# Debugging flashlight¶

## Common issues and pitfalls with the ArrayFire Backend¶

• When spawning new threads it’s required to call fl::setDevice() before using any ArrayFire operations.

• Although ArrayFire’s JIT generally works well, one may encounter bugs. One common issue is that due to JIT’s laziness, the input arrays accumulate in memory, leading to OOMs. One can add fl::eval() in the right places to sidestep this, but using it too liberally may degrade performance gains from JIT.

• ArrayFire’s arrays are copy-on-write. This provides value semantics without unnecssary copies. However, due to this, modifying an array may incur a potentially expensive copy operation. On the other hand, fl::Variable has shared pointer semantics; copying a Variable just creates another pointer to the same array and autograd node.

## Printing Info¶

• fl::getMemMgrInfo will write memory manager information to an output stream, if implemented by the active backend.

• Flashlight Tensors can be written directly to an arbitrary output stream via their operator<<(e.g. std::cout << "myTensor" << myTensor << std::endl;) or printed to standard out by using fl::print().

By including ArrayFire headers <arrayfire/arrayfire.h> directly, one has access to useful debugging functions: - af::info() and af::deviceInfo() can provide info about ArrayFire

version, GPU devices, compute capabilities, etc.

• af::printMemInfo() can be useful in debugging OOMs.

• af::print() can be used to inspect arrays. Note: the 1st argument should be a message/name, and the 2nd argument shoudld be an array, not Variable:

auto x = fl::constant({1}, 1.0);
auto y = x * x;
y.backward();
std::cout << "x" << x.tensor() << std::endl;

• Or, one can first convert them to vectors using fl::Tensor::host() and do more customized inspection. Do not use scalar() to print each element; that would be very slow.
flashlight uses standard C++ exceptions to indicate failure states. Unhandled exceptions will abort the program, producing a core dump which can be analyzed with gdb and similar tools. One can also use libraries like glog and Boost.Stacktrace to print out a stack trace upon crashing. For instance, with glog, one simply needs to add the line
google::InstallFailureSignalHandler();

to the main function.