c++ - thrust::raw_pointer_cast 和多个 GPU，奇怪的行为

Question

我在我的代码中经常使用 thrust，因为它是一个很棒的包装器并提供了非常有用的实用程序，因为添加了对异步行为的支持，我更加确信。

在我最近尝试在我的应用程序中添加多 GPU 支持之前，我的代码使用 cuda thrust 运行良好。我经历了烦人的事

CUDA Runtime API error 77 : an illegal memory access was encountered

我的部分代码以前从未出现过任何边界问题。

我在我的代码中添加了冗长的代码，看起来我的 thrust::device_vector 指针地址在执行过程中发生了变化，没有明显的原因，在手写内核中生成了错误 77。

我可能误解了 UVA 概念及其最终的“副作用”，但我仍然有兴趣了解导致指针更新的过程。

我无法准确重现我的问题，其中我没有使用临时主机变量来存储 cuda 内存指针，而是仅在内核包装器调用需要时即时使用 thrust::raw_pointer_cast。

但我已经编写了一个小程序来显示我可能遇到的错误类型，请注意这并不可靠，您的系统上至少需要有 2 个 gpu 才能运行它:

/********************************************************************************************
** Compile using nvcc ./test.cu -gencode arch=compute_35,code=sm_35 -std=c++11 -o test.exe **
********************************************************************************************/

//Standard Library
#include <iostream>
#include <vector>

//Cuda
#include "cuda_runtime.h"

//Thrust
#include <thrust/device_vector.h>
#include <thrust/functional.h>
#include <thrust/transform.h>

inline void __checkCudaErrors( cudaError err, const char *file, const int line )
{
    if( err != cudaSuccess )
    {
        printf("%s(%i) : CUDA Runtime API error %i : %s \n",file ,line, (int)err, cudaGetErrorString(err) );
    }
};

#define checkCudaErrors(err)    __checkCudaErrors (err, __FILE__, __LINE__)

__global__ void write_memory( float* buf, float value )
{
    printf("GPU TALK: Raw pointer is %p \n",buf);
    buf[0] = value;
}

int main( int argc, char* argv[] )
{
    //declare a vector of vector
    std::vector<thrust::device_vector<float> > v;
    float test;
    float* tmp;

    //Initialize first vector on GPU 0
    cudaSetDevice( 0 );
    v.emplace_back( 65536, 1.0f );
    tmp = thrust::raw_pointer_cast( v.at(0).data() );
    std::cout << " Host TALK: Raw pointer of vector 0 at step 0 " << (void*)tmp << std::endl;

    //Try to use it raw pointer
    write_memory<<<1,1,0,0>>>( tmp, 2.0f );
    checkCudaErrors( cudaStreamSynchronize( NULL ) );
    test = v.at(0)[0];
    std::cout << " Host TALK: After first kernel launch, value is " << test << std::endl;

    //Initialize second vector on GPU 1, but we do not use it
    cudaSetDevice( 1 );
    v.emplace_back( 65536, 1.0f );
    std::cout << " Host TALK: Raw pointer of vector 0 at step 1 is now " << (void*)thrust::raw_pointer_cast( v.at(0).data() ) << " != " << (void*)tmp << std::endl;
    std::cout << " Host TALK: Raw pointer of vector 1 at step 1 is " << (void*)thrust::raw_pointer_cast( v.at(1).data() ) << std::endl; 

    //Try to use the first vector : No segmentation fault ?
    test = v.at(0)[0];
    std::cout << " Host TALK: Before second kernel launch, value is " << test << std::endl;
    write_memory<<<1,1,0,0>>>( thrust::raw_pointer_cast( v.at(0).data() ), 3.0f );
    checkCudaErrors( cudaStreamSynchronize( NULL ) );
    test = v.at(0)[0];
    std::cout << " Host TALK: After second kernel launch, value is " << test << std::endl;

    //Raw pointer stored elsewhere: generates a segmentation fault
    write_memory<<<1,1,0,0>>>( tmp, 4.0f );
    checkCudaErrors( cudaStreamSynchronize( NULL ) );
    test = v.at(0)[0];
    std::cout << " Host TALK: After third kernel launch, value is " << test << std::endl;

    return 0;
}

这是它在我的机器上产生的输出示例:

主机对话:第 0 步 vector 0 的原始指针 0xb043c0000
GPU 对话:原始指针是 0xb043c0000
Host TALK:第一次内核启动后，值为 2
主机对话:第 1 步 vector 0 的原始指针现在是 0xb08000000 != 0xb043c0000
主机对话:第 1 步 vector 1 的原始指针是 0xb07fc0000
Host TALK:在第二次内核启动之前，值为 2
GPU 对话:原始指针是 0xb08000000
Host TALK:第二次内核启动后，值为 3
GPU 对话:原始指针是 0xb043c0000
./test.cu(68):CUDA 运行时 API 错误 77:遇到非法内存访问 在抛出“thrust::system::system_error”实例后调用终止 what():遇到非法内存访问

在此先感谢您的帮助，我也可能会在 thrust 的 github 上问这个问题。

编辑:感谢 m.s 和 Hiura，这是一个按预期工作的代码:

/********************************************************************************************
** Compile using nvcc ./test.cu -gencode arch=compute_35,code=sm_35 -std=c++11 -o test.exe **
********************************************************************************************/

//Standard Library
#include <iostream>
#include <vector>

//Cuda
#include "cuda_runtime.h"

//Thrust
#include <thrust/device_vector.h>
#include <thrust/functional.h>
#include <thrust/transform.h>

inline void __checkCudaErrors( cudaError err, const char *file, const int line )
{
    if( err != cudaSuccess )
    {
        printf("%s(%i) : CUDA Runtime API error %i : %s \n",file ,line, (int)err, cudaGetErrorString(err) );
    }
};

#define checkCudaErrors(err)    __checkCudaErrors (err, __FILE__, __LINE__)

__global__ void write_memory( float* buf, float value )
{
    printf("GPU TALK: Raw pointer is %p \n",buf);
    buf[0] = value;
}

int main( int argc, char* argv[] )
{
    //declare a vector of vector
    std::vector<thrust::device_vector<float> > v;
    v.reserve(2);
    float test;
    float* tmp;

    //Initialize first vector on GPU 0
    cudaSetDevice( 0 );
    v.emplace_back( 65536, 1.0f );
    tmp = thrust::raw_pointer_cast( v.at(0).data() );
    std::cout << " Host TALK: Raw pointer of vector 0 at step 0 " << (void*)tmp << std::endl;

    //Try to use it raw pointer
    write_memory<<<1,1,0,0>>>( tmp, 2.0f );
    checkCudaErrors( cudaStreamSynchronize( NULL ) );
    test = v.at(0)[0];
    std::cout << " Host TALK: After first kernel launch, value is " << test << std::endl;

    //Initialize second vector on GPU 1, but we do not use it
    cudaSetDevice( 1 );
    v.emplace_back( 65536, 1.0f );
    std::cout << " Host TALK: Raw pointer of vector 0 at step 1 is now " << (void*)thrust::raw_pointer_cast( v.at(0).data() ) << " != " << (void*)tmp << std::endl;
    std::cout << " Host TALK: Raw pointer of vector 1 at step 1 is " << (void*)thrust::raw_pointer_cast( v.at(1).data() ) << std::endl; 

    //Try to use the first vector : No segmentation fault ?
    cudaSetDevice( 0 );
    test = v.at(0)[0];
    std::cout << " Host TALK: Before second kernel launch, value is " << test << std::endl;
    write_memory<<<1,1,0,0>>>( thrust::raw_pointer_cast( v.at(0).data() ), 3.0f );
    checkCudaErrors( cudaStreamSynchronize( NULL ) );
    test = v.at(0)[0];
    std::cout << " Host TALK: After second kernel launch, value is " << test << std::endl;

    //Raw pointer stored elsewhere: generates a segmentation fault
    write_memory<<<1,1,0,0>>>( tmp, 4.0f );
    checkCudaErrors( cudaStreamSynchronize( NULL ) );
    test = v.at(0)[0];
    std::cout << " Host TALK: After third kernel launch, value is " << test << std::endl;

    return 0;
}

这是我代码中最后一个地方，为了简单起见，我没有使用指向对象的指针 vector 而不是对象 vector ，但我发现我应该避免这些烦人的移动/复制问题......

现在的输出是:

主机对话:第 0 步 vector 0 的原始指针 0xb043c0000
GPU 对话:原始指针是 0xb043c0000
Host TALK:第一次内核启动后，值为 2
主机对话:第 1 步 vector 0 的原始指针现在是 0xb043c0000 != xb043c0000
主机对话:第 1 步 vector 1 的原始指针是 0xb07fc0000
Host TALK:在第二次内核启动之前，值为 2
GPU 对话:原始指针是 0xb043c0000
Host TALK:第二次内核启动后，值为 3
GPU 对话:原始指针是 0xb043c0000
Host TALK:第三次内核启动后，值为 4

Answer 1

所以我快速安装了 CUDA 来检验我的假设:添加 reserve 语句可以保留地址。

//declare a vector of vector
std::vector<thrust::device_vector<float> > v;
v.reserve(2); // <<-- HERE
float test;
float* tmp;

以及输出，首先是没有补丁。

 $ nvcc thrust.cu  -std=c++11 -o test
 $ ./test 
  Host TALK: Raw pointer of vector 0 at step 0 0x700ca0000
 GPU TALK: Raw pointer is 0x700ca0000 
  Host TALK: After first kernel launch, value is 2
  Host TALK: Raw pointer of vector 0 at step 1 is now 0x700d20000 != 0x700ca0000
  Host TALK: Raw pointer of vector 1 at step 1 is 0x700ce0000
  Host TALK: Before second kernel launch, value is 2
 GPU TALK: Raw pointer is 0x700d20000 
  Host TALK: After second kernel launch, value is 3
 GPU TALK: Raw pointer is 0x700ca0000 
  Host TALK: After third kernel launch, value is 3

补丁:

 $ nvcc thrust.cu  -std=c++11 -o test
 $ ./test 
  Host TALK: Raw pointer of vector 0 at step 0 0x700ca0000
 GPU TALK: Raw pointer is 0x700ca0000 
  Host TALK: After first kernel launch, value is 2
  Host TALK: Raw pointer of vector 0 at step 1 is now 0x700ca0000 != 0x700ca0000
  Host TALK: Raw pointer of vector 1 at step 1 is 0x700ce0000
  Host TALK: Before second kernel launch, value is 2
 GPU TALK: Raw pointer is 0x700ca0000 
  Host TALK: After second kernel launch, value is 3
 GPU TALK: Raw pointer is 0x700ca0000 
  Host TALK: After third kernel launch, value is 4