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debugging: add instructions for debugging with DDT #266

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debugging: add instructions for debugging with DDT #266

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37 changes: 35 additions & 2 deletions jobs/debugging.rst
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Debugging Jobs
==============

Flux supports parallel debuggers such as Rogue Wave Software (RWS)'s
`TotalView parallel debugger <https://totalview.io>`_.
Debugging Flux jobs has been tested with Rogue Wave Software (RWS)'s
`TotalView parallel debugger <https://totalview.io>`_ and
Linaro `DDT <https://www.linaroforge.com/linaroDdt/>`_. More detailed
instructions for specific debuggers are included in the sections below.

----------------------------------
Parallel Debugging using TotalView
Expand Down Expand Up @@ -94,6 +96,37 @@ Notice that it is designed to support not only Flux but also Slurm's
srun and IBM JSM's jsrun commands. The ``regex`` syntax of
``exec_handling`` within TotalView can be found in `TotalView user guide`_.

---------------------------
Parallel Debugging with DDT
---------------------------

While at this time DDT does not have native support for Flux, small to
medium size jobs can be debugged with DDT using a combination of the
:core:man1:`flux job` :command:`hostpids` command and the :command:`ddt
--attach` option. For example, to attach :command:`ddt` to the previous
job

.. code-block:: console

$ ddt --attach=$(flux job hostpids $(flux job last))

Flux can launch jobs with every task stopped in :linux:man2:`exec` by
providing the ``stop-tasks-in-exec`` job shell option. Thus, launching a
job under control of DDT can be simulated by something like:

.. code-block:: console

$ ddt --attach=$(flux job hostpids $(flux submit -n 265 myapp))

The :command:`flux job hostpids` command will block until the job has started
running and the process IDs for all tasks are available, and therfore
:command:`ddt` will not launch until the job has started and is ready
for debugger attach. Since tasks have been stopped in :linux:man2:`exec`,
the debugger will have control of job tasks before execution begins.

.. note::

:command:`flux job hostpids` was added in flux-core v0.60.0.

------------
Known Issues
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