Numba includes a CUDA Simulator that implements most of the semantics in CUDA Python using the Python interpreter and some additional Python code. This can be used to debug CUDA Python code, either by adding print statements to your code, or by using the debugger to step through the execution of an individual thread.
Execution of kernels is performed by the simulator one block at a time. One thread is spawned for each thread in the block, and scheduling of the execution of these threads is left up to the operating system.
The simulator is enabled by setting the environment variable
NUMBA_ENABLE_CUDASIM to 1. CUDA Python code may then be executed as
normal. The easiest way to use the debugger inside a kernel is to only stop a
single thread, otherwise the interaction with the debugger is difficult to
handle. For example, the kernel below will stop in the thread
@cuda.jit def vec_add(A, B, out): x = cuda.threadIdx.x bx = cuda.blockIdx.x bdx = cuda.blockDim.x if x == 1 and bx == 3: from pdb import set_trace; set_trace() i = bx * bdx + x out[i] = A[i] + B[i]
when invoked with a one-dimensional grid and one-dimensional blocks.
The simulator aims to provide as complete a simulation of execution on a real GPU as possible - in particular, the following are supported:
syncthreads()is supported - however, in the case where divergent threads enter different
syncthreads()calls, the launch will not fail, but unexpected behaviour will occur. A future version of the simulator may detect this condition.
device_array_like(). The APIs for pinned memory
pinned_array()are also supported, but no pinning takes place.
cuda.cudadrv.devices.gpus) is supported, and reports a single GPU context. This context can be closed and reset as the real one would.
detect()function is supported, and reports one device called SIMULATOR.
Some limitations of the simulator include:
Obviously, the speed of the simulator is also much lower than that of a real device. It may be necessary to reduce the size of input data and the size of the CUDA grid in order to make debugging with the simulator tractable.