shmem_size for LayoutStride

[bfzinser]

We have a multidimensional view that should be LayoutRight (int order [] = {2, 1, 0}) on CPU and LayoutStride (int order [] = {2, 0, 1}) on GPU. (We are using the first dimension to switch between the type of element information in order to reduce the number of views we create. Examples of element information include vertices and height vectors.) We decided to implement both as LayoutStride. We also want this view in shared memory, but we cannot call shmem_size for a view with LayoutStride. We can provide a short example, but we were told that shmem_size is not implemented for LayoutStride so an example does not seem necessary.

Our current toy problem stores doubles in the view. In the interest of a workaround, what downsides can we expect by calculating the shared memory requirement as (number of doubles) * (size of doubles)? Failing that, we can temporarily use LayoutLeft or LayoutRight on GPU, but this sounds expensive in terms of performance. In the future, we are interested in replacing doubles with floats or structs that represent complex numbers or 3D points.

[crtrott]

I see. Yeah this makes sense. I will have a look at this.

[crtrott]

Or rather I will find someone to take a look at this ;-)

[ibaned]

The main issue here is that LayoutStride needs to be given the array of strides (or an ordering array), and shmem_size is a static method. With some enable_if I could in theory get the following to work:

Kokkos::View<double***, Kokkos::LayoutStride>::shmem_size(extent0, stride0, extent1, stride1, extent2, stride2);
int rank;
int order[3];
int extents[3];
Kokkos::View<double***, Kokkos::LayoutStride>::shmem_size(rank, order, extents);

Does that sound like the right approach?

[bfzinser]

We think that this looks like a good approach. Thanks!

As you pointed out, shmem_size is a static method so we just need a way to get the relevant information to it. Our primary motivation in using shmem_size instead of calculating the memory requirement ourselves is to have Kokkos calculate any required padding for us.

[ibaned]

On second thought, to be consistent with constructors, maybe a better approach is this:

Kokkos::View<double***, Kokkos::LayoutStride>::shmem_size(Kokkos::LayoutStride(extent0, stride0, extent1, stride1, extent2, stride2));
int rank;
int order[3];
int extents[3];
Kokkos::View<double***, Kokkos::LayoutStride>::shmem_size(Kokkos::LayoutStride::order_dimensions(rank, order, extents));

Although its more verbose, I think its more consistent with the idea that the arguments to shmem_size() should be similar to the arguments to the View constructor.