I have written a piece of scientific code in python, mainly using the numpy library (especially Fast Fourier Transforms), and a bit of Cython. Nothing in CUDA or anything GPU related that I am aware of. There is no graphic interface, everything runs in the terminal (I'm using WSL2 on Windows). The whole code is mostly about number crunching, nothing fancy at all.

When I run my program, I see that CPU usage is ~ 100% (to be expected of course), but GPU usage also rises, to around 5%.

Is it possible that a part of the work gets offloaded automatically to the GPU? How else can I explain this small but consistent increase in GPU usage ?

Thanks for the help

CodePudding user response：

No, there is no automatic offloading in Numpy, at least not with the standard Numpy implementation. Note that some specific FFT libraries can use the GPU, but the standard implementation of Numpy uses its own implementation of FFT called PocketFFT based on FFTPack that do not use the GPU. Cython do not perform any automatic implicit GPU offloading. The code need to do that explicitly/manually.

No GPU offloading are automatically performed because GPUs are not faster than CPUs for all tasks and offloading data to the GPU is expensive, especially with small arrays (due to the relatively high-latency of the PCI bus and kernel calls in such a case). Moreover, this is hard to do efficiently even in case where the GPUs could be theoretically faster.

The 5% GPU usage is relative to the frequency of the GPU which is often configured to use an adaptative frequency. For example my discrete Nv-1660S GPU frequency is currently 300 MHz while it can automatically reach 1.785 GHz. Using actually 5% of a GPU running at a 17% of its maximum frequency with a 2D rendering of a terminal is totally possible. On my machine, printing lines in a for loop at 10 FPS in a Windows terminal takes 6% of my GPU still running at low-frequency (0-1% without running anything).

If you want to check the frequency of your GPU and the load there are plenty of tools for that starting from vendor tools often installed with the driver to softwares like GPU-z on Windows. For Nvidia GPU, you can list the processes currently using you GPU with nvidia-smi (it should be rocm-smi on AMD GPUs).