debugging

Debugging

General notes

• Code needs to be compiled with debugging option -g
• Compiler optimizations might complicate debugging (dead code elimination, loop transformations, etc.), recommended to compile without optimizations with -O0
  • Sometimes bugs show up only with optimizations

Launching Arm DDT

LUMI

Set up VNC for smoother GUI performance:

module load lumi-vnc
start-vnc

Follow the instructions from the command output to set up SSH port forwarding and opening VNC on a browser.

Load module and start debugger in an interactive session:

module load ARMForge
export SLURM_OVERLAP=1
salloc -A project_465000536 --nodes=1 --ntasks-per-node=2 --time=00:30:00 --partition=debug
ddt srun ./buggy

The debugger GUI will open in VNC in the browser window.

Note: you can also skip the VNC step and use X11 forwarding for GUI (might be slow).

Puhti and Mahti

Launch a desktop session on a browser for smoother GUI performance (see the documentation for detailed instructions):

• Login to https://www.puhti.csc.fi or https://www.mahti.csc.fi
• Launch Desktop with 1 core
• Open a terminal window in the desktop session and proceed there

Load module and start debugger in an interactive session:

module load ddt
export SLURM_OVERLAP=1
salloc -A project_2007995 --nodes=1 --ntasks-per-node=2 --time=00:15:00 --partition=test
ddt srun ./buggy

Note: you can also skip the desktop session step and use X11 forwarding for GUI (might be slow).

Examples

These example codes can be built with make all.

Message exchange revisited

Debug the exchange.cpp code similar to the earlier exercise.

The following will be demoed with DDT:

• Launching DDT
• Examining per-process status
• Setting build configuration
• Fixing the code within DDT

Collective operations revisited

Debug the collective.cpp code similar to the earlier exercise.

The following will be demoed with DDT:

• Setting breakpoints
• Stepping execution
• Using distributed array view

Bonus 1

Memory debugging with sanitizer. On LUMI:

CC -g -fsanitize=address collective.cpp -o collective.exe
srun -A project_465000536 --nodes=1 --ntasks-per-node=4 --time=00:05:00 --partition=debug --label ./collective.exe

Note the --label option that prepends task number to lines of stdout/err.

Bonus 2

Memory debugging with valgrind4hpc. On LUMI:

module load valgrind4hpc

CC -g collective.cpp -o collective.exe
valgrind4hpc --num-ranks=4 --launcher-args="-A project_465000536 --nodes=1 --ntasks-per-node=4 --time=00:05:00 --partition=debug" ./collective.exe