[FEA] Add a argument or flag to control whether the constructor and operator= should be async

[JustPlay]

Add a argument or flag to control whether the constructor and operator= should be async

https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_buffer.hpp#L118

https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_buffer.hpp#L198

https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_buffer.hpp#L438

the copy() is async and the above ctor and operator= do not wait for the copy to be done, so the caller may need to sync t he stream explicitly (e.g. if the caller operate on more than two different stream(s))

I think it's better to add a extra parameter or flag to indicate whether we need a sync patten，and we can give it a default argument so no code need to be changed for RMM user (e.g. cuDF and rapids)

[JustPlay]

https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_scalar.hpp#L127

https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_scalar.hpp#L154

https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_scalar.hpp#L190

Why the set operator is async by default? （i thinks this put too much work onto the caller side）

[harrism]

I think the intended design is that everything is async and stream-ordered. A user is free to add a synchronous wrapper if needed, but in general in RAPIDS we need asynchronicity.

You could use a thrust::device_vector but you may find, like we did, that the convenience it provides comes at a performance cost.

[jrhemstad]

I think the intended design is that everything is async and stream-ordered. A user is free to add a synchronous wrapper if needed, but in general in RAPIDS we need asynchronicity.

You could use a thrust::device_vector but you may find, like we did, that the convenience it provides comes at a performance cost.

I think the original issue brings up valid points. There's a lot of things I don't like about device_buffer that I fixed in device_uvector:

• There is no constructor to copy from arbitrary host/device data (only from another device_uvector)
  • I don't like controlling the (a)synchrony of a constructor with a parameter. Instead, I would make the constructor synchronous and use a factory for the async version, e.g., auto buff = device_buffer::make_async(n, stream).
• operator= is deleted
• Modifiers are split based on asynchrony, e.g., set_element and set_element_async
• No default stream arguments

I would like to make similar changes to device_buffer.

[JustPlay]

https://github.com/rapidsai/rmm/issues/570

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[harrism]

I think the original issue brings up valid points. There's a lot of things I don't like about device_buffer that I fixed in device_uvector:

1. There is no constructor to copy from arbitrary host/device data (only from another `device_uvector`)

  * I don't like controlling the (a)synchrony of a constructor with a parameter. Instead, I would make the constructor synchronous and use a factory for the async version, e.g., `auto buff = device_buffer::make_async(n, stream)`.

2. `operator=` is deleted

3. Modifiers are split based on asynchrony, e.g., [`set_element`](https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_uvector.hpp#L193) and [`set_element_async`](https://github.com/rapidsai/rmm/blob/6a3a62fce9c002c8390561566a71f544d7a017c7/include/rmm/device_uvector.hpp#L230)

4. No default stream arguments

I would like to make similar changes to device_buffer.

775 addresses #2 and #4 in Jake's list. It deletes the copy assignment operator and copy constructor, and requires an explicit stream everywhere possible (e.g. resize() and shrink_to_fit()).

@jrhemstad are you suggesting we remove this constructor? https://github.com/rapidsai/rmm/blob/0cc1380523d31b2f044ecc74b457b228f8aea0c8/include/rmm/device_buffer.hpp#L124-L132

And when you say "I would make the constructor synchronous ", you don't mean to synchronize the specified stream, do you?

[harrism]

@jrhemstad are you suggesting we remove this constructor?

@jrhemstad can you comment? I'd like to make progress on closing or implementing this FEA.

[jrhemstad]

@jrhemstad are you suggesting we remove this constructor?

@jrhemstad can you comment? I'd like to make progress on closing or implementing this FEA.

I think I meant making that constructor synchronous (sync the specified stream) and have an async factory.

[harrism]

Wouldn't an sync factory need to call an async constructor?

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