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POEM ID: 002
Title: New capability for user to send a termination signal to an OpenMDAO process so that SNOPT in pyoptsparse can terminate cleanly.
Authors: [Kenneth-T-Moore]
Competing POEMs: [N/A]
Related POEMs: [N/A]
Associated implementation PR: OpenMDAO/OpenMDAO#1138 (43dcc0a3e0ca5ebcce9f8712f1be620b8b1d3089)

Status:

  • Active
  • Requesting decision
  • Accepted
  • Rejected
  • Integrated

Motivation

The SNOPT optimizer provides a way to cleanly terminate an optimization by returning a status of -2 or lower from the model function and gradient evaluation functions. This allows for a clean and graceful exit from optimization. To take advantage of this under a wide range of operating scenarios (serial, MPI, job queue submission), we need a way to send a signal to a running OpenMDAO model to tell it to return an error code to pyoptsparse/SNOPT so that we can terminate cleanly.

Description

Sending Signals from the OS

Unix provides a selection of signals [1] that are used for interprocess communication. Microsoft Windows also provides a smaller set of signals [2] for the same purpose. The discussion here will focus on Unix signals, as these cover Unix and OSX systems. Many of the signals already have defined behavior, such as SIGINT which interrupts execution, SIGTERM which terminates the process, and SIGSTP which temporarily suspends execution.

Unix provides the kill [4] command, which can be used to send any signal to any process. Similarly, job queue submission systems also provide a way to forward a specified signal to all processes in the job (e.g., Slurm [5] and PBS [6])

It is important to check how an MPI implementation handles signals as well. For example, OpenMPI forwards SIGUSR1 and SIGUSR2 to all processes, while HPE MPT does not forward those signals, but does forward SIGURG. Because the signal forwarding behavior is different depending on operating system, mpi implementation, and scheduler, the user must retain the ability to choose which signal will be used to initiate clean termination. A reasonable default value is SIGUSR1.

Handling Signals in pyOptSparseDriver

The Python standard library includes a library [3] for dealing with signals, including a way to associate a handler function with a particular signal, so that when that signal is detected, the handler function is called. When this feature is enabled by the user, a handler will be placed on the expected signal. This handler will raise a new exception UserRequestedException that will be caught in _objfunc and _gradfunc, where the return code self.fail will be set to -2.

User Interface for Enabling New Capability

A new option named "user_terminate_signal" will be added to the pyOptSparseDriver, with the default value of signals.SIGUSR1. The user can change this to any other valid signal name or number. The user can also set it to False to prevent the addition of any handlers.

Changes needed to pyoptsparse

This POEM is tied to release 1.1 of pyoptsparse.

In pyoptsparse 1.1, The Optimizer class was upated to allow a value of 2 to be passed back to the SNOPT callback function wrapper.

References

  1. https://en.wikipedia.org/wiki/Signal_(IPC)
  2. https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/signal?view=vs-2019
  3. https://docs.python.org/3/library/signal.html
  4. https://en.wikipedia.org/wiki/Kill_(command)
  5. https://slurm.schedmd.com/scancel.html