Implementation of compounded vectors.

[nychiang]

It's better to implement something for blocked vectors, e.g., a wrapper for multiple single vectors in a compound vector.
By doing this, we can have better memory management in HiOp.
For instance, we can avoid unnecessary data transfer in this function .

[pelesh]

It might be a good idea to consider vector views that do not own the data, but are used for user/application to read/write from appropriate locations. In the example you are showing, you would allocate only *ir_x0_ and all the other vectors would be views of different sections of *ir_x0_. Vector views are lightweight objects and can be allocated on the stack, so the implementation could be quite efficient.

My concern is if you go with a wrapper of multiple vectors, it is difficult to control where in the memory these vectors will be allocated. If parts of *ir_x0_ are allocated "everywhere", one could run into excessive cache misses.

[cnpetra]

Vector views is definitely a good idea for performance. But I think the "views" need to implement hiopVector interface, otherwise code gets messy. Modifying existing hiopVector implementations so that they don't own the data would do this without much effort (plus some methods to create such views based on (start,length) pairs).

But, imo, compound vectors (of vectors, including vector views) will still be useful to keep the code simple.

[pelesh]

The way I see it, implementing vector view could be as easy as adding another constructor or two to the existing vector class plus a flag indicating if the object owns the data. I agree, having a block (compound) vector object in addition to vector views would indeed help remove explicit pointer arithmetic from algorithm classes without significant performance hit.

[nychiang]

Closed by #621