TimeHistory cannot output scalar information

[TotoGaz]

We'd like to output quantities like mean pressure in a reservoir, or the number of iterations in the loops.
Those are scalar quantities which are currently not supported.
This is a tool which would bring a lot of value to GEOSX's users.

Three examples can be used:

• Computation of the well bhp (registered here) as a function of time.
• Computation of linear iterations (registered here) as a function of time.
• Computation of mean pressure (registered here) as a function of time.
  For this one, RegionStatistics surely needs to be adapted.

[TotoGaz]

With the current implementation, we end up in the function https://github.com/GEOSX/GEOSX/blob/develop/src/coreComponents/dataRepository/BufferOpsDevice.hpp#L154-L163
which does not allow the packing of scalar information.

On a CPU target (non GPU), implementing a quick and dirty packing function makes the TimeHistory/PackCollection process work: scalar information is written into the hdf5 output file.

This open some questions about the packing: is it legit to impose the onDevice flag or should the geosx fwk deal it for us? Should we ignore the flag if not relevant/possible?

[untereiner]

I a not quite understanding why scalar information can't be memcopied from gpu to cpu.
I found a work for HDF5 on a virtual File Driver for GPUDirect Storage: https://github.com/hpc-io/vfd-gds
Maybe a silly question but I am wondering changing the VFD depending on the target would not help to write to an HDF5 file... probably not for the pack/unpack operations...

[klevzoff]

Scalar data doesn't live in the GPU memory space in the first place, because there is no good reason to bring it there. It's only ever updated on the host, and if a read-only value is needed in a kernel it is much more efficiently passed via kernel arguments (e.g. embedded in the closure object passed to RAJA).

We should just ignore the onDevice flag when it's not meaningful as a quick solution. A more general approach, as discussed, would be to query the last-modified memory space for each piece of data and dispatch to the appropriate packing function.

[wrtobin]

WIP #2369.

This PR should resolve any scalar packing where the scalar resides on host using the legacy packing functions.

[paveltomin]

knock knock

[paveltomin]

who is there?