TL;DR

For me, it was caused by the network adapter power saving on the Pi and my Mac.

Long story

I’m working on a realtime hardware porject with a wifi connection from Unity on my MBP to my Raspberry Pi. Realtime as in “the less lag, the better”. Today I pinged the Pi, to get a rough estimate of the connection latency.

The results were shocking. The ping time was highly erratic and waaay to long

64 bytes from 192.168.0.2: icmp_seq=2 ttl=64 time=266.042 ms
64 bytes from 192.168.0.2: icmp_seq=3 ttl=64 time=166.670 ms
64 bytes from 192.168.0.2: icmp_seq=4 ttl=64 time=66.558 ms
64 bytes from 192.168.0.2: icmp_seq=5 ttl=64 time=7.362 ms
64 bytes from 192.168.0.2: icmp_seq=6 ttl=64 time=6.677 ms
64 bytes from 192.168.0.2: icmp_seq=7 ttl=64 time=5.854 ms
64 bytes from 192.168.0.2: icmp_seq=8 ttl=64 time=172.650 ms
64 bytes from 192.168.0.2: icmp_seq=9 ttl=64 time=75.915 ms

I pinged around
MBP ⇄ Pi: really bad
MBP → AP: blazingly fast
MBP → Rounter: blazingly fast
Pi → AP: blazingly fast
Pi → Rounter: blazingly fast

This didn’t make any sense to me, so I did, what every tinkerer does: I googled.

To my dismay, there are gazillion of possible causes. The most common suggestion is to check the power supply and ensure it’s strong enough. Pretty sure it is — my Pi is the single consumer on a full blown ATX power supply.

Next also power related possible cause, is the power saving mode of the network interface. I turned it off by adding wireless-power off to the Pi’s /etc/network/interfaces and checked iwconfig to say Power Management:off. I pinged my MBP again and nothing had changed. It later turned out, I made a mistake here :-/

I tried a different AP. Just two meters away, no walls in between, a separate network. Same same, not different.

I tried to set up an ad hoc network. Man, this sucks – I gave up.

Finally I questioned my Mac Book Pro. And it turns out Mavericks puts the network interface into power save mode, too. At this point my mistake became obvious: When I deactivated the power saving on the Pi, I pinged my MBP, not the other way round. I pinged the Pi and got decent results. A stack exchange discussion mentions it’s possible to prevent the power save mode with a higher ping freqency. With the ping Pi → Mac running, I started pinging Mac → AP every 200ms and instantly the the Pi → Mac ping was below 10ms average.