Name

    ARM_thread_limit_hint

Name Strings

    cl_arm_thread_limit_hint

Contributors

    Robert Elliott, ARM Ltd.
    Kévin Petit, ARM Ltd.

Contact

    Kévin Petit, ARM Ltd. (kevin.petit 'at' ARM.com)

IP Status

    No claims or disclosures are known to exist.

Version

    Revision: #3, Sept 28th, 2017

Number

    OpenCL Extension #41

Status

    Complete.

Extension Type

    OpenCL device extension

Dependencies

    Requires OpenCL version 1.0 or later.

Overview

    This extension enables an application to provide a hint for the maximum
    number of threads allowed to run concurrently on a compute unit. This
    results in a limit in the threads used by a kernel instance on devices
    that support it, lowering pressure on caches.

Header File

    No host changes needed.

Glossary

    No new terminology is introduced by this extension.

New Types

    None

New Procedures and Functions

    The new kernel qualifier

      __attribute__((arm_thread_limit_hint(N)))

    Description

      The attribute can be specified as part of the declaration of a kernel and
      provides a hint to the implementation that using fewer threads is desired.

      The implementation will accept any number between 0 and
      CL_DEVICE_MAX_WORK_GROUP_SIZE and choose the closest number that can be
      used.

      If the hint is larger than the maximum workgroup size supported by the
      kernel for that device, it is not honored.

      If the hint is smaller than the requested workgroup size for the kernel-
      instance, it is not honored.

      If the hint is not honored, a warning will be produced on context_notify.

      The hint will be honored on devices which support this feature.

New Tokens

    OpenCL kernel code now has access to:

      #pragma OPENCL EXTENSION cl_arm_thread_limit_hint : enable
      The define cl_arm_thread_limit_hint is also present.

Interactions with other extensions

    None

Issues

    None

Sample Code

    The following is a basic example of use, nothing else is required for
    the extension to function:

      // Check for extension and define a throttle value if it is present. This
      // is portable to drivers or devices without support for the extension.
      #ifdef cl_arm_thread_limit_hint
        #pragma OPENCL EXTENSION cl_arm_thread_limit_hint : enable
        #define THROTTLE_ATTRIBUTE __attribute__((arm_thread_limit_hint(64)))
      #else
        #define THROTTLE_ATTRIBUTE
      #endif

      kernel THROTTLE_ATTRIBUTE void throttled_kernel( global int* in, global int *out )
      {
        // Kernel body ...
      }

Conformance Tests

    None

Revision History

    Revision: #1, Feb 2nd, 2015 - Initial revision
    Revision: #2, Feb 23rd, 2015 - Tidied up some of the language, added _hint
                  to the extension to be more consistent with other extensions.
    Revision: #3, Sept 28th, 2017 - Relaxed the constraints on the number of
                  threads accepted. Clarified the wording.