Non-moving GC does not free nursery area quickly by performGC.

Summary

I am not sure this is a bug.

Our application(hasktorch) manages many c++ objects by ForeignPtr. (hasktorch is FFI of torch.) The pointers are short-lived.

Each c++ object needs large memory area on GPU. The application frequently calls performGC, because I want it released quickly after each epoch.

When we do not use non-moving GC, the area is released by calling performGC soon. Memory usage does not increase.The program finishes in a few minutes.

When we use non-moving GC, the area is not released by calling performGC soon. Memory usage increases, then we can not allocate memory to GPU.

Is there a correct way to force the release of short lived objects?

Steps to reproduce

https://github.com/hasktorch/hasktorch/blob/master/examples/static-mnist-cnn/Main.hs This program runs on CUDA.

I'm sorry I haven't provided an example that can be easily reproduced.

Expected behavior

The program can run without overflow.

Environment

• GHC version used: 8.10.1

Optional:

• Operating System: ubuntu 18.04
• System Architecture: x86_64