Add initial cupy tests

[jacobtomlinson]

Added some initial unit tests for cupy. Mainly to create a place for cupy tests to go and to check some basic functionality.

I've created a fixture which constructs the dataset from the Toy weather data example and converts the underlying arrays to cupy. Then I've added a test which checks that after calling operations such as mean and groupby the resulting DataArray is still backed by a cupy array.

The main penalty with working on GPUs is accidentally shunting data back and forth between the GPU and system memory. Copying data over the PCI bus is slow compared to the rest of the work so should be avoided. So this first test is checking that we are leaving things on the GPU.

Because this data copying is so expensive cupy have intentionally broken the __array__ method and introduced a .get method instead. This means that users have to be explicit in converting back to numpy and copying back to the main memory. Therefore we will need to add some logic to xarray to use .get in appropriate situations such as plotting.

• Releated to Add cupy support #4212
• Tests added
• Passes isort -rc . && black . && mypy . && flake8

[TomNicholas]

Hi Jacob, would it make sense to add a new CI job that has cupy so that these tests actually get run automatically on future PRs? At the moment they're just going to get skipped, but if you're planning to work on this it might be a good idea to automate the cupy integration tests early on?

[jacobtomlinson]

@TomNicholas I've been trying to find a way to do this. In order for the cupy tests to be run the CI machine will need an NVIDIA GPU. But setting up a GPU CI would be a burden on the xarray team both technically and financially. This is why I'm just skipping for now.

For projects over in RAPIDS we have have a private GPU CI. We aren't in a position to integrate this here but we may be able to run nightlies. I'll update when I have more info.

[jacobtomlinson]

I think what we can offer at the moment is to run a nightly against the default branch on some GPU hardware. If the nightly fails we will open an issue and look into resolving it.

Not as neat as hooking into PRs like the rest of the CI though. Hopefully this will improve over time.

[dcherian]

run a nightly against the default branch on some GPU hardware. If the nightly fails we will open an issue and look into resolving it.

This is great! Thanks @jacobtomlinson

[jacobtomlinson]

Ok great! Once this is merged we will get that set up on our end.

[dcherian]

LGTM

[dcherian]

I'm merging so that @jacobtomlinson can keep moving forward.

It isn't breaking anything too :)

[jacobtomlinson]

Many thanks! We will start running nightlies from today.

[jacobtomlinson]

Nightlies are running. Sorry we can't do more than this right now.