CLCREATECOMMANDQUEUE(3clc)

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version 1.0~svn33624 · 3 versions
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💖 CLCREATECOMMANDQUEUE 💖

okay bestie, so this one lives in the OpenCL Manual as CLCREATECOMMANDQUEUE(3clc) and it's literally the function that spins up your command-queue. it's giving "let's make a line of stuff for the GPU to do." slay.

✨ NAME ✨

clCreateCommandQueue — create a command-queue on a specific device. that's it, that's the whole job 💅

cl_command_queue clCreateCommandQueue(cl_context context,
                                      cl_device_id device,
                                      cl_command_queue_properties properties,
                                      cl_int *errcode_ret);

🌸 PARAMETERS 🌸

  • context — has to be a valid OpenCL context. no random nonsense, please.
  • device — has to be a device associated with context. and it can be either one of the devices you listed when context got created with clCreateContext(3clc), or it can have the same device type as the device type you specified when the context was created with clCreateContextFromType(3clc). both paths are valid, pick your fighter 🫶
  • properties — this is your list of properties for the command-queue, and it's a bit-field, described in the lil' table below. important: only the command-queue properties in that table can be set in properties — if you put anything else in there, the value you specified in properties is considered not valid. periodt.
    • 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 the note below for more info 👀
    • CL_QUEUE_PROFILING_ENABLE — enable or disable profiling of commands in the command-queue. if set, profiling of commands is enabled. otherwise profiling of commands is disabled. see clGetEventProfilingInfo(3clc) for more info. (iykyk)
  • errcode_ret — returns an appropriate error code. if errcode_ret is NULL, no error code is returned. it's giving optional, bestie.

🦋 NOTES 🦋

OpenCL objects like memory, program and kernel objects are created using a context. operations on those objects are performed using a command-queue. the command-queue can be used to queue a set of operations (we call them commands) in order. having multiple command-queues lets applications queue multiple independent commands without needing synchronization. lowkey important caveat: this should work as long as these objects are not being shared. sharing objects across multiple command-queues means the application has to do the appropriate synchronization itself. this whole thing is described in Appendix A of the specification. the section is titled Out-of-order Execution of Kernels and Memory Object Commands.

The OpenCL functions that are submitted to a command-queue are enqueued in the order the calls are made, but they can be configured to execute in-order or out-of-order. the properties argument in clCreateCommandQueue is how you specify the execution order. 🎀

⭐ In-order vibes

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 the memory objects produced by execution of kernel A. so polite, so orderly 💜

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. 😭 chaos, honestly.

🔥 Out-of-order 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. and because there's no guarantee 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. wild, right? 💀

To guarantee a specific order of execution of kernels, a wait on a particular event (in this case event A) can be used. that wait for event A can be specified in the event_wait_list argument to clEnqueueNDRangeKernel(3clc) for kernel B. problem solved, bestie. ✨

🌈 Waits and barriers

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.

🧚 Memory commands, be careful

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. don't skip this, fr fr 🙈

💀 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 © 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

and that's the whole page, bestie 💅 the Khronos Group wrote this one, it's dated 01/14/2021, and it ships as part of the CLCREATECOMMANDQUEUE(3clc) manual page in the OpenCL Manual. go forth and queue responsibly 🫶✨

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