RE: bug 1918 - openmpi broken due to rdma-cm changes

Sean Hefty
2010-02-05T16:22:48Z

>Also note that trying to bind rdma cm to all interface ip addresses was the way
>that we were advised by openfabrics to figure out which devices are rdma-
>capable.
>
>As such, it is highly desirable to get the fix transparently in rdmacm and
>preserve the old semantic. More specifically, it seems undesirable to change
>this semantic in a minor ofed point release.

I think the issue is larger than just the rdma_cm.

First, it sounds like openmpi tries to bind to 127.0.0.1, which now works. If
opemmpi uses shared memory for connections on the same machine, I'm not sure why
this is a problem, unless it is passing that address to another machine to use
for a connection. If this is the case, then that is a bug in openmpi.

Second, I still don't understand whether iwarp is limited to 'loopback'
connections that are not bound to 127.0.0.1. For instance, if the RDMA device
is associated with 192.168.0.1, then can it handle a connection from 192.168.0.1
<-> 192.168.0.1? If it can't, then the rdma_cm can't help in this case when
bind is called. The failure has to come during connect, which sounds like the
behavior that's seen today with 127.0.0.1.

So, while the rdma_cm can fail binds to 127.0.0.1 if the RDMA device doesn't
support loopback, I'm still not sure how much of a fix this is.

- Sean