Device object and device `kwarg` for creation

[emcastillo]

Hi,

When reading the section regarding devices, I stumbled upon the following note:

The only way to obtain a Device object is from the .device property on the array object, hence there is no Device object in the array API itself that can be instantiated to point to a specific physical or logical device.

I understood this, as users can never instantiate a Device object themselves.
But in the paragraph inmediately above there are sentences like:

A device=None keyword for array creation functions, which takes an instance of a Device object.

A .to_device(device) method on the array object, with device again being a Device object, to move an array to a different device.

And both require a Device object, that according to the note, can't be directly instantiated, or be obtained from elsewhere than a created array.

[leofang]

That's right. AFAIK the intention is to use device=a.device, i.e. when you already have a compliant array a and so you can use its device attribute. As for how to determine the device for the very first array, it is currently listed as out of scope, and each library will use its existing strategy for the choice of the default device, performing device management, etc.

[rgommers]

We first had a separate Device object, but after discussion found that there were too many differences between libraries to make it straightforward/sensible to standardize.

For library code, typically what you want to do is:

def somefunc(x, y, ...):
    device = x.device
    new_array = xp.array([...], device=device)
    if not x.device == y.device:
        # either raise exception, or
        y = y.to_device(device)

rather than using an explicit device = Device('cuda:0') or some such thing. The use of explicit devices is more appropriate for end user code, and there each library already has its own way of creating new arrays on a specific device.

[rgommers]

Discussion on this API is in gh-96

[emcastillo]

Crystal clear!
Thanks😄

[leofang]

Feel free to reopen if you have further questions on the device aspect, @emcastillo!

[leofang]

As for how to determine the device for the very first array, it is currently listed as out of scope, and each library will use its existing strategy for the choice of the default device, performing device management, etc.

The use of explicit devices is more appropriate for end user code, and there each library already has its own way of creating new arrays on a specific device.

I think we should keep this issue open until we revise the description in question

array-api/spec/design_topics/device_support.md

Lines 52 to 54 in 0941067

	The only way to obtain a `Device` object is from the `.device` property on
	the array object, hence there is no `Device` object in the array API itself
	that can be instantiated to point to a specific physical or logical device.

as it wasn't clear to either library developers nor end users what to do. In particular, for CuPy this implies the Device object needs to have a cpu flavor so that we can facilitate host-device transfer, a point I overlooked.