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. */ ``` — Hui Zhou On Dec 2, 2019, at 3:14 PM, Mccall, Kurt E. (MSFC-EV41) via discuss <[email protected]<mailto:[email protected]>> wrote: My application uses mainly inter-communicators rather than intra-communicators for fault tolerance. A particular process might have 20 inter-communicators active at one time. I’m receiving the warning [[email protected]<mailto:[email protected]>] WARNING: too many ssh connections to n009.cluster.com<http://n009.cluster.com/>; waiting 6 seconds What is the cause of this? I have several guesses: 1) MPICH has an internal limit on the number of connections 2) I’m bumping up against a Linux limit on the number of connections 3) Non-blocking communication using MPI_Isend() creates a temporary ssh connection (not likely) The other question is, what are the consequences of “waiting 6 seconds”? Are some non-blocking messages dropped? I’m using MPICH 3.3.2, CentOS 3.10 and the Portland Group compiler pgc++ 19.5.0. _______________________________________________ discuss mailing list [email protected]<mailto:[email protected]> To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss