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Using Valgrind to debug, verify and optimize your code
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What is Valgrind?¶
Valgrind is:
- A memory error detector
- A thread error detector
- A MPI error detector
- A cache and branch-prediction profiler
- A call-graph generating cache profiler
- A heap profiler
- Very easy to use
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Valgrind is the best thing for developers since the invention of pre-sliced bread!
How does Valgrind work?¶
Valgrind works by emulating one or more CPUs. Hence it can intercept and inspect your unmodified, running program in ways which would not be otherwise possible. It can for example check that all variables have actually been assigned before use, that all memory references are within their allowed space, even for static arrays and arrays on the stack.
What makes Valgrind so extremely powerful is that it will tell exactly where in the program a problem, error or violation occurred. It will also give you information on how many allocates/deallocates the program performed, and whether there is any unreleased memory or memory leaks at program exit. In fact, it goes even further and will tell you on what line the unreleased/leaking memory was allocated. The cache profiler will give you information about cache misses and where they occur.
The biggest downside to Valgrind is that it will make your program run much slower. How much slower depends on what kind of, and how much, information you request. Typically the program will run 10-100 times slower under Valgrind.
Using Valgrind¶
Checking memory
As stated earlier, Valgrind is very easy to use; Simply start Valgrind with the path to the binary program to be tested:
$ module load valgrind $ valgrind /path/to/prog
This runs Valgrind with the default "tool" which is called memcheck,
which checks memory consistency. When run without any extra flags,
Valgrind will produce a balanced, not overly detailed and informative
output. If you need a more detailed (but slower) report, run Valgrind
with:
$ valgrind --leak-check=full --track-origins=yes --show-reachable=yes /path/to/prog
Of course, if you want to get all possible information about where in the program something was inconsistent you must compile the program with debugging flags switched on.
Checking threads
If you have a multi-threaded program (e.g. OpenMP, pthreads), and
you are unsure if there might be possible deadlocks or data races
lurking in your program, the Valgrind thread checker is your best
friend. The thread checking tool is called helgrind:
$ export OMP_NUM_THREADS=2 $ valgrind --tool=helgrind /path/to/prog
Further information
For more information on using Valgrind please refer to the man pages and the Valgrind manual which can be found on the Valgrind website:
