当对输入数据使用各种偏移时，CUDA内核启动失败

Question

我的代码给出了一条错误消息，我正在努力找出原因。为了更容易找到问题，我删除了与导致错误消息无关的代码。如果你能告诉我为什么下面的简单代码会产生错误消息，那么我认为我应该能够修复我的原始代码：

#include "cuComplex.h"
#include <cutil.h>
__device__ void compute_energy(void *data, int isample, int nsamples) {
  cuDoubleComplex * const nminusarray          = (cuDoubleComplex*)data;
  cuDoubleComplex * const f                    = (cuDoubleComplex*)(nminusarray+101);
  double          * const abs_est_errorrow_all = (double*)(f+3);
  double          * const rel_est_errorrow_all = (double*)(abs_est_errorrow_all+nsamples*51);
  int             * const iid_all              = (int*)(rel_est_errorrow_all+nsamples*51);
  int             * const iiu_all              = (int*)(iid_all+nsamples*21);
  int             * const piv_all              = (int*)(iiu_all+nsamples*21);
  cuDoubleComplex * const energyrow_all        = (cuDoubleComplex*)(piv_all+nsamples*12);
  cuDoubleComplex * const refinedenergyrow_all = (cuDoubleComplex*)(energyrow_all+nsamples*51);
  cuDoubleComplex * const btplus_all           = (cuDoubleComplex*)(refinedenergyrow_all+nsamples*51);
  cuDoubleComplex * const btplus           = btplus_all+isample*21021;
  btplus[0] = make_cuDoubleComplex(0.0, 0.0);
}
__global__ void computeLamHeight(void *data, int nlambda) {
  compute_energy(data, blockIdx.x, nlambda);
}
int main(int argc, char *argv[]) {
  void *device_data;
  CUT_DEVICE_INIT(argc, argv);
  CUDA_SAFE_CALL(cudaMalloc(&device_data, 184465640));
  computeLamHeight<<<dim3(101, 1, 1), dim3(512, 1, 1), 45000>>>(device_data, 101);
  CUDA_SAFE_CALL(cudaThreadSynchronize());
}

我使用的是GeForce GTX 480，我正在编译这样的代码：

nvcc -L /soft/cuda-sdk/4.0.17/C/lib -I /soft/cuda-sdk/4.0.17/C/common/inc -lcutil_x86_64 -arch sm_13 -O3 -Xopencc "-Wall" Main.cu

输出为：

Using device 0: GeForce GTX 480
Cuda error in file 'Main.cu' in line 31 : unspecified launch failure.

编辑：我现在已经进一步简化了代码。以下更简单的代码仍然会产生错误消息：

#include <cutil.h>
__global__ void compute_energy(void *data) {
  *(double*)((int*)data+101) = 0.0;
}
int main(int argc, char *argv[]) {
  void *device_data;
  CUT_DEVICE_INIT(argc, argv);
  CUDA_SAFE_CALL(cudaMalloc(&device_data, 101*sizeof(int)+sizeof(double)));
  compute_energy<<<dim3(1, 1, 1), dim3(1, 1, 1)>>>(device_data);
  CUDA_SAFE_CALL(cudaThreadSynchronize());
}

现在很容易看出偏移量应该是有效的。我试着运行cuda memcheck，它显示如下：

========= CUDA-MEMCHECK
Using device 0: GeForce GTX 480
Cuda error in file 'Main.cu' in line 13 : unspecified launch failure.
========= Invalid __global__ write of size 8
=========     at 0x00000020 in compute_energy
=========     by thread (0,0,0) in block (0,0,0)
=========     Address 0x200200194 is misaligned
=========
========= ERROR SUMMARY: 1 error

我试着在互联网上搜索地址错位是什么意思，但我找不到解释。怎么回事？

Answer 1

使用所有这些神奇的常量解析原始代码非常困难，但更新后的repo案例使问题立即变得显而易见。GPU体系结构要求所有指针都与单词边界对齐。您的内核包含一个未正确对齐单词的指针访问。Double是一种64位类型，并且您的寻址不与偶数64位边界对齐。此：

*(double*)((int*)data+100) = 0.0; // 50th double

或者这个：

*(double*)((int*)data+102) = 0.0; // 51st double

都是合法的。此：

*(double*)((int*)data+101) = 0.0; // not aligned to a 64 bit boundary

不是。

Answer 2

错误表示内存访问越界，请检查偏移量值。