[PATCH 0/9][RFC] KVM virtio_net performance

[Mark McLoughlin <markmc@redhat.com>]

Hey,
      Here's a bunch of patches attempting to improve the performance
of virtio_net. This is more an RFC rather than a patch submission
since, as can be seen below, not all patches actually improve the
perfomance measurably.

      I've tried hard to test each of these patches with as stable and
informative a benchmark as I could find. The first benchmark is a
netperf[1] based throughput benchmark and the second uses a flood
ping[2] to measure latency differences.

      Each set of figures is min/average/max/standard deviation. The
first set is Gb/s and the second is milliseconds.

      The network configuration used was very simple - the guest with
a virtio_net interface and the host with a tap interface and static
IP addresses assigned to both - e.g. there was no bridge in the host
involved and iptables was disable in both the host and guest.

      I used:

  1) kvm-71-26-g6152996 with the patches that follow

  2) Linus's v2.6.26-5752-g93ded9b with Rusty's virtio patches from
     219:bbd2611289c5 applied; these are the patches have just been
     submitted to Linus

      The conclusions I draw are:

  1) The length of the tx mitigation timer makes quite a difference to
     throughput achieved; we probably need a good heuristic for
     adjusting this on the fly.

  2) Using the recently merged GSO support in the tun/tap driver gives
     a huge boost, but much more so on the host->guest side.

  3) Adjusting the virtio_net ring sizes makes a small difference, but
     not as much as one might expect

  4) Dropping the global mutex while reading GSO packets from the tap
     interface gives a nice speedup. This highlights the global mutex
     as a general perfomance issue.

  5) Eliminating an extra copy on the host->guest path only makes a
     barely measurable difference.

        Anyway, the figures:

  netperf, 10x20s runs (Gb/s)  |       guest->host          |       host->guest
  -----------------------------+----------------------------+---------------------------
  baseline                     | 1.520/ 1.573/ 1.610/ 0.034 | 1.160/ 1.357/ 1.630/ 0.165
  50us tx timer + rearm        | 1.050/ 1.086/ 1.110/ 0.017 | 1.710/ 1.832/ 1.960/ 0.092
  250us tx timer + rearm       | 1.700/ 1.764/ 1.880/ 0.064 | 0.900/ 1.203/ 1.580/ 0.205
  150us tx timer + rearm       | 1.520/ 1.602/ 1.690/ 0.044 | 1.670/ 1.928/ 2.150/ 0.141
  no ring-full heuristic       | 1.480/ 1.569/ 1.710/ 0.066 | 1.610/ 1.857/ 2.140/ 0.153
  VIRTIO_F_NOTIFY_ON_EMPTY     | 1.470/ 1.554/ 1.650/ 0.054 | 1.770/ 1.960/ 2.170/ 0.119
  recv NO_NOTIFY               | 1.530/ 1.604/ 1.680/ 0.047 | 1.780/ 1.944/ 2.190/ 0.129
  GSO                          | 4.120/ 4.323/ 4.420/ 0.099 | 6.540/ 7.033/ 7.340/ 0.244
  ring size == 256             | 4.050/ 4.406/ 4.560/ 0.143 | 6.280/ 7.236/ 8.280/ 0.613
  ring size == 512             | 4.420/ 4.600/ 4.960/ 0.140 | 6.470/ 7.205/ 7.510/ 0.314
  drop mutex during tapfd read | 4.320/ 4.578/ 4.790/ 0.161 | 8.370/ 8.589/ 8.730/ 0.120
  aligouri zero-copy           | 4.510/ 4.694/ 4.960/ 0.148 | 8.430/ 8.614/ 8.840/ 0.142

  ping -f -c 100000 (ms)       |       guest->host          |       host->guest
  -----------------------------+----------------------------+---------------------------
  baseline                     | 0.060/ 0.459/ 7.602/ 0.846 | 0.067/ 0.331/ 2.517/ 0.057
  50us tx timer + rearm        | 0.081/ 0.143/ 7.436/ 0.374 | 0.093/ 0.133/ 1.883/ 0.026
  250us tx timer + rearm       | 0.302/ 0.463/ 7.580/ 0.849 | 0.297/ 0.344/ 2.128/ 0.028
  150us tx timer + rearm       | 0.197/ 0.323/ 7.671/ 0.740 | 0.199/ 0.245/ 7.836/ 0.037
  no ring-full heuristic       | 0.182/ 0.324/ 7.688/ 0.753 | 0.199/ 0.243/ 2.197/ 0.030
  VIRTIO_F_NOTIFY_ON_EMPTY     | 0.197/ 0.321/ 7.447/ 0.730 | 0.196/ 0.242/ 2.218/ 0.032
  recv NO_NOTIFY               | 0.186/ 0.321/ 7.520/ 0.732 | 0.200/ 0.233/ 2.216/ 0.028
  GSO                          | 0.178/ 0.324/ 7.667/ 0.736 | 0.147/ 0.246/ 1.361/ 0.024
  ring size == 256             | 0.184/ 0.323/ 7.674/ 0.728 | 0.199/ 0.243/ 2.181/ 0.028
  ring size == 512             |             (not measured) |             (not measured)
  drop mutex during tapfd read | 0.183/ 0.323/ 7.820/ 0.733 | 0.202/ 0.242/ 2.219/ 0.027
  aligouri zero-copy           | 0.185/ 0.325/ 7.863/ 0.736 | 0.202/ 0.245/ 7.844/ 0.036

Cheers,
Mark.

[1] - I used netperf trunk from:

  http://www.netperf.org/svn/netperf2/trunk

      and simply ran:

  $> i=0; while [ $i -lt 10 ]; do ./netperf -H <host> -f g -l 20 -P 0 | netperf-collect.py; i=$((i+1)); done

      where netperf-collect.py is just a script to calculate the
      average across the runs:

  http://markmc.fedorapeople.org/netperf-collect.py

[2] - ping -c 100000 -f <host>