[mpich-discuss] Hydra WARNING: too many ssh connections

Mccall, Kurt E. (MSFC-EV41) kurt.e.mccall at nasa.gov
Fri Apr 1 16:08:56 CDT 2022

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.


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