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NAME
clCreateCommandQueue - Create a command-queue on a specific device.
cl_command_queue clCreateCommandQueue(cl_context context, cl_device_id device,
cl_command_queue_properties properties,
cl_int *errcode_ret);
PARAMETERS
context
Must be a valid OpenCL context.
device
Must be a device associated with context. It can either be in the list of devices
specified when context is created using clCreateContext(3clc) or have the same device
type as the device type specified when the context is created using
clCreateContextFromType(3clc).
properties
Specifies a list of properties for the command-queue. This is a bit-field described in
the table below. Only command-queue properties specified in the table below can be set
in properties; otherwise the value specified in properties is considered to be not
valid.
┌───────────────────────────────────────┬──────────────────────────────────┐
│Command-Queue Properties │ Description │
├───────────────────────────────────────┼──────────────────────────────────┤
│CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE │ Determines whether the commands │
│ │ queued in the command-queue are │
│ │ executed in-order or │
│ │ out-of-order. If set, the │
│ │ commands in the command-queue │
│ │ are executed out-of-order. │
│ │ Otherwise, commands are executed │
│ │ in-order. See note below for │
│ │ more information. │
├───────────────────────────────────────┼──────────────────────────────────┤
│CL_QUEUE_PROFILING_ENABLE │ Enable or disable profiling of │
│ │ commands in the command-queue. │
│ │ If set, the profiling of │
│ │ commands is enabled. Otherwise │
│ │ profiling of commands is │
│ │ disabled. See │
│ │ clGetEventProfilingInfo(3clc) │
│ │ for more information. │
└───────────────────────────────────────┴──────────────────────────────────┘
errcode_ret
Returns an appropriate error code. If errcode_ret is NULL, no error code is returned.
NOTES
OpenCL objects such as memory, program and kernel objects are created using a context.
Operations on these objects are performed using a command-queue. The command-queue can be
used to queue a set of operations (referred to as commands) in order. Having multiple
command-queues allows applications to queue multiple independent commands without
requiring synchronization. Note that this should work as long as these objects are not
being shared. Sharing of objects across multiple command-queues will require the
application to perform appropriate synchronization. This is described in Appendix A of the
specification. Out-of-order Execution of Kernels and Memory Object Commands.PP The OpenCL
functions that are submitted to a command-queue are enqueued in the order the calls are
made but can be configured to execute in-order or out-of-order. The properties argument in
clCreateCommandQueue can be used to specify the execution order.
If the CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property of a command-queue is not set, the
commands enqueued to a command-queue execute in order. For example, if an application
calls clEnqueueNDRangeKernel(3clc) to execute kernel A followed by a
clEnqueueNDRangeKernel(3clc) to execute kernel B, the application can assume that kernel A
finishes first and then kernel B is executed. If the memory objects output by kernel A are
inputs to kernel B then kernel B will see the correct data in memory objects produced by
execution of kernel A. If the CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property of a
commandqueue is set, then there is no guarantee that kernel A will finish before kernel B
starts execution.
Applications can configure the commands enqueued to a command-queue to execute
out-of-order by setting the CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property of the
command-queue. This can be specified when the command-queue is created. In out-of-order
execution mode there is no guarantee that the enqueued commands will finish execution in
the order they were queued. As there is no guarantee that kernels will be executed in
order, that is based on when the clEnqueueNDRangeKernel(3clc) calls are made within a
command-queue, it is therefore possible that an earlier clEnqueueNDRangeKernel(3clc) call
to execute kernel A identified by event A may execute and/or finish later than a
clEnqueueNDRangeKernel(3clc) call to execute kernel B which was called by the application
at a later point in time. To guarantee a specific order of execution of kernels, a wait on
a particular event (in this case event A) can be used. The wait for event A can be
specified in the event_wait_list argument to clEnqueueNDRangeKernel(3clc) for kernel B.
In addition, a wait for events (clEnqueueMarkerWithWaitList(3clc)) or a barrier
(clEnqueueBarrierWithWaitList(3clc)) command can be enqueued to the command-queue. The
wait for events command ensures that previously enqueued commands identified by the list
of events to wait for have finished before the next batch of commands is executed. The
barrier command ensures that all previously enqueued commands in a command-queue have
finished execution before the next batch of commands is executed.
Similarly, commands to read, write, copy or map memory objects that are enqueued after
clEnqueueNDRangeKernel(3clc), clEnqueueTask(3clc) or clEnqueueNativeKernel(3clc) commands
are not guaranteed to wait for kernels scheduled for execution to have completed (if the
CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE property is set). To ensure correct ordering of
commands, the event object returned by clEnqueueNDRangeKernel(3clc), clEnqueueTask or
clEnqueueNativeKernel(3clc) can be used to enqueue a wait for event or a barrier command
can be enqueued that must complete before reads or writes to the memory object(s) occur.
ERRORS
clCreateCommandQueue returns a valid non-zero command-queue and errcode_ret is set to
CL_SUCCESS if the command-queue is created successfully. Otherwise, it returns a NULL
value with one of the following error values returned in errcode_ret:
• CL_INVALID_CONTEXT if context is not a valid context.
• CL_INVALID_DEVICE if device is not a valid device or is not associated with context.
• CL_INVALID_VALUE if values specified in properties are not valid.
• CL_INVALID_QUEUE_PROPERTIES if values specified in properties are valid but are not
supported by the device.
• 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.
SPECIFICATION
OpenCL Specification[1]
SEE ALSO
clGetCommandQueueInfo(3clc), clReleaseCommandQueue(3clc), clRetainCommandQueue(3clc),
clCreateContext(3clc), clCreateContextFromType(3clc), classDiagram(3clc)
AUTHORS
The Khronos Group
COPYRIGHT
Copyright © 2007-2011 The Khronos Group Inc.
Permission is hereby granted, free of charge, to any person obtaining a copy of this
software and/or associated documentation files (the "Materials"), to deal in the Materials
without restriction, including without limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of the Materials, and to permit
persons to whom the Materials are furnished to do so, subject to the condition that this
copyright notice and permission notice shall be included in all copies or substantial
portions of the Materials.
NOTES
1. OpenCL Specification
page 61, section 5.1 - Command Queues