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[John-Michael <thesaxophonist@xxxxxxxxx>]

This appears to be an upstream kernel bug, and not limited to just
Ubuntu/Debian. I'm currently running Arch Linux and this regression
showed up in the 4.4 kernel. The last known good kernel version was
4.3.3. Kernel versions that have the issue include 4.4.1 and 4.5rc1.

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https://bugs.launchpad.net/bugs/1536837

Title:
  Severe performance regression in Infiniband or IPoIB

Status in linux package in Ubuntu:
  Incomplete

Bug description:
  When upgrading to the latest 4.4 test kernel from mainline
  development, all IPoIB links showed a severe performance regression as
  compared with kernel 4.2.  The bandwidth of each link dropped from
  around 3Gbps to around 3Mbps (3 orders of magnitude worse!); reverting
  to the 4.2 kernel immediately fixed this problem.

  The 4.3 kernel from Debian does not appear to suffer from this
  regression.  The regression shows up on both IB ports of the Mellanox
  MT25208 controller.  Other hardware has not been tested.

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