Add Method to Resize View without Initialization

[stanmoore1]

Can a way to resize a view without any initialization please be added? The values are just going to be overwritten in the next parallel region.

[stanmoore1]

See also #1877.

[Char-Aznable]

I asked the same question on the Slack channel. It seems the answer for now is to use unmanaged view

[mhoemmen]

You don't have to use an unmanaged View. Just create a new View, uninitialized, and assign to the old one. That's what Kokkos::resize does anyway.

[stanmoore1]

Can only resize managed views, so can't use unmanaged in this case. A static assert prevents this, see https://github.com/kokkos/kokkos/blob/master/core/src/Kokkos_CopyViews.hpp#L1839.

That's what Kokkos::resize does anyway

No, I don't think that is true, for example see https://github.com/kokkos/kokkos/blob/master/core/src/Kokkos_CopyViews.hpp#L470.

[stanmoore1]

@mhoemmen unless by "assign" you mean copy element by element...

[stanmoore1]

Maybe I need to explain what I need a little better. When I call Kokkos::resize on a view, I want to preserve the existing values, without initializing the extra empty space, since the new space will just be overwritten in the next parallel region. So the existing values in the old view have to be copied element by element in the new view.

[stanmoore1]

@mhoemmen pretty sure you are thinking of Kokkos::realloc instead of Kokkos::resize.

[Char-Aznable]

@stanmoore1 I didn't mean resizing the unmanaged directly. Can you create an unmanaged view with the desired dimension by wrapping the pointer from the old view?

[stanmoore1]

Can you create an unmanaged view with the desired dimension by wrapping the pointer from the old view?

I could create a new larger view that is uninitialized and copy element by element manually, but then I'm recreating the Kokkos::resize method on my own, which is not convenient. I'm not trying to get around the element by element copy, I don't think that is possible except for really special cases. I am trying to get around the initialization that wastes another CUDA kernel launch, the overhead of which can be significant when you are launch latency bound.

[stanmoore1]

Just to be 100% clear, Kokkos::resize preserves old values, while Kokkos::realloc does not. Kokkos::resize copies all the values from the old view into the new view element-by-element by launching a parallel_for on the device. It also initializes the new values in the new view by launching another parallel_for on the device, which is what I want to avoid.

[stanmoore1]

Actually, it is worse than I thought. Kokkos isn't just initializing the "extra" in the new view, it is initializing the entire new view, even though most of the values will just be copied over in the next operation. You can see that using this code.

#include <Kokkos_Core.hpp>
#include <cstdio>

typedef Kokkos::View<int*> view_type;

int main (int argc, char* argv[]) {
  Kokkos::initialize (argc, argv);
  {
    const int N = 10;
    view_type a (Kokkos::view_alloc("A",Kokkos::WithoutInitializing), N);
    Kokkos::resize(a,2*N);
  }
  Kokkos::finalize ();
}

I used kernel_logger and also put a printf statement in the view initialization function:

KokkosP: Allocate<Cuda> name: A pointer: 0x7fff5a200080 size: 40
KokkosP: Allocate<Cuda> name: A pointer: 0x7fff5a200280 size: 80
KokkosP: Executing parallel-for kernel on device 0 with unique execution identifier 0
KokkosP: Kokkos::View::initialization
Initialized 80
KokkosP: Execution of kernel 0 is completed.
KokkosP: Executing parallel-for kernel on device 0 with unique execution identifier 1
KokkosP: Kokkos::ViewCopy-1D
KokkosP: Execution of kernel 1 is completed.
KokkosP: Deallocate<Cuda> name: A pointer: 0x7fff5a200080 size: 40
KokkosP: Deallocate<Cuda> name: A pointer: 0x7fff5a200280 size: 80

[mhoemmen]

pretty sure you are thinking of Kokkos::realloc instead of Kokkos::resize

my bad; thanks!

[ibaned]

I opened a duplicate earlier, (#2028), but I'll close that one in favor of this one because this one has user data showing why this is expensive.

[ibaned]

@dhollman volunteered on #2028, so I'll assign him to this one

[ibaned]

The CoPA/Cabana team also needs this feature.

[vqd8a]

Is adding something like Kokkos::resize( Kokkos::view_resize (WithoutInitializing), A, 2*N ) okay?

[stanmoore1]

Is adding something like Kokkos::resize( Kokkos::view_resize (WithoutInitializing), A, 2*N ) okay?

Yes, seems fine to me.