Adding IPC Shared Memory docs (#910)

docs/sphinx/cookbook.rst

@@ -29,4 +29,5 @@ that have introspection disabled for improved performance, and applying CUDA
    cookbook/mixed_pool.rst
    cookbook/thread_safe.rst
    cookbook/file_allocation.rst
-   cookbook/strategy_name.rst
+   cookbook/shared_memory.rst
+   cookbook/strategy_name.rst

docs/sphinx/cookbook/shared_memory.rst

@@ -0,0 +1,79 @@
+.. _shared_memory:
+
+=======================
+Using IPC Shared Memory 
+=======================
+
+Umpire supports the use of Inter-Process Communication (IPC) Shared Memory on the HOST memory resource. IPC Shared Memory refers to 
+the mechanisms that allow processes to communicate with each other and synchronize their actions and involves a method where multiple 
+processes can access a common memory space.
+
+To use Umpire's IPC Shared Memory allocators, the ``UMPIRE_ENABLE_IPC_SHARED_MEMORY`` flag 
+should be set to ``On``. Note that you can use IPC Shared Memory with MPI enabled or disabled.
+
+First, to get started with the shared memory allocator, set up the traits. For example:
+
+.. code-block:: cpp
+
+    auto traits{umpire::get_default_resource_traits("SHARED")};
+
+The ``traits`` above is a struct of different properties for your shared allocator. You can
+set the maximum size of the allocator with ``traits.size`` and set the scope of the allocator.
+
+For example, you can set the scope to socket:
+
+.. code-block:: cpp
+
+   traits.scope = umpire::MemoryResourceTraits::shared_scope::socket;
+
+However, by default the scope will be set to "node".
+
+Next, create the shared memory allocator:
+
+.. code-block:: cpp
+
+   auto node_allocator{rm.makeResource("SHARED::node_allocator", traits)};
+
+.. note::
+   The name of the Shared Memory allocators MUST have "SHARED" in the name. This will help
+   Umpire distinguish the allocators as Shared Memory allocators. It is also used for discovery 
+   by other ranks on node.
+
+Now you can allocate and deallocate shared memory with:
+
+.. code-block:: cpp
+
+   void* ptr{node_allocator.allocate("allocation_name_2", sizeof(uint64_t))};
+   ...
+   node_allocator.deallocate(ptr);
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Important Notes About Shared Memory
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Because we are dealing with shared memory there are a few unique characteristics of the Shared Memory allocators
+which set it apart from other Umpire allocators.
+
+1. Once you allocate shared memory, that block of memory is fixed. If you need a bigger size, you will have to create a new one.
+2. If you want to see how much memory is available for a shared memory allocator, use the ``getActualSize()`` function.
+3. File descriptors are used for the shared memory. These files will be under ``/dev/shm``.
+4. Although Umpire does not need to have MPI enabled in order to provide IPC Shared Memory, if users wish to associate shared memory with MPI communicators, Umpire will need to be built with MPI enabled.
+
+There are a few helper functions provided in the ``Umpire.hpp`` header that will be useful when working with 
+Shared Memory allocators. For example, you can grab the MPI communicator for a particular Shared Memory allocator with:
+
+.. code-block:: cpp
+
+   MPI_Comm shared_allocator_comm = umpire::get_communicator_for_allocator(node_allocator, MPI_COMM_WORLD);
+
+Note that the ``node_allocator`` is the Shared Memory allocator we created above.
+Additionally, we can double check that an allocator has the ``SHARED`` memory resource by asserting:
+
+.. code-block:: cpp
+
+  UMPIRE_ASSERT(node_allocator.getAllocationStrategy()->getTraits().resource == umpire::MemoryResourceTraits::resource_type::shared);
+
+You can see a full example here:
+
+.. literalinclude:: ../../../examples/cookbook/recipe_shared_memory.cpp
+   :language: cpp