I use parallel-ssh(gevent + paramiko) to execute command or scp files in parallel, as I have a lot of servers, so just use a process with parallel-ssh can't exhaust the performance of the scp server.

As proposed in the investigation of #10789, this issue is the short term version of #10790. This implementation requires a change in the dapi.py module to use ProcessPoolExecutor limited to 1 worker.