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问题描述

众所周知,AMD-OpenCL支持WaveFront(2015年8月): http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_OpenCL_Programming_Optimization_Guide2.pdf

As known, AMD-OpenCL supports WaveFront (August 2015): http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_OpenCL_Programming_Optimization_Guide2.pdf


但是为什么在OpenCL标准1.0/2.0/2.2中没有提及WaveFront?


But why in the OpenCL standards 1.0/2.0/2.2 there is no mention about the WaveFront?

PDF都没有一个单词 WaveFront : https ://www.khronos.org/registry/OpenCL/specs/

None of the PDF has not a word WaveFront: https://www.khronos.org/registry/OpenCL/specs/

我也发现:

  • 2013年: https://stackoverflow.com/a/19874984/1558037
    • 2013: https://stackoverflow.com/a/19874984/1558037
      • 2011: https://forums.khronos.org/showthread.php/7211-How-can-i-split-my-work-load-in-a-GPU-with-OpenCL
        • 2011: https://forums.khronos.org/showthread.php/7211-How-can-i-split-my-work-load-in-a-GPU-with-OpenCL
          • 2011: https://www.cvg.ethz .ch/teaching/2011spring/gpgpu/GPU-Optimization.pdf
            • 2011: https://www.cvg.ethz.ch/teaching/2011spring/gpgpu/GPU-Optimization.pdf
            • 确实是官方的OpenCL 2.2标准仍不支持WaveFront吗?

              Indeed the official OpenCL 2.2 standard still does not support the WaveFront?

              结论:

              OpenCL标准中没有WaveFront,但是在OpenCL-2.0中,有类似于WaveFronts的SIMD执行模型的子组.

              There is no WaveFront in OpenCL standard, but in OpenCL-2.0 there is Sub-groups with SIMD execution model akin to WaveFronts.

              OpenCL 2.0引入了Khronos 子组扩展.子组是 类似于硬件SIMD执行模型的逻辑抽象 波前,扭曲或矢量,并允许更靠近 与供应商无关的硬件.此扩展包括一组 跨子组内置函数的集合 上面指定的跨工作组内置函数.

              OpenCL 2.0 introduces a Khronos sub-group extension. Sub-groups are a logical abstraction of the hardware SIMD execution model akin to wavefronts, warps, or vectors and permit programming closer to the hardware in a vendor-independent manner. This extension includes a set of cross-sub-group built-in functions that match the set of the cross-work-group built-in functions specified above.

              推荐答案

              他们必须采用一种称为sub-group的更具动态性的方法: https://www.khronos.org/registry/OpenCL/specs/opencl-2.2.pdf

              They must have gone to a more dynamical approach called sub-group: https://www.khronos.org/registry/OpenCL/specs/opencl-2.2.pdf

              Sub-group: Sub-groups are an implementation-dependent grouping of work-items within a
              work-group. The size and number of sub-groups is implementation-defined.
              

              Work-groups are further divided into sub-groups,
              which provide an additional level of control over execution.
              

              The mapping of work-items to
              sub-groups is implementation-defined and may be queried at runtime.
              

              所以即使它不被称为wavefront,它现在也可以在运行时和

              so even if its not called wavefront, its now queryable in run-time and

              甚至锁步方式有时也会丢失.

              even lockstep manner may be lost at times.

              最重要的是

               sub_group_all() and
              sub_group_broadcast() and are described in OpenCL C++ kernel language and IL specifications.
              The use of these sub-group functions implies sequenced-before relationships between statements
              within the execution of a single work-item in order to satisfy data dependencies.
              

              说存在某种类型的子组内部通信.因为现在opencl具有子内核定义:

              saying that some kind of intra-sub-group communication exists. Because now opencl has child-kernel definition:

              Device-side enqueue: A mechanism whereby a kernel-instance is enqueued by a kernel-instance
              running on a device without direct involvement by the host program. This produces nested
              parallelism; i.e. additional levels of concurrency are nested inside a running kernel-instance.
              The kernel-instance executing on a device (the parent kernel) enqueues a kernel-instance (the
              child kernel) to a device-side command queue. Child and parent kernels execute asynchronously
              though a parent kernel does not complete until all of its child-kernels have completed.
              


              最终,类似


              Ultimately, with something like

              kernel void launcher()
              {
                  ndrange_t ndrange = ndrange_1D(1);
                  enqueue_kernel(get_default_queue(), CLK_ENQUEUE_FLAGS_WAIT_KERNEL, ndrange,
                  ^{
                  size_t id = get_global_id(0);
                  }
                  );
              }
              

              您应该能够生成所需大小的自己的(已升级?)波前,并且它们与父内核同时工作(并且可以与子组内的线程进行通信),但是由于它们没有经过硬编码,因此它们不被称为波前.通过硬件恕我直言.

              you should be able to spawn your own (upgraded?)wavefronts with any size you need and they work concurrently with parent kernel(and can communicate intra-sub-group threads) but they are not called wavefronts because they are not hardcoded by hardware imho.

              2.0 api规范说:

              2.0 api specs saying:

              Extreme care should be exercised when writing code that uses
              subgroups if the goal is to write portable OpenCL applications.
              

              让我们想起amd的16宽simds和nvidia的32宽simds与一些虚构的fpga的95宽计算核心.也许是伪波前?

              which reminds amd's 16-wide simds and nvidia's 32-wide simds versus some imaginary fpga's 95-wide compute cores. Pseudo-wavefront maybe?

              这篇关于官方的OpenCL 2.2标准是否支持WaveFront?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!

08-19 23:31