Iza*_*din 0 parallel-processing cuda dynamic
这是我第一次尝试用 CUDA 实现递归。目标是使用 CUDA 的强大功能从一组字符“12345”中提取所有组合以动态并行化任务。这是我的内核：
__device__ char route[31] = { "_________________________"};
__device__ char init[6] = { "12345" };
__global__ void Recursive(int depth) {
// up to depth 6
if (depth == 5) return;
// newroute = route - idx
int x = depth * 6;
printf("%s\n", route);
int o = 0;
int newlen = 0;
for (int i = 0; i<6; ++i)
{
if (i != threadIdx.x)
{
route[i+x-o] = init[i];
newlen++;
}
else
{
o = 1;
}
}
Recursive<<<1,newlen>>>(depth + 1);
}
__global__ void RecursiveCount() {
Recursive <<<1,5>>>(0);
}
这个想法是在每个不同的线程中排除1个项目（与threadIdx对应的项目）。在每次递归调用中，使用变量深度，它在路由设备变量上的不同基数（变量 x）上工作。
我希望内核提示如下：
第 2345 章
第 1345 章
第1245章
第1234章
2345_345_________________
2345_245_________________
2345_234_________________
2345_345__45_____________
2345_345__35_____________
2345_345__34_____________
..
2345_245__45_____________
..
但是提示...
·_____________
·_____________
·_____________
·_____________
·_____________
·2345
·2345
·2345
·2345
...
我做错了什么？
我做错了什么？
我可能不会清楚地说明您的代码的所有问题，但这些项目应该会让您更接近。
我建议提供一个完整的例子。在我看来，它基本上是 Stack Overflow 所必需的，请参阅此处的第1 项，注意“必须”一词的使用。您的示例缺少任何主机代码，包括原始内核调用。这只是几行额外的代码，为什么不包括它呢？当然，在这种情况下，我可以推断出调用必须是什么，但为什么不包括它呢？无论如何，根据您指出的输出，主机启动的启动配置似乎很明显必须是<<<1,1>>>.
这对我来说似乎不合逻辑：
我希望内核提示如下：
第 2345 章
您的内核所做的第一件事就是route在对其进行任何更改之前打印出该变量，因此我希望_____________________. 然而，我们可以通过将打印输出移动到内核的末尾来“修复”这个问题。
您可能对什么是__device__变量感到困惑。它是一个全局变量，并且只有一个副本。因此，当您在内核代码中修改它时，每个内核中的每个线程都试图同时修改同一个全局变量。在任何线程并行环境中，这不可能有有序的结果。我选择通过为每个线程制作一个本地副本来“修复”这个问题。
您有一个 off-by-1 错误，以及此循环中的范围错误：
for (int i = 0; i<6; ++i)
off-by-1 错误是由于您正在迭代 6 个可能的项目（即i可以达到 5 的值），但init变量中只有 5 个项目（第 6 个项目是空终止符。该正确的索引从 0-4 开始（其中一个被跳过）。在后续迭代深度上，有必要将此索引范围减少 1。请注意，我选择通过增加init.当然，还有其他方法可以修复。我的方法_在结果的深度之间插入了一个额外的值。
您假设在每个迭代深度，项目的正确选择是相同的，并且顺序相同，即init。然而，这种情况并非如此。在每个深度，项目的选择必须不是从不变的init变量中选择，而是从前一个深度传递的选择中选择。因此我们需要一个本地的、每线程的init也是。
关于CUDA 动态并行性(CDP) 的其他一些评论。将指向数据的指针从一个内核作用域传递到子作用域时，不能使用本地空间指针。因此，我route从堆中分配了本地副本，因此可以将其传递给子内核。 init可以从 推导出来route，所以我们可以使用一个普通的局部变量 for myinit。
如果您继续这样做，您将在这里快速达到一些动态并行性（可能还有内存）限制。我相信为此启动的内核总数是 5^5，也就是 3125（我这样做很快，我可能会弄错）。默认情况下，CDP 的挂起启动限制为 2000 个内核。根据我所见，我们并没有在这里解决这个问题，但是如果您增加此操作的深度或宽度，您迟早会遇到这个问题。此外，来自设备堆的内核分配默认限制为 8KB。我似乎没有达到这个限制，但可能我是，所以我的设计可能应该修改以解决这个问题。
最后，内核printf输出受限于特定缓冲区的大小。如果此技术尚未达到该限制，则如果您增加宽度或深度，它很快就会达到。
这是一个工作示例，试图解决上述各种问题。我并不是说它没有缺陷，但我认为输出更接近您的期望。请注意，由于 SO 答案的字符限制，我已截断/摘录了一些输出。
$ cat t1639.cu
#include <stdio.h>
__device__ char route[31] = { "_________________________"};
__device__ char init[7] = { "12345_" };
__global__ void Recursive(int depth, const char *oroute) {
char *nroute = (char *)malloc(31);
char myinit[7];
if (depth == 0) memcpy(myinit, init, 6);
else memcpy(myinit, oroute+(depth-1)*6, 6);
myinit[6] = 0;
if (nroute == NULL) {printf("oops\n"); return;}
memcpy(nroute, oroute, 30);
nroute[30] = 0;
// up to depth 6
if (depth == 5) return;
// newroute = route - idx
int x = depth * 6;
//printf("%s\n", nroute);
int o = 0;
int newlen = 0;
for (int i = 0; i<(6-depth); ++i)
{
if (i != threadIdx.x)
{
nroute[i+x-o] = myinit[i];
newlen++;
}
else
{
o = 1;
}
}
printf("%s\n", nroute);
Recursive<<<1,newlen>>>(depth + 1, nroute);
}
__global__ void RecursiveCount() {
Recursive <<<1,5>>>(0, route);
}
int main(){
RecursiveCount<<<1,1>>>();
cudaDeviceSynchronize();
}
$ nvcc -o t1639 t1639.cu -rdc=true -lcudadevrt -arch=sm_70
$ cuda-memcheck ./t1639
========= CUDA-MEMCHECK
2345_____________________
1345_____________________
1245_____________________
1235_____________________
1234_____________________
2345__345________________
2345__245________________
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...
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