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README.md

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 DDT

LUMI

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

  • Login to https://www.lumi.csc.fi
  • Launch Desktop with 1 core on interactive partition
  • Open a terminal window in the desktop session
  • Bug fix: run module reload in the terminal
  • Follow the commands below

Run DDT for a buggy program:

module load ARMForge
export SLURM_OVERLAP=1
salloc -A project_465001194 --nodes=1 --ntasks-per-node=2 --time=00:30:00 --partition=debug
ddt srun ./buggy.exe

Note: you can also skip the desktop session step and run DDT using X11 forwarding (might be slow).

Puhti and Mahti

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

Run DDT for a buggy program:

module load ddt
export SLURM_OVERLAP=1
salloc -A project_2010554 --nodes=1 --ntasks-per-node=2 --time=00:15:00 --partition=test
ddt srun ./buggy.exe

Note: you can also skip the desktop session step and run DDT using X11 forwarding (might be slow).

Demos

Note to the instructor: rm -r ~/.allinea/ to reset DDT configs on LUMI.

Buggy message exchange

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

...
CC -g exchange.cpp -o exchange.exe
ddt srun ./exchange.exe

The following will be demoed with DDT:

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

Bonus exercises

Buggy collective operations

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

...
CC -g collective.cpp -o collective.exe
ddt srun ./collective.exe

Memory debugging with sanitizer

Memory debugging with sanitizer. On LUMI:

CC -g -fsanitize=address collective.cpp -o collective.exe
srun -A project_465001194 --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.

Memory debugging with valgrind4hpc

Memory debugging with valgrind4hpc. On LUMI:

module load valgrind4hpc

CC -g collective.cpp -o collective.exe
valgrind4hpc --num-ranks=4 --launcher-args="-A project_465001194 --nodes=1 --ntasks-per-node=4 --time=00:05:00 --partition=debug" ./collective.exe