Every time you call MPI_Comm_spawn, hydra will launch a ssh (for each host) to create a proxy. It is certainly not ideal for applications relying on spawning many processes. ________________________________ From: Mccall, Kurt E. (MSFC-EV41) via discuss <[email protected]> Sent: Friday, April 1, 2022 4:08 PM To: [email protected] <[email protected]> Cc: Mccall, Kurt E. (MSFC-EV41) <[email protected]> Subject: [mpich-discuss] Hydra WARNING: too many ssh connections Hi, you provided the following information about the warning “too many ssh connections”: The particular warning is issued by hydra, MPICH’s process manager. Following excerpt is the comment in that source code: /* ssh has many types of security controls that do not allow a * user to ssh to the same node multiple times very * quickly. If this happens, the ssh daemons disables ssh * connections causing the job to fail. This is basically a * hack to slow down ssh connections to the same node. We * check for offset == 0 before applying this hack, so we only * slow down the cases where ssh is being used, and not the * cases where we fall back to fork. */ Is this just during an initial ssh connection attempt? I’m trying to figure out where my code is triggering this warning. Could it be from 1. MPI_Intercomm_create 2. MPI_Comm_spawn 3. others? I’m calling mpiexec with the “—launcher ssh” option, MPICH 4.0.1. Thanks, Kurt