From this mailing list:
this error message is being printed by the asynchronous progress
thread because of receving an IBV_EVENT_CLIENT_REREGISTER event (event
#17).
It is suggested that you update to the latest version. The mail I linked to suggest MVAPICH2 1.4 (which is newer than yours), despite that the fact that the mail is from 2009.
The code, that probably generates that is:
switch (event.event_type) {
...
break;
default:
NEM_IB_ERR("Got unknown event %d ... continuing ...",
event.event_type);
}
where you can find the full code here.
As indicated in the comment section:
IBV_EVENT_CLIENT_REREGISTER
The SM requests that the client will reregister to all subscriptions
previously requested from this port, for example (but not limited to)
join a multicast group. This event may be generated when the SM
suffered from a failure, which caused it to lose his records or when
there is new SM in the subnet.
This event will be generated by the device only if the bit that
indicates that client reregister is supported set in
port_attr.port_cap_flags
.
Source
I wouldn't be happy with that event, so if I were you, I would update. If the issue persists, I would contact the MVAPICH2 people.