[openib-general] ipoib mtu problem with UDP

[Michael S. Tsirkin]

I tried using ifconfig to limit the ipoib mtu.
Once I do this on *either* both server and client, or only on the client side,
UDP seems to stop working:

#ifconfig ib0 mtu 512
#netperf -c -C -H 11.4.3.68 -f M -t UDP_STREAM
UDP UNIDIRECTIONAL SEND TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 11.4.3.68
(11.4.3.68) port 0 AF_INET : demo
Socket  Message  Elapsed      Messages                   CPU      Service
Size    Size     Time         Okay Errors   Throughput   Util     Demand
bytes   bytes    secs            #      #   MBytes/sec % SS     us/KB

118784   65507   10.00       27582      0      172.2     26.33    inf
118784           10.00           0               0.0     23.40    inf

Things work fine if the mtu on the client side is 2044:
# ifconfig ib0 mtu 2044
# netperf -c -C -H 11.4.3.68 -f M -t UDP_STREAM
UDP UNIDIRECTIONAL SEND TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 11.4.3.68 (11.4.3.68) port 0 AF_INET : demo
Socket  Message  Elapsed      Messages                   CPU      Service
Size    Size     Time         Okay Errors   Throughput   Util     Demand
bytes   bytes    secs            #      #   MBytes/sec % SS     us/KB

118784   65507   10.00       78488      0      490.1     25.31    2.310
118784           10.00       68534             428.0     24.55    2.241

Tested with kernel 2.6.19-rc4 and netperf 2.4.2.

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MST