All MTT testing looks good for 1.6.4. There seems to be an MPI dynamics problem when --enable-spare-groups is used, but this does not look like a regression to me.
I put out a final rc, because there was one more minor change to accommodate an MXM API change; it's in the usual place:
Unless something disastrous happens, I plan to release this as the final 1.6.4 tomorrow.
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