cudaSetDevice（）对CUDA设备有什么作用；什么是上下文堆栈？

假设我有一个与设备

I

关联的活动CUDA上下文，现在我调用

cudaSetDevice（I）

。发生了什么事

什么都没有

主上下文是否替换堆栈的顶部

是否将主上下文推送到堆栈上

事实上，这似乎是不一致的。我编写了这个程序，在一台只有一个设备的机器上运行：

#include <cuda.h>
#include <cuda_runtime_api.h>
#include <cassert>
#include <iostream>

int main()
{
        CUcontext ctx1, primary;
        cuInit(0);
        auto status = cuCtxCreate(&ctx1, 0, 0);
        assert (status == (CUresult) cudaSuccess);
        cuCtxPushCurrent(ctx1);
        status = cudaSetDevice(0);
        assert (status == cudaSuccess);
        void* ptr1;
        void* ptr2;
        cudaMalloc(&ptr1, 1024);
        assert (status == cudaSuccess);
        cuCtxGetCurrent(&primary);
        assert (status == (CUresult) cudaSuccess);
        assert(primary != ctx1);
        status = cuCtxPushCurrent(ctx1);
        assert (status == (CUresult) cudaSuccess);
        cudaMalloc(&ptr2, 1024);
        assert (status == (CUresult) cudaSuccess);
        cudaSetDevice(0);
        assert (status == (CUresult) cudaSuccess);
        int i = 0;
        while (true) {
                status = cuCtxPopCurrent(&primary);
                if (status != (CUresult) cudaSuccess) { break; }
                std::cout << "Next context on stack (" << i++ << ") is " << (void*) primary << '\n';
        }
}

这种行为似乎有时是一种替代，有时是一种推动

---

发生了什么事；DR：根据您提供的代码，在这两种特定用法中，

cudaSetDevice（）

似乎正在替换堆栈顶部的上下文

让我们稍微修改一下代码，然后看看我们可以推断出代码中每个API调用对上下文堆栈的影响：

$ cat t1759.cu
#include <cuda.h>
#include <cuda_runtime_api.h>
#include <cassert>
#include <iostream>
void check(int j, CUcontext ctx1, CUcontext ctx2){
  CUcontext ctx0;
  int i = 0;
  while (true) {
                auto status = cuCtxPopCurrent(&ctx0);
                if (status != CUDA_SUCCESS) { break; }
                if (ctx0 == ctx1) std::cout << j << ":Next context on stack (" << i++ << ") is ctx1:" << (void*) ctx0 << '\n';
                else if (ctx0 == ctx2) std::cout << j << ":Next context on stack (" << i++ << ") is ctx2:" << (void*) ctx0 << '\n';
                else std::cout << j << ":Next context on stack (" << i++ << ") is unknown:" << (void*) ctx0 << '\n';
  }
}
void runtest(int i)
{
        CUcontext ctx1, primary = NULL;
        cuInit(0);
        auto dstatus = cuCtxCreate(&ctx1, 0, 0);    // checkpoint 1
        assert (dstatus == CUDA_SUCCESS);
        if (i == 1) {check(i,ctx1,primary); return;}// checkpoint 1
        dstatus = cuCtxPushCurrent(ctx1);           // checkpoint 2
        assert (dstatus == CUDA_SUCCESS);
        if (i == 2) {check(i,ctx1,primary); return;}// checkpoint 2
        auto rstatus = cudaSetDevice(0);            // checkpoint 3
        assert (rstatus == cudaSuccess);
        if (i == 3) {check(i,ctx1,primary); return;}// checkpoint 3
        void* ptr1;
        void* ptr2;
        rstatus = cudaMalloc(&ptr1, 1024);          // checkpoint 4
        assert (rstatus == cudaSuccess);
        if (i == 4) {check(i,ctx1,primary); return;}// checkpoint 4
        dstatus = cuCtxGetCurrent(&primary);        // checkpoint 5
        assert (dstatus == CUDA_SUCCESS);
        assert(primary != ctx1);
        if (i == 5) {check(i,ctx1,primary); return;}// checkpoint 5
        dstatus = cuCtxPushCurrent(ctx1);           // checkpoint 6
        assert (dstatus == CUDA_SUCCESS);
        if (i == 6) {check(i,ctx1,primary); return;}// checkpoint 6
        rstatus = cudaMalloc(&ptr2, 1024);          // checkpoint 7
        assert (rstatus == cudaSuccess);
        if (i == 7) {check(i,ctx1,primary); return;}// checkpoint 7
        rstatus = cudaSetDevice(0);                 // checkpoint 8
        assert (rstatus == cudaSuccess);
        if (i == 8) {check(i,ctx1,primary); return;}// checkpoint 8
        return;
}

int main(){
        for (int i = 1; i < 9; i++){
          cudaDeviceReset();
          runtest(i);}
}
$ nvcc -o t1759 t1759.cu -lcuda -std=c++11
$ ./t1759
1:Next context on stack (0) is ctx1:0x11087e0
2:Next context on stack (0) is ctx1:0x1741160
2:Next context on stack (1) is ctx1:0x1741160
3:Next context on stack (0) is unknown:0x10dc520
3:Next context on stack (1) is ctx1:0x1c5aa70
4:Next context on stack (0) is unknown:0x10dc520
4:Next context on stack (1) is ctx1:0x23eaa00
5:Next context on stack (0) is ctx2:0x10dc520
5:Next context on stack (1) is ctx1:0x32caf30
6:Next context on stack (0) is ctx1:0x3a44ed0
6:Next context on stack (1) is ctx2:0x10dc520
6:Next context on stack (2) is ctx1:0x3a44ed0
7:Next context on stack (0) is ctx1:0x41cfd90
7:Next context on stack (1) is ctx2:0x10dc520
7:Next context on stack (2) is ctx1:0x41cfd90
8:Next context on stack (0) is ctx2:0x10dc520
8:Next context on stack (1) is ctx2:0x10dc520
8:Next context on stack (2) is ctx1:0x4959c70
$

上下文创建还将新创建的上下文推送到堆栈上，如前所述

二,

毫不奇怪，在堆栈上推送相同的上下文会为其创建另一个堆栈条目

三,

cudaSetDevice（）

调用已将堆栈顶部替换为“未知”上下文。（此时未知，因为我们尚未检索“其他”上下文的句柄值）

四,

由于此调用，堆栈配置没有差异

五,

由于此调用，堆栈配置没有差异，但我们现在知道堆栈顶部上下文是当前上下文（我们可以推测它是主上下文）

六,

这里没有真正的惊喜。我们在堆栈上按下

ctx1

，因此堆栈有3个条目，第一个条目是驱动程序API创建的上下文，接下来的两个条目与步骤5中的堆栈配置相同，只是向下移动了一个堆栈位置

七,

同样，此调用对堆栈配置没有影响

八,

我们再次看到，这里的行为是

cudaSetDevice（）

调用已将堆栈顶部上下文替换为主上下文

我从您的测试代码中得出的结论是，当与代码中的各种运行时和驱动程序API调用混合时，

cudaSetDevice（）

调用的行为没有不一致

---

在我看来，这种编程范式是疯狂的。我无法想象为什么你会希望以这种方式混合驱动程序API和运行时API代码。

哦，不，我不想这样做，这是一个人工示例，我只是想了解其行为，这样当我将API包装扩展到驱动程序API时，我就不会因为错误的假设而把事情搞砸。现在，我有了“推”和“弹出”的代码当前设备-假设仅使用运行时API。既然您已经阐明了

cudaSetDevice（）

的行为方式，我可以将该代码更改为：1。查看当前上下文。2.把它放在一边。3.

cudaSetDevice（）

到我想使用运行时API 4使用的设备。做我的工作。5.

cuCtxSetCurrent（）

到我保存的上下文中。通过编辑我的答案，您做出了一个我不满意的声明，并有效地将该声明归因于我。如果你不编辑我的答案可能会更好（除非我将它们标记为社区维基）。我对此感到不舒服，我建议今后不要对我的答案如此随意。我很乐意对我观察到的东西进行陈述。就这样吧。如果您希望进一步了解，我的建议是通过developer.nvidia.com上的bug归档门户提出此类请求。如果您希望澄清，请使用评论。此外，我不会对您现在在评论中列出的5个步骤进行说明。我将尊重您的建议和要求。

$ cat t1759.cu
#include <cuda.h>
#include <cuda_runtime_api.h>
#include <cassert>
#include <iostream>
void check(int j, CUcontext ctx1, CUcontext ctx2){
  CUcontext ctx0;
  int i = 0;
  while (true) {
                auto status = cuCtxPopCurrent(&ctx0);
                if (status != CUDA_SUCCESS) { break; }
                if (ctx0 == ctx1) std::cout << j << ":Next context on stack (" << i++ << ") is ctx1:" << (void*) ctx0 << '\n';
                else if (ctx0 == ctx2) std::cout << j << ":Next context on stack (" << i++ << ") is ctx2:" << (void*) ctx0 << '\n';
                else std::cout << j << ":Next context on stack (" << i++ << ") is unknown:" << (void*) ctx0 << '\n';
  }
}
void runtest(int i)
{
        CUcontext ctx1, primary = NULL;
        cuInit(0);
        auto dstatus = cuCtxCreate(&ctx1, 0, 0);    // checkpoint 1
        assert (dstatus == CUDA_SUCCESS);
        if (i == 1) {check(i,ctx1,primary); return;}// checkpoint 1
        dstatus = cuCtxPushCurrent(ctx1);           // checkpoint 2
        assert (dstatus == CUDA_SUCCESS);
        if (i == 2) {check(i,ctx1,primary); return;}// checkpoint 2
        auto rstatus = cudaSetDevice(0);            // checkpoint 3
        assert (rstatus == cudaSuccess);
        if (i == 3) {check(i,ctx1,primary); return;}// checkpoint 3
        void* ptr1;
        void* ptr2;
        rstatus = cudaMalloc(&ptr1, 1024);          // checkpoint 4
        assert (rstatus == cudaSuccess);
        if (i == 4) {check(i,ctx1,primary); return;}// checkpoint 4
        dstatus = cuCtxGetCurrent(&primary);        // checkpoint 5
        assert (dstatus == CUDA_SUCCESS);
        assert(primary != ctx1);
        if (i == 5) {check(i,ctx1,primary); return;}// checkpoint 5
        dstatus = cuCtxPushCurrent(ctx1);           // checkpoint 6
        assert (dstatus == CUDA_SUCCESS);
        if (i == 6) {check(i,ctx1,primary); return;}// checkpoint 6
        rstatus = cudaMalloc(&ptr2, 1024);          // checkpoint 7
        assert (rstatus == cudaSuccess);
        if (i == 7) {check(i,ctx1,primary); return;}// checkpoint 7
        rstatus = cudaSetDevice(0);                 // checkpoint 8
        assert (rstatus == cudaSuccess);
        if (i == 8) {check(i,ctx1,primary); return;}// checkpoint 8
        return;
}

int main(){
        for (int i = 1; i < 9; i++){
          cudaDeviceReset();
          runtest(i);}
}
$ nvcc -o t1759 t1759.cu -lcuda -std=c++11
$ ./t1759
1:Next context on stack (0) is ctx1:0x11087e0
2:Next context on stack (0) is ctx1:0x1741160
2:Next context on stack (1) is ctx1:0x1741160
3:Next context on stack (0) is unknown:0x10dc520
3:Next context on stack (1) is ctx1:0x1c5aa70
4:Next context on stack (0) is unknown:0x10dc520
4:Next context on stack (1) is ctx1:0x23eaa00
5:Next context on stack (0) is ctx2:0x10dc520
5:Next context on stack (1) is ctx1:0x32caf30
6:Next context on stack (0) is ctx1:0x3a44ed0
6:Next context on stack (1) is ctx2:0x10dc520
6:Next context on stack (2) is ctx1:0x3a44ed0
7:Next context on stack (0) is ctx1:0x41cfd90
7:Next context on stack (1) is ctx2:0x10dc520
7:Next context on stack (2) is ctx1:0x41cfd90
8:Next context on stack (0) is ctx2:0x10dc520
8:Next context on stack (1) is ctx2:0x10dc520
8:Next context on stack (2) is ctx1:0x4959c70
$

        auto dstatus = cuCtxCreate(&ctx1, 0, 0);    // checkpoint 1
1:Next context on stack (0) is ctx1:0x11087e0

        dstatus = cuCtxPushCurrent(ctx1);           // checkpoint 2
2:Next context on stack (0) is ctx1:0x1741160
2:Next context on stack (1) is ctx1:0x1741160

        auto rstatus = cudaSetDevice(0);            // checkpoint 3
3:Next context on stack (0) is unknown:0x10dc520
3:Next context on stack (1) is ctx1:0x1c5aa70

        rstatus = cudaMalloc(&ptr1, 1024);          // checkpoint 4
4:Next context on stack (0) is unknown:0x10dc520
4:Next context on stack (1) is ctx1:0x23eaa00

        dstatus = cuCtxGetCurrent(&primary);        // checkpoint 5
5:Next context on stack (0) is ctx2:0x10dc520
5:Next context on stack (1) is ctx1:0x32caf30

        dstatus = cuCtxPushCurrent(ctx1);           // checkpoint 6
6:Next context on stack (0) is ctx1:0x3a44ed0
6:Next context on stack (1) is ctx2:0x10dc520
6:Next context on stack (2) is ctx1:0x3a44ed0

        rstatus = cudaMalloc(&ptr2, 1024);          // checkpoint 7
7:Next context on stack (0) is ctx1:0x41cfd90
7:Next context on stack (1) is ctx2:0x10dc520
7:Next context on stack (2) is ctx1:0x41cfd90

        rstatus = cudaSetDevice(0);                 // checkpoint 8
8:Next context on stack (0) is ctx2:0x10dc520
8:Next context on stack (1) is ctx2:0x10dc520
8:Next context on stack (2) is ctx1:0x4959c70