):
> 1x:
> p50 21 21
> p99 27 27
> Gcycles 180 167
>
> 10x:
> p50 40 39
> p99 52 52
> Gcycles 214 211
>
> 100x:
> p50 281 241
> p99 411 337
> Gcycles 218 226
>
> TCP_RR 4K:
> 1x:
> p50 28 29
> p99 34 36
> Gcycles 177 167
>
> 10x:
> p50 70 71
> p99 85 134
> Gcycles 213 214
>
> 100x:
> p50 442 611
> p99 802 785
> Gcycles 237 216
>
> UDP_STREAM:
> 1x:
> Mbps 29468 26800
> Gcycles 284 293
>
> 10x:
> Mbps 29891 29978
> Gcycles 285 312
>
> 100x:
> Mbps 30269 30304
> Gcycles 318 316
>
> UDP_RR:
> 1x:
> p50 19 19
> p99 23 23
> Gcycles 180 173
>
> 10x:
> p50 35 40
> p99 54 64
> Gcycles 245 237
>
> 100x:
> p50 234 286
> p99 484 473
> Gcycles 224 214
>
> Note that GSO is enabled, so 4K RR still translates to one packet
> per request.
>
> Lower throughput at 100x vs 10x can be (at least in part)
> explained by looking at bytes per packet sent (nstat). It likely
> also explains the lower throughput of 1x for some variants.
>
> upstream:
>
> N=1 bytes/pkt=16581
> N=10 bytes/pkt=61513
> N=100 bytes/pkt=51558
>
> at_rx:
>
> N=1 bytes/pkt=65204
> N=10 bytes/pkt=65148
> N=100 bytes/pkt=56840
>
> Willem de Bruijn (5):
> virtio-net: napi helper functions
> virtio-net: transmit napi
> virtio-net: move free_old_xmit_skbs
> virtio-net: clean tx descriptors from rx napi
> virtio-net: keep tx interrupts disabled unless kick
>
> drivers/net/virtio_net.c | 193 ++++++++++++++++++++++++++++++++---------------
> 1 file changed, 132 insertions(+), 61 deletions(-)
>
> --
> 2.12.2.816.g2cccc81164-goog
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