HI hritikesh semwal, The error says a connection error: Connection reset by peer. Are you running 8 processes on a single node or 8 nodes? -- Hui Zhou From: hritikesh semwal via discuss <[email protected]> Reply-To: "[email protected]" <[email protected]> Date: Tuesday, April 28, 2020 at 8:45 PM To: "[email protected]" <[email protected]> Cc: hritikesh semwal <[email protected]> Subject: [mpich-discuss] Error in MPI call. Hello all, Greetings of the day. I am working on the development of a parallel CFD solver in which I am using distributed graph communicator with MPI neighbourhood collective operation MPI_Neighbor_alltoallw. While running on 2 and 4 processors, the code is working fine but when I tried it on 8 processors it is showing following error, Fatal error in PMPI_Neighbor_alltoallw: Unknown error class, error stack: PMPI_Neighbor_alltoallw(299)..............: MPI_Neighbor_alltoallw(sendbuf=0x2fa8c20, sendcounts=0x1cd2300, sdispls=0x1cd2270, sendtypes=0x1cd2180, recvbuf=0x2fac1d0, recvcounts=0x7f167615d010, rdispls=0x7f1667ace010, recvtypes=0x7f1660786010, comm=comm=0x84000002) PMPI_Neighbor_alltoallw(285)..............: MPIR_Neighbor_alltoallw_impl(174).........: MPIR_Neighbor_alltoallw_intra_auto(93)....: MPIR_Neighbor_alltoallw_nb(33)............: MPIR_Wait_impl(77)........................: MPIDI_CH3i_Progress_wait(242).............: an error occurred while handling an event returned by MPIDI_CH3I_Sock_Wait() MPIDI_CH3I_Progress_handle_sock_event(479): MPIDI_CH3I_Socki_handle_read(4013)........: connection failure (set=0,sock=1,errno=104:Connection reset by peer) Can anyone tell me what is the solution to this problem? This would be a great help. Thank you.