libgfortran/caf_shared/README.native_coarrays - gcc - Git at Google

 Each image is its own process, that is forked from the master process at the
 start of the program. The number of images is determined by the environment
 variable GFORTRAN_NUM_IMAGES or, alternatively, the number of processors.

 Each coarray is identified by its address. Since coarrays always behave as if
 they had the SAVE attribute, this works even for allocatable coarrays. ASLR is
 not an issue, since the addresses are assigned at startup and remain valid over
 forks. If, on two different images, the allocation function is called with the
 same descriptor address, the same piece of memory is allocated.

 Internally, the allocator (alloc.c) uses a shared hashmap (hashmap.c) to
 remember with which ids pieces of memory allocated. If a new piece of memory is
 needed, a simple relatively allocator (allocator.c) is used. If the allocator
 doesn't hold any previously free()d memory, it requests it from the shared
 memory object (shared_memory.c), which also handles the translation of
 shared_mem_ptr's to pointers in the address space of the image. At the moment
 shared_memory relies on double-mapping pages for this (which might restrict the
 architectures on which this will work, I have tested this on x86 and POWER),
 but since any piece of memory should only be written to through one address
 within one alloc/free pair, it shouldn't matter that much performance-wise.

 The entry points in the library with the exception of master are defined in
 wrapper.c, master(), the function handling launching the images, is defined in
 coarraynative.c, and the other files shouldn't require much explanation.

 To compile a program to run with native coarrays, compile with -fcoarray=shared
 -lcaf_shared -lrt (I've not yet figured out how to automagically link against
 the library if -fcoarray=shared is specified).

 TODO (the list is probably incomplete):

 - make STAT and ERRMSG work for ALLOCATE, DEALLOCATE, SYNC and locks
 - Teams
 - Types containing allocatable components in coarrays
 - Events
 - MOVE_ALLOC
 - automatic linking of relevant libraries with -fcoarray=shared
	Each image is its own process, that is forked from the master process at the
	start of the program. The number of images is determined by the environment
	variable GFORTRAN_NUM_IMAGES or, alternatively, the number of processors.

	Each coarray is identified by its address. Since coarrays always behave as if
	they had the SAVE attribute, this works even for allocatable coarrays. ASLR is
	not an issue, since the addresses are assigned at startup and remain valid over
	forks. If, on two different images, the allocation function is called with the
	same descriptor address, the same piece of memory is allocated.

	Internally, the allocator (alloc.c) uses a shared hashmap (hashmap.c) to
	remember with which ids pieces of memory allocated. If a new piece of memory is
	needed, a simple relatively allocator (allocator.c) is used. If the allocator
	doesn't hold any previously free()d memory, it requests it from the shared
	memory object (shared_memory.c), which also handles the translation of
	shared_mem_ptr's to pointers in the address space of the image. At the moment
	shared_memory relies on double-mapping pages for this (which might restrict the
	architectures on which this will work, I have tested this on x86 and POWER),
	but since any piece of memory should only be written to through one address
	within one alloc/free pair, it shouldn't matter that much performance-wise.

	The entry points in the library with the exception of master are defined in
	wrapper.c, master(), the function handling launching the images, is defined in
	coarraynative.c, and the other files shouldn't require much explanation.

	To compile a program to run with native coarrays, compile with -fcoarray=shared
	-lcaf_shared -lrt (I've not yet figured out how to automagically link against
	the library if -fcoarray=shared is specified).

	TODO (the list is probably incomplete):

	- make STAT and ERRMSG work for ALLOCATE, DEALLOCATE, SYNC and locks
	- Teams
	- Types containing allocatable components in coarrays
	- Events
	- MOVE_ALLOC
	- automatic linking of relevant libraries with -fcoarray=shared