veths often slow to come up

[Shaun Crampton]

Please CC me on any responses, thanks.

Setting both ends of a veth to be oper UP completes very quickly but I
find that pings only start flowing over the veth after about a second.
This seems to correlate with the NO-CARRIER flag being set or the
interface being in "state UNKNOWN" or "state DOWN² for about a second
(demo script below).

If I run the script repeatedly then sometimes it completes very quickly on
subsequent runs as if there¹s a hot cache somewhere.

Could this be a bug or is there a configuration to speed this up?  Seems
odd that it¹s almost exactly 1s on the first run.

Seen on these kernels:
* 3.13.0-57-generic #95-Ubuntu SMP Fri Jun 19 09:28:15 UTC 2015 x86_64
x86_64 x86_64 GNU/Linux
* 4.0.9-coreos #2 SMP Thu Jul 30 01:07:55 UTC 2015 x86_64 Intel(R) Xeon(R)
CPU @ 2.50GHz GenuineIntel GNU/Linux

Regards,

-Shaun


Running my test script below (Assumes veth0/1 do not already exist):

$ sudo ./veth-test.sh
Time to create veth:

real	0m0.019s
user	0m0.002s
sys	0m0.010s

Time to wait for carrier:

real	0m1.005s
user	0m0.007s
sys	0m0.123s



# veth-test.sh

#!/bin/bash
function create_veth {
  ip link add type veth
  ip link set veth0 up
  ip link set veth1 up
}
function wait_for_carrier {
  while ! ip link show | grep -qE 'veth[01]';
  do
    sleep 0.05
  done
  while ip link show | grep -E 'veth[01]¹ | \
        grep -Eq 'NO-CARRIER|state DOWN|state UNKNOWN';
  do
    sleep 0.05
  done
}
echo "Time to create veth:"
time create_veth
echo
echo "Time to wait for carrier:"
time wait_for_carrier
ip link del veth0

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