并行缩减无法正常工作

Question

我在 OpenCL 上编写了以下并行内核缩减。我只想对 BlockSum 数组中的所有值求和。在使用 work_group_reduce_add(BlockSum[GetIndex]); 时它工作得很好，使用我从 https://www.fz-juelich.de/SharedDocs/Downloads/IAS/JSC/EN/slides/opencl/opencl-05-reduction.pdf?__blob=publicationFile （幻灯片 11）中读取的优化代码不能正常工作。这里似乎有什么错误？ global_work_size 设置为 {16,16} 以及 local_work_size（意味着每个工作组总共有 256 个线程）。在 work_group_reduce_add 的情况下，我得到 255 这是正确的，但使用优化代码我得到 0

__kernel void Reduction()
{
        unsigned char GetThreadX = get_local_id(0); //it takes values from 0..15
        unsigned char GetThreadY = get_local_id(1); //it takes values from 0..15
        unsigned char GetGroup   = get_local_size(0); //16
        unsigned short  BlockSum[256];      
        int SumOfAll= 0;            
        
        unsigned short GetIndex = GetThreadX + (GetGroup * GetThreadY); // takes values 0..255, group=16        
        
        BlockSum[GetIndex] = 1;             
        barrier(CLK_LOCAL_MEM_FENCE);       
        
        SumOfAll= work_group_reduce_add(BlockSum[GetIndex]); //works great  
        
        // BUT CODE BELOW DOES NOT SUM CORRECTLY
        /*
        for(unsigned short stride=128; stride>1; stride >>= 1) {
            
            if(GetIndex < stride)
                BlockSum[GetIndex] += BlockSum[GetIndex + stride];          
            barrier(CLK_LOCAL_MEM_FENCE);           
        }               
        if(GetIndex==0)             
            SumOfAll = BlockSum[0] + BlockSum[1];       
        barrier(CLK_LOCAL_MEM_FENCE);
        */
        printf("SumOfAll=%d\n",SumOfAll);
}

Answer 1

好的，问题已解决。 BlockSum[256]; 未声明为 __local 而是私有内存（没有 __local 地址 Space 限定符），这意味着每个线程（或核心）都有自己的副本这些数据，但优化的缩减代码正在寻找 shared local memory 线程之间的数据，以求和值。此外，变量 int SumOfAll; 也应声明为带初始化的 __local 或 private 在我的情况下之前没有任何初始化。你选。

所以工作内核现在看起来像这样。

我希望这种类型的错误能帮助像我这样不谨慎的人。

__kernel void Reduction()
{
        unsigned char GetThreadX = get_local_id(0); //it takes values from 0..15
        unsigned char GetThreadY = get_local_id(1); //it takes values from 0..15
        unsigned char GetGroup   = get_local_size(0); //16

        //*********************************************************
        //below was the offending code and the root of the problem 
        //**********************************************************
        __local unsigned short  BlockSum[256];      
        int SumOfAll;           
        //**********************************************************
        
        unsigned short GetIndex = GetThreadX + (GetGroup * GetThreadY); // takes values 0..255, group=16        
        
        BlockSum[GetIndex] = 1;             
        barrier(CLK_LOCAL_MEM_FENCE);       
        
        //SumOfAll = work_group_reduce_add(BlockSum[GetIndex]); 
        
        // OPTIMIZED CODE BELOW NOW SUM UP CORRECTLY
        
        for(unsigned short stride=128; stride>1; stride >>= 1) {
            
            if(GetIndex < stride)
                BlockSum[GetIndex] += BlockSum[GetIndex + stride];          
            barrier(CLK_LOCAL_MEM_FENCE);           
        }               
        if(GetIndex==0)             
            SumOfAll = BlockSum[0] + BlockSum[1];       
        barrier(CLK_LOCAL_MEM_FENCE);
        
        printf("SumOfAll=%d\n",SumOfAll);
        


        
    
}

并行缩减无法正常工作

Parallel Reduction does not work correctly

kernel

reduction

opencl