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ddt-skillshare

This repo uses git submodules. To clone the repository, run

git clone --recurse-submodules https://github.com/SeanBryan51/ddt-skillshare.git

What is DDT?

DDT is a graphical debugger used for debugging parallelised programs.

Supports debugging C, C++, Fortran and Python (limited to Python 3.5 to 3.8 on Gadi, ref)

Running DDT

See the NCI opus documentation on how to run DDT on Gadi.

Note: the instructions on NCI Opus are a bit out of date. ARM sold Forge (that includes DDT) to Linaro. So to get the latest version on Gadi: module load linaro-forge/23.0.1. And on the Linaro website, you can download a remote client for DDT which is faster than running through the X-client.

Interactive Debugging

Interactive debugging is typically done via a VDI session or by submitting an interactive PBS job as it will require using the GUI. For the demo, we will use an interactive job.

cd demo_integrate_mpi
./build.sh # compile the executable
qsub -I -X debug.pbs

Note, you will need to have X11 forwarding enabled when ssh'ing onto Gadi.

After the interactive job starts, run:

./debug.pbs

This will launch the DDT debugger for the compiled executable.

Stepping through a program

Like most debuggers, DDT can:

  • Play (continue execution)
  • Pause execution
  • Step through each line of execution
  • Step in out functions

Process groups

When debugging programs which run multiple processes, DDT allows you to group processes together into process groups. This is useful if we only want to debug a subset of processes.

Locals

Shows the variables in the current context. Value of each variables across each process is shown qualitatively.

Breakpoints

Breakpoints can be set globally or per process group.

Tracepoints

Tracepoints log the value of a variable at a specific execution point. They can also be set globally or per process group.

Evaluating variables

The 'Evaluate' panel provides a way to modify variables in the debugger at runtime.

Assembly Mode

DDT's 'Assembly Mode' shows the assembly of the current source file.

Memory debugging

DDT provides some functionality for memory debugging. These include:

  • Heap debugging
  • Heap overflow/underflow detection
  • Heap consistency checks
  • Backtraces for memory allocations
  • Current memory usage across processes (via Tools > Overall Memory Usage)
  • Memory statistics (via Tools > Overall Memory Stats)

Note, memory debugging functionality must be enabled on start up.

See the ARM MAP tool for more comprehensive memory profiling.

Logbook

To encourage planning when debugging.

Offline Debugging

Offline debugging simply runs the program with DDT without a GUI. Once the program exits, DDT produces a debug report as a html file. This is often useful when the program is too computationally expensive to debug interactively.

Offline debugging can be configured either via command line arguments or by specifying a sesssion file.

In offline mode, breakpoints don't halt execution but dump the state of the program to the debug report.

Creating a session file

We can create a session file from an interactive debugging session by clicking File > Save Session.

Note, launching the interactive session does not require a large amount of resources as we only want to set relevant breakpoints, etc.

Running an offline debugging session with a session file

The following command runs an offline a debugging session with a generated session file:

ddt -offline --ddtsession=<session-file-name> mpirun -n <nproc> <prog>

Other General Notes:

  • I recommend disabling optimisation flags when compiling for easier use.

Resources: