Sharing topologies between processes

Functions
int	hwloc_shmem_topology_get_length (hwloc_topology_t topology, size_t *lengthp, unsigned long flags)
int	hwloc_shmem_topology_write (hwloc_topology_t topology, int fd, hwloc_uint64_t fileoffset, void *mmap_address, size_t length, unsigned long flags)
int	hwloc_shmem_topology_adopt (hwloc_topology_t *topologyp, int fd, hwloc_uint64_t fileoffset, void *mmap_address, size_t length, unsigned long flags)

Detailed Description

These functions are used to share a topology between processes by duplicating it into a file-backed shared-memory buffer.

The master process must first get the required shared-memory size for storing this topology with hwloc_shmem_topology_get_length().

Then it must find a virtual memory area of that size that is available in all processes (identical virtual addresses in all processes). On Linux, this can be done by comparing holes found in /proc/<pid>/maps for each process.

Once found, it must open a destination file for storing the buffer, and pass it to hwloc_shmem_topology_write() together with virtual memory address and length obtained above.

Other processes may then adopt this shared topology by opening the same file and passing it to hwloc_shmem_topology_adopt() with the exact same virtual memory address and length.

Function Documentation

hwloc_shmem_topology_adopt()

int hwloc_shmem_topology_adopt	(	hwloc_topology_t *	topologyp,
		int	fd,
		hwloc_uint64_t	fileoffset,
		void *	mmap_address,
		size_t	length,
		unsigned long	flags
	)

Adopt a shared memory topology stored in a file.

Map a file in virtual memory and adopt the topology that was previously stored there with hwloc_shmem_topology_write().

The returned adopted topology in topologyp can be used just like any topology. And it must be destroyed with hwloc_topology_destroy() as usual.

However the topology is read-only. For instance, it cannot be modified with hwloc_topology_restrict() and object userdata pointers cannot be changed.

The segment of the file pointed by descriptor fd, starting at offset fileoffset, and of length length (in bytes), will be mapped at virtual address mmap_address.

The file pointed by descriptor fd, the offset fileoffset, the requested mapping virtual address mmap_address and the length length must be identical to what was given to hwloc_shmem_topology_write() earlier.

Note

Flags flags are currently unused, must be 0.

The object userdata pointer should not be used unless the process that created the shared topology also placed userdata-pointed buffers in shared memory.

This function takes care of calling hwloc_topology_abi_check().

Returns

0 on success.

-1 with errno set to EBUSY if the virtual memory mapping defined by mmap_address and length isn't available in the process.

-1 with errno set to EINVAL if fileoffset, mmap_address or length aren't page-aligned, or do not match what was given to hwloc_shmem_topology_write() earlier.

-1 with errno set to EINVAL if the layout of the topology structure is different between the writer process and the adopter process.

hwloc_shmem_topology_get_length()

int hwloc_shmem_topology_get_length	(	hwloc_topology_t	topology,
		size_t *	lengthp,
		unsigned long	flags
	)

Get the required shared memory length for storing a topology.

This length (in bytes) must be used in hwloc_shmem_topology_write() and hwloc_shmem_topology_adopt() later.

Returns

the length, or -1 on error, for instance if flags are invalid.

Note

Flags flags are currently unused, must be 0.

hwloc_shmem_topology_write()

int hwloc_shmem_topology_write	(	hwloc_topology_t	topology,
		int	fd,
		hwloc_uint64_t	fileoffset,
		void *	mmap_address,
		size_t	length,
		unsigned long	flags
	)

Duplicate a topology to a shared memory file.

Temporarily map a file in virtual memory and duplicate the topology topology by allocating duplicates in there.

The segment of the file pointed by descriptor fd, starting at offset fileoffset, and of length length (in bytes), will be temporarily mapped at virtual address mmap_address during the duplication.

The mapping length length must have been previously obtained with hwloc_shmem_topology_get_length() and the topology must not have been modified in the meantime.

Note

Flags flags are currently unused, must be 0.

The object userdata pointer is duplicated but the pointed buffer is not. However the caller may also allocate it manually in shared memory to share it as well.

Returns

0 on success.

-1 with errno set to EBUSY if the virtual memory mapping defined by mmap_address and length isn't available in the process.

-1 with errno set to EINVAL if fileoffset, mmap_address or length aren't page-aligned.