Understanding cuda heap memory limitations per thr

问题:

This question is about heap size limitation in cuda. Having visited some questions concerning this topic, including this one: new operator in kernel .. strange behaviour I've made some tests. Given a kernel as follow:

#include <cuda.h>
#include <cuda_runtime.h>
#define CUDA_CHECK( err ) __cudaSafeCall( err, __FILE__, __LINE__ )
#define CUDA_CHECK_ERROR()    __cudaCheckError( __FILE__, __LINE__ )
inline void __cudaSafeCall( cudaError err, const char *file, const int line )
{
    if ( cudaSuccess != err )
    {
        fprintf( stderr, "cudaSafeCall() failed at %s:%i : %s\n",
                file, line, cudaGetErrorString( err ) );
        exit( -1 );
    }
    return;
}

inline void __cudaCheckError( const char *file, const int line )
{
    cudaError err = cudaGetLastError();
    if ( cudaSuccess != err )
    {
        fprintf( stderr, "cudaCheckError() failed at %s:%i : %s\n",
                file, line, cudaGetErrorString( err ) );
        exit( -1 );
    }
    return;
}
#include <stdio>
#define NP 900000
__device__ double *temp;
__device__ double *temp2;
__global__
void test(){
int i = blockDim.x*blockIdx.x + threadIdx.x;
if(i==0){
    temp = new double[NP];
    //temp2 = new double[NP];
}

if(i==0){
    for(int k=0;k<NP;k++){
        temp[i] = 1.;
        if(k%1000 == 0){
            printf("%d : %g\n", k, temp[i]);
        }
    }
}
if(i==0){
    delete(temp);
    //delete(temp2);
}
}
int main(){
    //cudaDeviceSetLimit(cudaLimitMallocHeapSize, 32*1024*1024);
    //for(int k=0;k<2;k++){
        test<<<ceil((float)NP/512), 512>>>();
        CUDA_CHECK_ERROR();
    //}
    return 0;
}

I want to test the heap size limitation.

1. Dynamically allocating one array (temp) with one thread which size is roughly over 960,000*sizeof(double) (close to 8MB, which is the default limit of the heap size) gives an error : ok. 900,000 works. (does someone know how to calculate the true limit?)
2. Rising the heap size limit allows to allocate more memory : normal, ok.
3. Back to a 8MB heap size, allocating one array per thread with TWO threads (so, replacing if (i==0) by if(i==0 || i==1), each one 900,000 * sizeof(double) fails. But 450,000*sizeof(double) each, works. Still ok.
4. Here comes my problem : allocating TWO arrays with ONE thread (so, temp and temp2 for thread 0), each 900,000 * sizeof(double) works too, but it should not? Indeed when I try to write in both arrays, it fails. But anyone has an idea why this different behaviour in allocation when using two arrays with one thread instead of two arrays with two threads?

EDIT : another test, which I find interesting for those who, like me, would be learning the usage of heap : 5. Executing the kernel two times, with one array of size 900,000 * sizeof(double) allocated by the single thread 0, works if there is the delete. If delete is omitted, it will fail the second time, but the first call will be executed.

EDIT 2 : how to allocate a device-wide variable once but writable by all threads (not from host, using dynamic allocation in device code)?

回答1:

Probably you are not testing for a returned null-pointer on the new operation, which is a valid method in C++ for the operator to report a failure.

When I modify your code as follows, I get the message "second new failed":

#include <stdio.h>

 #define NP 900000
__device__ double *temp;
__device__ double *temp2;
__global__
void test(){
int i = blockDim.x*blockIdx.x + threadIdx.x;
if(i==0){
    temp = new double[NP];
    if (temp == 0) {printf("first new failed\n"); return;}
    temp2 = new double[NP];
    if (temp2 == 0) {printf("second new failed\n"); return;}
}

if(i==0){
    for(int k=0;k<NP;k++){
        temp[i] = 1.;
        if(k%1000 == 0){
            printf("%d : %g\n", k, temp[i]);
        }
    }
}
if(i==0){
    delete(temp);
    delete(temp2);
}
}

int main() {

  test<<<1,1>>>();
  cudaDeviceSynchronize();
  return 0;
}

It's convenient if you provide a complete, compilable code, for others to work with, just as I have.

For your first EDIT question, it's not surprising that the second new will work if the first is deleted. The first allocates nearly all of the 8MB available. If you delete that allocation, then the second one will succeed. Referring to the documentation, we see that memory allocated dynamically in this fashion lives for the entire lifetime of the cuda context, or until a corresponding delete operation is performed (i.e. not just a single kernel call. The completion of the kernel does not necessarily free the allocation.)

For your second EDIT question, you are already demonstrating a method, using your __device__ double *temp; pointer, by which one thread can allocate storage which all threads can access. You will have a problem across blocks, however, because there is no guarantee of synchronization order amongst blocks or execution order amongst blocks, so if you allocate from thread 0 in block 0, that is only useful if block 0 executes before other blocks. You could come up with a complicated scheme to check if the variable allocation was already done (perhaps by testing the pointer for NULL, and also perhaps using atomics) but it creates fragile code. It's better to plan your global allocations ahead of time and allocate accordingly from the host.