Let's say I have a hyperparameter optimization task where I have to tune a simulation to some spec by varying 2-4 input parameters, and the output is a single number. I have no analytical gradient, though it's probably OK to assume the domain is smooth. Each sim takes hours to run, the entire search could take days, and I would like something that works well in parallel so I can speed up the search. What's the state of art here? Are there anything close-to-state-of-art that's useable out of the box? I've read a few papers but they don't tend to come with software.