Global Names

HPX implements an Active Global Address Space (AGAS) which is exposing a single uniform address space spanning all localities an application runs on. AGAS is a fundamental component of the ParalleX execution model. Conceptually, there is no rigid demarcation of local or global memory in AGAS; all available memory is a part of the same address space. AGAS enables named objects to be moved (migrated) across localities without having to change the object's name, i.e., no references to migrated objects have to be ever updated. This feature has significance for dynamic load balancing and in applications where the workflow is highly dynamic, allowing work to be migrated from heavily loaded nodes to less loaded nodes. In addition, immutability of names ensures that AGAS does not have to keep extra indirections ("bread crumbs") when objects move, hence minimizing complexity of code management for system developers as well as minimizing overheads in maintaining and managing aliases.

The AGAS implementation in HPX does not automatically expose every local address to the global address space. It is the responsibility of the programmer to explicitly define which of the objects have to be globally visible and which of the objects are purely local.

In HPX global addresses (global names) are represented using the hpx::id_type data type. This data type is conceptually very similar to void* pointers as it does not expose any type information of the object it is referring to.

The only predefined global addresses are assigned to all localities. The following HPX API functions allow one to retrieve the global addresses of localities:

hpx::find_here(): retrieve the global address of the locality this function is called on.
hpx::find_all_localities(): retrieve the global addresses of all localities available to this application (including the locality the function is being called on).
hpx::find_remote_localities(): retrieve the global addresses of all remote localities available to this application (not including the locality the function is being called on)
hpx::get_num_localities(): retrieve the number of localities available to this application.
hpx::find_locality(): retrieve the global address of any locality supporting the given component type.
hpx::get_colocation_id(): retrieve the global address of the locality currently hosting the object with the given global address.

Additionally, the global addresses of localities can be used to create new instances of components using the following HPX API function:

hpx::new_<Component>(): Create a new instance of the given Component type on the specified locality.

	Note
	HPX does not expose any functionality to delete component instances. All global addresses (as represented using `hpx::id_type`) are being automatically garbage collected. When the last (global) reference to a particular component instance goes out of scope the corresponding component instance is automatically deleted.