It appears as if the problem is the sentinel FIFO opened by (for example) multiprocessing.popen_fork.Popen._launch(). It registers a finalization class to close the sentinel on garbage collection. Instead, it should be closed in poll() or wait() when the child process is reaped and known to be dead. The sentinel serves no purpose after the child is reaped, and waiting till garbage collection means that programs forking large numbers of processes cannot control file descriptor utilization.