如何使用推力和 CUDA 流将内存从主机异步复制到设备

How to asynchronously copy memory from the host to the device using thrust and CUDA streams(如何使用推力和 CUDA 流将内存从主机异步复制到设备)

问题描述

我想使用推力将内存从主机复制到设备

I would like to copy memory from the host to the device using thrust as in

thrust::host_vector<float> h_vec(1 << 28);
thrust::device_vector<float> d_vec(1 << 28);
thrust::copy(h_vec.begin(), h_vec.end(), d_vec.begin());

使用 CUDA 流类似于使用流将内存从设备复制到设备:

using CUDA streams analogously to how you would copy memory from the device to the device using streams:

cudaStream_t s;
cudaStreamCreate(&s);

thrust::device_vector<float> d_vec1(1 << 28), d_vec2(1 << 28);
thrust::copy(thrust::cuda::par.on(s), d_vec1.begin(), d_vec1.end(), d_vec2.begin());

cudaStreamSynchronize(s);
cudaStreamDestroy(s);

问题是我无法将执行策略设置为 CUDA 以在从主机复制到设备时指定流，因为在这种情况下，thrust 会假定两个向量都存储在设备上.有没有办法解决这个问题?我正在使用来自 github 的最新推力版本(它在 version.h 文件中显示为 1.8).

The problem is that I can't set the execution policy to CUDA to specify the stream when copying from the host to the device, because, in that case, thrust would assume that both vectors are stored on the device. Is there a way to get around this problem? I'm using the latest thrust version from github (it says 1.8 in the version.h file).

推荐答案

正如评论中所指出的，我认为直接使用 thrust::copy 是不可能的.但是我们可以在推力应用中使用 cudaMemcpyAsync 来实现异步复制和复制与计算重叠的目标.

As indicated in the comments, I don't think this will be possible directly with thrust::copy. However we can use cudaMemcpyAsync in a thrust application to achieve the goal of asynchronous copies and overlap of copy with compute.

这是一个有效的例子:

#include <thrust/host_vector.h>
#include <thrust/device_vector.h>
#include <thrust/system/cuda/experimental/pinned_allocator.h>
#include <thrust/system/cuda/execution_policy.h>
#include <thrust/fill.h>
#include <thrust/sequence.h>
#include <thrust/for_each.h>
#include <iostream>

// DSIZE determines duration of H2D and D2H transfers
#define DSIZE (1048576*8)
// SSIZE,LSIZE determine duration of kernel launched by thrust
#define SSIZE (1024*512)
#define LSIZE 1
// KSIZE determines size of thrust kernels (number of threads per block)
#define KSIZE 64
#define TV1 1
#define TV2 2

typedef int mytype;
typedef thrust::host_vector<mytype, thrust::cuda::experimental::pinned_allocator<mytype> > pinnedVector;

struct sum_functor
{
  mytype *dptr;
  sum_functor(mytype* _dptr) : dptr(_dptr) {};
  __host__ __device__ void operator()(mytype &data) const
    {
      mytype result = data;
      for (int j = 0; j < LSIZE; j++)
        for (int i = 0; i < SSIZE; i++)
          result += dptr[i];
      data = result;
    }
};

int main(){

  pinnedVector hi1(DSIZE);
  pinnedVector hi2(DSIZE);
  pinnedVector ho1(DSIZE);
  pinnedVector ho2(DSIZE);
  thrust::device_vector<mytype> di1(DSIZE);
  thrust::device_vector<mytype> di2(DSIZE);
  thrust::device_vector<mytype> do1(DSIZE);
  thrust::device_vector<mytype> do2(DSIZE);
  thrust::device_vector<mytype> dc1(KSIZE);
  thrust::device_vector<mytype> dc2(KSIZE);

  thrust::fill(hi1.begin(), hi1.end(),  TV1);
  thrust::fill(hi2.begin(), hi2.end(),  TV2);
  thrust::sequence(do1.begin(), do1.end());
  thrust::sequence(do2.begin(), do2.end());

  cudaStream_t s1, s2;
  cudaStreamCreate(&s1); cudaStreamCreate(&s2);

  cudaMemcpyAsync(thrust::raw_pointer_cast(di1.data()), thrust::raw_pointer_cast(hi1.data()), di1.size()*sizeof(mytype), cudaMemcpyHostToDevice, s1);
  cudaMemcpyAsync(thrust::raw_pointer_cast(di2.data()), thrust::raw_pointer_cast(hi2.data()), di2.size()*sizeof(mytype), cudaMemcpyHostToDevice, s2);

  thrust::for_each(thrust::cuda::par.on(s1), do1.begin(), do1.begin()+KSIZE, sum_functor(thrust::raw_pointer_cast(di1.data())));
  thrust::for_each(thrust::cuda::par.on(s2), do2.begin(), do2.begin()+KSIZE, sum_functor(thrust::raw_pointer_cast(di2.data())));

  cudaMemcpyAsync(thrust::raw_pointer_cast(ho1.data()), thrust::raw_pointer_cast(do1.data()), do1.size()*sizeof(mytype), cudaMemcpyDeviceToHost, s1);
  cudaMemcpyAsync(thrust::raw_pointer_cast(ho2.data()), thrust::raw_pointer_cast(do2.data()), do2.size()*sizeof(mytype), cudaMemcpyDeviceToHost, s2);

  cudaDeviceSynchronize();
  for (int i=0; i < KSIZE; i++){
    if (ho1[i] != ((LSIZE*SSIZE*TV1) + i)) { std::cout << "mismatch on stream 1 at " << i << " was: " << ho1[i] << " should be: " << ((DSIZE*TV1)+i) << std::endl; return 1;}
    if (ho2[i] != ((LSIZE*SSIZE*TV2) + i)) { std::cout << "mismatch on stream 2 at " << i << " was: " << ho2[i] << " should be: " << ((DSIZE*TV2)+i) << std::endl; return 1;}
    }
  std::cout << "Success!" << std::endl;
  return 0;
}

对于我的测试用例，我使用了 RHEL5.5、Quadro5000 和 cuda 6.5RC.此示例旨在让推力创建非常小的内核(只有单个线程块，只要 KSIZE 很小，例如 32 或 64)，因此推力创建的内核从 thrust::for_each 可以同时运行.

For my test case, I used RHEL5.5, Quadro5000, and cuda 6.5RC. This example is designed to have thrust create very small kernels (only a single threadblock, as long as KSIZE is small, say 32 or 64), so that the kernels that thrust creates from thrust::for_each are able to run concurrently.

当我分析这段代码时，我看到:

When I profile this code, I see:

这表明我们正在实现推力内核之间以及复制操作和推力内核之间的适当重叠，以及内核完成时的异步数据复制.请注意，cudaDeviceSynchronize() 操作填充"了时间线，表明所有异步操作(数据复制、推力功能)都是异步发出的，并且在任何操作进行之前控制返回给主机线程.所有这些都是预期的，主机、GPU 和数据复制操作之间完全并发的正确行为.

This indicates that we are achieving proper overlap both between thrust kernels, and between copy operations and thrust kernels, as well as asynchronous data copying at the completion of the kernels. Note that the cudaDeviceSynchronize() operation "fills" the timeline, indicating that all the async operations (data copying, thrust functions) were issued asynchronously and control returned to the host thread before any of the operations were underway. All of this is expected, proper behavior for full concurrency between host, GPU, and data copying operations.

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