nvptx (GNU libgomp)

### 12.2 nvptx ¶

On the hardware side, there is the hierarchy (fine to coarse):

- thread
- warp
- thread block
- streaming multiprocessor

All OpenMP and OpenACC levels are used, i.e.

- OpenMP’s simd and OpenACC’s vector map to threads
- OpenMP’s threads (“parallel”) and OpenACC’s workers map to warps
- OpenMP’s teams and OpenACC’s gang use a threadpool with the size of the number of teams or gangs, respectively.

The used sizes are

- The `warp_size` is always 32
- CUDA kernel launched: `dim={#teams,1,1}, blocks={#threads,warp_size,1}`.
- The number of teams is limited by the number of blocks the device can host simultaneously.

Additional information can be obtained by setting the environment variable to `GOMP_DEBUG=1` (very verbose; grep for `kernel.*launch` for launch parameters).

GCC generates generic PTX ISA code, which is just-in-time compiled by CUDA, which caches the JIT in the user’s directory (see CUDA documentation; can be tuned by the environment variables `CUDA_CACHE_{DISABLE,MAXSIZE,PATH}`.

Note: While PTX ISA is generic, the `-mptx=` and `-march=` commandline options still affect the used PTX ISA code and, thus, the requirements on CUDA version and hardware.

The implementation remark:

- I/O within OpenMP target regions and OpenACC compute regions is supported using the C library `printf` functions. Additionally, the Fortran `print`/`write` statements are supported within OpenMP target regions, but not yet within OpenACC compute regions.
- Compilation OpenMP code that contains `requires reverse_offload` requires at least `-march=sm_35`, compiling for `-march=sm_30` is not supported.
- For code containing reverse offload (i.e. `target` regions with `device(ancestor:1)`), there is a slight performance penalty for all target regions, consisting mostly of shutdown delay Per device, reverse offload regions are processed serially such that the next reverse offload region is only executed after the previous one returned.
- OpenMP code that has a `requires` directive with `self_maps` or `unified_shared_memory` runs on nvptx devices if and only if all of those support the `pageableMemoryAccess` property;5 otherwise, all nvptx device are removed from the list of available devices (“host fallback”).
- The default per-warp stack size is 128 kiB; see also `-msoft-stack` in the GCC manual.
- Low-latency memory (`omp_low_lat_mem_space`) is supported when the the `access` trait is set to `cgroup`, and libgomp has been built for PTX ISA version 4.1 or higher (such as in GCC’s default configuration). The default pool size is 8 kiB per team, but may be adjusted at runtime by setting environment variable `GOMP_NVPTX_LOWLAT_POOL=bytes`. The maximum value is limited by the available hardware, and care should be taken that the selected pool size does not unduly limit the number of teams that can run simultaneously.
- `omp_low_lat_mem_alloc` cannot be used with true low-latency memory because the definition implies the `omp_atv_all` trait; main graphics memory is used instead.
- `omp_cgroup_mem_alloc`, `omp_pteam_mem_alloc`, and `omp_thread_mem_alloc`, all use low-latency memory as first preference, and fall back to main graphics memory when the low-latency pool is exhausted.
- Managed memory allocated on the host with the `ompx_gnu_managed_mem_alloc` allocator or in the `ompx_gnu_managed_mem_space` (both GNU extensions) allocate memory in the CUDA Managed Memory space using `cuMemAllocManaged`. This memory is accessible by both the host and the device at the same address, so it need not be mapped with `map` clauses. Instead, use the `is_device_ptr` clause or `has_device_addr` clause to indicate that the pointer is already accessible on the device. The CUDA runtime will automatically handle data migration between host and device as needed. If managed memory is not supported by the default device, as configured at the moment the allocator is called, then the allocator will use the fall-back setting. If the default device is configured differently when the memory is freed, via `omp_free` or `omp_realloc`, the result may be undefined.
- Pinned memory allocated using `omp_alloc` with the `ompx_gnu_pinned_mem_alloc` allocator or the `pinned` trait is obtained via the CUDA API when an NVPTX device is present. This provides a performance boost for NVPTX offload code and also allows unlimited use of pinned memory regardless of the OS `ulimit`/`rlimit` settings.
- The OpenMP routines `omp_target_memcpy_rect` and `omp_target_memcpy_rect_async` and the `target update` directive for non-contiguous list items use the 2D and 3D memory-copy functions of the CUDA library. Higher dimensions call those functions in a loop and are therefore supported.
- The unique identifier (UID), used with OpenMP’s API UID routines, consists of the ‘`GPU-`’ prefix followed by the 16-bytes UUID as returned by the CUDA runtime library. This UUID is output in grouped lower-case hex digits; the grouping of those 32 digits is: 8 digits, hyphen, 4 digits, hyphen, 4 digits, hyphen, 16 digits. This leads to a string like `GPU-a8081c9e-f03e-18eb-1827-bf5ba95afa5d`. The output matches the format used by `nvidia-smi`.

#### Footnotes

##### (5)

https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#um-requirements