As we've discussed here several times before, PETSc's --download-mpich option uses ch3:sock by default because this issue http://trac.mpich.org/projects/mpich/ticket/79 makes nemesis unusable when oversubscribed. We're also checking for MPI_Ibarrier and using Torsten's "nonblocking consensus" algorithm when possible. Unfortunately, MPI_Ibarrier deadlocks when run over ch3:sock. (Both procs have posted the Ibarrier, but the requests never complete.) I can prepare a reduced test case if you'd like. Are you still supporting ch3:sock? Is there some test we can do to check whether ch3:sock is being used so that we can choose a safe algorithm?