Re: [PATCH v3 1/3] libgomp, nvptx: low-latency memory allocator

[Tobias Burnus <tobias@codesourcery.com>]

On 03.12.23 01:32, Andrew Stubbs wrote:
> This patch adds support for allocating low-latency ".shared" memory on
> NVPTX GPU device, via the omp_low_lat_mem_space and omp_alloc.  The memory
> can be allocated, reallocated, and freed using a basic but fast algorithm,
> is thread safe and the size of the low-latency heap can be configured using
> the GOMP_NVPTX_LOWLAT_POOL environment variable.
>
> The use of the PTX dynamic_smem_size feature means that low-latency allocator
> will not work with the PTX 3.1 multilib.
>
> For now, the omp_low_lat_mem_alloc allocator also works, but that will change
> when I implement the access traits.

...

LGTM, however, I about the following:

> diff --git a/libgomp/libgomp.texi b/libgomp/libgomp.texi
> index e5fe7af76af..39d0749e7b3 100644
> --- a/libgomp/libgomp.texi
> +++ b/libgomp/libgomp.texi
> @@ -3012,11 +3012,14 @@ value.
>   @item omp_const_mem_alloc       @tab omp_const_mem_space
>   @item omp_high_bw_mem_alloc     @tab omp_high_bw_mem_space
>   @item omp_low_lat_mem_alloc     @tab omp_low_lat_mem_space
> -@item omp_cgroup_mem_alloc      @tab --
> -@item omp_pteam_mem_alloc       @tab --
> -@item omp_thread_mem_alloc      @tab --
> +@item omp_cgroup_mem_alloc      @tab omp_low_lat_mem_space (implementation defined)
> +@item omp_pteam_mem_alloc       @tab omp_low_lat_mem_space (implementation defined)
> +@item omp_thread_mem_alloc      @tab omp_low_lat_mem_space (implementation defined)
>   @end multitable
>
> +The @code{omp_low_lat_mem_space} is only available on supported devices.
> +See @ref{Offload-Target Specifics}.
> +

Whether it would be clearer to have this wording not here for the OMP_ALLOCATOR env, i.e.
https://gcc.gnu.org/onlinedocs/libgomp/OMP_005fALLOCATOR.html
but just a simple crossref like:

--- a/libgomp/libgomp.texi
+++ b/libgomp/libgomp.texi
@@ -3061,5 +3061,5 @@ OMP_ALLOCATOR=omp_low_lat_mem_space:pinned=true,partition=nearest
  @item @emph{See also}:
  @ref{Memory allocation}, @ref{omp_get_default_allocator},
-@ref{omp_set_default_allocator}
+@ref{omp_set_default_allocator}, @ref{Offload-Target Specifics}

  @item @emph{Reference}:


And add your wording to:
   https://gcc.gnu.org/onlinedocs/libgomp/Memory-allocation.html

As this sections mentions that "omp_low_lat_mem_space maps to omp_default_mem_space" in general.
Hence, mentioning in this section in addition that  omp_low_lat_mem_space  is honored on devices
seems to be the better location.

Tobias

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