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clEnqueueMigrateMemObjects.adoc

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clEnqueueMigrateMemObjects

Enqueues a command to indicate which device a set of memory objects should be associated with.

cl_int clEnqueueMigrateMemObjects(cl_command_queue command_queue,
                                  cl_uint num_mem_objects,
                                  const cl_mem *mem_objects,
                                  cl_mem_migration_flags flags,
                                  cl_uint num_events_in_wait_list,
                                  const cl_event *event_wait_list,
                                  cl_event *event)

Parameters

command_queue

A valid host command-queue. The specified set of memory objects in mem_objects will be migrated to the OpenCL device associated with command_queue or to the host if the CL_MIGRATE_MEM_OBJECT_HOST has been specified.

num_mem_objects

The number of memory objects specified in mem_objects.

mem_objects

A pointer to a list of memory objects.

flags

A bit-field that is used to specify migration options. The table below describes the possible values for flags.

cl_mem_migration flags Description

CL_MIGRATE_MEM_OBJECT_HOST

This flag indicates that the specified set of memory objects are to be migrated to the host, regardless of the target command-queue.

CL_MIGRATE_MEM_OBJECT_- CONTENT_UNDEFINED

This flag indicates that the contents of the set of memory objects are undefined after migration. The specified set of memory objects are migrated to the device associated with command_queue without incurring the overhead of migrating their contents.
event_wait_list, num_events_in_wait_list

Specify events that need to complete before this particular command can be executed. If event_wait_list is NULL, then this particular command does not wait on any event to complete. If event_wait_list is NULL, num_events_in_wait_list must be 0. If event_wait_list is not NULL, the list of events pointed to by event_wait_list must be valid and num_events_in_wait_list must be greater than 0. The events specified in event_wait_list act as synchronization points. The context associated with events in event_wait_list and command_queue must be the same. The memory associated with event_wait_list can be reused or freed after the function returns.

event

Returns an event object that identifies this particular command and can be used to query or queue a wait for this particular command to complete. event can be NULL in which case it will not be possible for the application to query the status of this command or queue a wait for this command to complete. If the event_wait_list and the event arguments are not NULL, the event argument should not refer to an element of the event_wait_list array.

Notes

This section describes a mechanism for assigning which device an OpenCL memory object resides. A user may wish to have more explicit control over the location of their memory objects on creation. This could be used to:

  • Ensure that an object is allocated on a specific device prior to usage.

  • Preemptively migrate an object from one device to another.

Typically, memory objects are implicitly migrated to a device for which enqueued commands, using the memory object, are targeted. clEnqueueMigrateMemObjects allows this migration to be explicitly performed ahead of the dependent commands. This allows a user to preemptively change the association of a memory object, through regular command queue scheduling, in order to prepare for another upcoming command. This also permits an application to overlap the placement of memory objects with other unrelated operations before these memory objects are needed potentially hiding transfer latencies. Once the event, returned from clEnqueueMigrateMemObjects, has been marked CL_COMPLETE the memory objects specified in mem_objects have been successfully migrated to the device associated with command_queue. The migrated memory object shall remain resident on the device until another command is enqueued that either implicitly or explicitly migrates it away.

clEnqueueMigrateMemObjects can also be used to direct the initial placement of a memory object, after creation, possibly avoiding the initial overhead of instantiating the object on the first enqueued command to use it.

The user is responsible for managing the event dependencies, associated with this command, in order to avoid overlapping access to memory objects. Improperly specified event dependencies passed to clEnqueueMigrateMemObjects could result in undefined results.

Errors

clEnqueueMigrateMemObjects returns CL_SUCCESS if the function is executed successfully. Otherwise, it returns one of the following errors:

  • CL_INVALID_COMMAND_QUEUE if command_queue is not a valid host command_queue.

  • CL_INVALID_CONTEXT if the context associated with command_queue and memory objects in memobj are not the same or if the context associated with command_queue and events in event_wait_list are not the same.

  • CL_INVALID_MEM_OBJECT if any of the memory objects in mem_objs is not a valid memory object.

  • CL_INVALID_VALUE if num_mem_objects is zero or if mem_objects is NULL.

  • CL_INVALID_VALUE if flags is not 0 or is not any of the values described in the table above.

  • CL_INVALID_EVENT_WAIT_LIST if event_wait_list is NULL and num_events_in_wait_list > 0, or event_wait_list is not NULL and num_events_in_wait_list is 0, or if event objects in event_wait_list are not valid events.

  • CL_MEM_OBJECT_ALLOCATION_FAILURE if there is a failure to allocate memory for the specified set of memory objects in mem_objects.

  • CL_OUT_OF_RESOURCES if there is a failure to allocate resources required by the OpenCL implementation on the device.

  • CL_OUT_OF_HOST_MEMORY if there is a failure to allocate resources required by the OpenCL implementation on the host.