Document cudaMallocAsync as an experimental feature.

Author: nouiz

docs/gpu_memory_allocation.rst

@@ -70,3 +70,31 @@ Common causes of OOM failures
   memory. Note however, that the algorithm is basic and you can often get better 
   trade-off between compute and memory by disabling the automatic remat pass and doing
   it manually with `the jax.remat API <https://jax.readthedocs.io/en/latest/jep/11830-new-remat-checkpoint.html>`_
+
+
+Experimental features
+---------------------
+
+Features here are experimental and must be tried with caution.
+
+``TF_GPU_ALLOCATOR=cuda_malloc_async``
+  This replace XLA's own BFC memory allocator with `cudaMallocAsync
+  <https://docs.nvidia.com/cuda/cuda-runtime-api/group__CUDART__MEMORY__POOLS.html>`_.
+  This will remove the big fixed pre-allocation and use a memory pool that grows.
+  The expected benefit is no need to set `XLA_PYTHON_CLIENT_MEM_FRACTION`.
+
+  The risk are:
+
+  - that memory fragmentation is different, so if you are close to the
+    limit, the exact OOM case due to fragmentation will be different.
+  - The allocation time won't be all paid at the start, but be incurred
+    when the memory pool need to be increased. So you could
+    experience less speed stability at the start and for benchmarks
+    it will be even more important to ignore the first few iterations.
+
+  The risks can be mitigated by pre-allocating a signigicant chunk and
+  still get the benefit of having a growing memory pool. This can be
+  done with `TF_CUDA_MALLOC_ASYNC_SUPPORTED_PREALLOC=N`. If N is `-1`
+  it will preallocate the same as what was allocatedy by
+  default. Otherwise, it is the size in bytes that you want to
+  preallocate.