From: Willem de Bruijn <willemb@xxxxxxxxxx>
Add napi for virtio-net transmit completion processing.
Changes:
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 28985 34492
Gcycles 278 269
10x:
Mbps 42267 42420
Gcycles 301 303
100x:
Mbps 29710 31131
Gcycles 286 282
TCP_RR Latency (us):
1x:
p50 22 21
p99 27 26
Gcycles 176 171
10x:
p50 40 40
p99 51 51
Gcycles 215 211
100x:
p50 272 235
p99 414 360
Gcycles 228 233
TCP_RR 4K:
1x:
p50 27 29
p99 34 36
Gcycles 184 170
10x:
p50 67 66
p99 82 82
Gcycles 217 218
100x:
p50 435 485
p99 813 626
Gcycles 242 226
UDP_STREAM:
1x:
Mbps 29802 25600
Gcycles 305 295
10x:
Mbps 29901 29684
Gcycles 287 295
100x:
Mbps 29952 29863
Gcycles 336 325
UDP_RR:
1x:
p50 18 18
p99 23 22
Gcycles 183 181
10x:
p50 35 38
p99 54 62
Gcycles 252 236
100x:
p50 240 279
p99 462 519
Gcycles 228 219
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 | 211 +++++++++++++++++++++++++++++++++--------------
1 file changed, 150 insertions(+), 61 deletions(-)
--
2.12.2.816.g2cccc81164-goog
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