.. _shared_memory_allocators_recipe:

==============================
Using Shared Memory Allocators
==============================

Umpire provides two different kinds of Shared Memory capabilities.
First, Umpire provides Inter-Process Communication (IPC) Shared Memory
which can be used with or without MPI. Secondly, Umpire provides
MPI3 Shared Memory which requires MPI. Although both kinds of Shared
Memory provide a convenient way to share memory across nodes/sockets,
each type has a few unique characteristics and usage details which
will be outlined in this section of the documentation.

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::POSIX")};

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);

.. note::
   A name is required in order to allocate memory with IPC Shared Memory allocators. However, if that isn't feasible, you
   can instead use the :class:`umpire::strategy::NamingShim` strategy. This allows you to call allocate with only 1 argument
   for the size in bytes. Check out the :doc:`cookbook recipe <../cookbook/naming_shim>` to learn more.

See the bottom of this page for a full example of how to use IPC Shared Memory Allocators with Umpire.

MPI3 Shared Memory
------------------

In addition to IPC Shared Memory, Umpire also supports MPI3 Shared Memory on the HOST memory resource. As the name suggests, this allocator
uses the MPI3 API for its Shared Memory mechanisms that allow processes to communicate with each other and synchronize their actions.

To use Umpire's MPI3 Shared Memory allocators, the ``UMPIRE_ENABLE_MPI3_SHARED_MEMORY`` flag 
should be set to ``On``. Note that if you are using MPI3 Shared Memory, then MPI must be enabled.

To create an allocator with the MPI3 Shared Memory resource, you can do the following:

.. code-block:: cpp

   auto traits{umpire::get_default_resource_traits("SHARED::MPI3")};
   auto node_allocator{rm.makeResource("SHARED::mpi3_alloc", traits)};

See the bottom of this page for a full example of how to use MPI3 Shared Memory Allocators with Umpire.

Using Both IPC and MPI3 Shared Memory Allocators
------------------------------------------------

It is possible to enable both IPC and MPI3 Shared Memory Allocators at the same time.

To create these Shared Memory allocators, you can do the following:

.. code-block:: cpp

   auto mpi3_traits{umpire::get_default_resource_traits("SHARED::MPI3")};
   // or
   auto ipc_traits{umpire::get_default_resource_traits("SHARED::POSIX")};

   // then create an allocator:
   auto mpi3_node_allocator{rm.makeResource("SHARED::mpi3_alloc", traits)};
   // or
   auto ipc_node_allocator{rm.makeResource("SHARED::ipc_alloc", traits)};

   // and allocate with
   mpi3_node_allocator.allocate(1024 * sizeof(double));
   // or
   ipc_node_allocator.allocate("my_SHARED_alloc", 1024 * sizeof(double));

.. note::
   It is best practice to use the full name, "SHARED::MPI3" or "SHARED::POSIX", when
   setting up the `traits` for a shared memory allocator. However, when both IPC and MPI3
   resources are enabled, using "SHARED" will default to the MPI3 memory resource. Additionally,
   the name used with the `makeResource` call could also just be "SHARED", but it must
   include either the "SHARED" or the "SHARED::" prefix. Finally, while a name is not needed
   for MPI3 allocate calls, it is required for IPC allocations.


Full Shared Memory Examples
--------------------------

This section shows two full code examples, one for IPC Shared Memory and one for MPI3 Shared Memory.

The following example shows how to create, use, and destruct the IPC Shared Memory Allocator. (Can be used with or without MPI, as shown in the example). 
Note that this example could be easily adapted to the MPI3 Shared Memory type if needed.

.. literalinclude:: ../../../examples/cookbook/recipe_shared_memory.cpp
   :language: cpp

The following example shows how to create, use, and verify the MPI3 Shared Memory Allocator. Note that although a name is needed when
when creating the MPI3 Shared Memory allocator, a name is not needed when allocating memory.

.. literalinclude:: ../../../examples/mpi3_shared_memory.cpp
   :language: cpp