Memory usage of optimize_mp increases over time

When running a large batch of optimizations in parrallel, the memory usage gradually increases over time.
(Ubuntu, 6.7k cells, i7-8700K, 12 processes)

context manager with close() does not seem to help.

https://stackoverflow.com/questions/18414020/memory-usage-keep-growing-with-pythons-multiprocessing-pool
https://chase-seibert.github.io/blog/2013/08/03/diagnosing-memory-leaks-python.html