How to solve CUDA Thrust library - for_each synchronization error? - foreach

Question

I'm trying to modify a simple dynamic vector in CUDA using the thrust library of CUDA. But I'm getting "launch_closure_by_value" error on the screen indicatiing that the error is related to some synchronization process.
A simple 1D dynamic array modification is not possible due to this error.
My code segment which is causing the error is as follows.
from a .cpp file I call setIndexedGrid, which is defined in System.cu
float* a= (float*)(malloc(8*sizeof(float)));
a[0]= 0; a[1]= 1; a[2]= 2; a[3]= 3; a[4]= 4; a[5]= 5; a[6]= 6; a[7]= 7;
float* b = (float*)(malloc(8*sizeof(float)));
setIndexedGridInfo(a,b);
The code segment at System.cu:
void
setIndexedGridInfo(float* a, float*b)
{
thrust::device_ptr<float> d_oldData(a);
thrust::device_ptr<float> d_newData(b);
float c = 0.0;
thrust::for_each(
thrust::make_zip_iterator(thrust::make_tuple(d_oldData,d_newData)),
thrust::make_zip_iterator(thrust::make_tuple(d_oldData+8,d_newData+8)),
grid_functor(c));
}
grid_functor is defined in _kernel.cu
struct grid_functor
{
float a;
__host__ __device__
grid_functor(float grid_Info) : a(grid_Info) {}
template <typename Tuple>
__device__
void operator()(Tuple t)
{
volatile float data = thrust::get<0>(t);
float pos = data + 0.1;
thrust::get<1>(t) = pos;
}
};
I also get these on the Output window (I use Visual Studio):
First-chance exception at 0x000007fefdc7cacd in Particles.exe:
Microsoft C++ exception: cudaError_enum at memory location
0x0029eb60.. First-chance exception at 0x000007fefdc7cacd in
smokeParticles.exe: Microsoft C++ exception:
thrust::system::system_error at memory location 0x0029ecf0.. Unhandled
exception at 0x000007fefdc7cacd in Particles.exe: Microsoft C++
exception: thrust::system::system_error at memory location
0x0029ecf0..
What is causing the problem?

Answer 1

You are trying to use host memory pointers in functions expecting pointers in device memory. This code is the problem:
float* a= (float*)(malloc(8*sizeof(float)));
a[0]= 0; a[1]= 1; a[2]= 2; a[3]= 3; a[4]= 4; a[5]= 5; a[6]= 6; a[7]= 7;
float* b = (float*)(malloc(8*sizeof(float)));
setIndexedGridInfo(a,b);
.....
thrust::device_ptr<float> d_oldData(a);
thrust::device_ptr<float> d_newData(b);
The thrust::device_ptr is intended for "wrapping" a device memory pointer allocated with the CUDA API so that thrust can use it. You are trying to treat a host pointer directly as a device pointer. That is illegal. You could modify your setIndexedGridInfo function like this:
void setIndexedGridInfo(float* a, float*b, const int n)
{
thrust::device_vector<float> d_oldData(a,a+n);
thrust::device_vector<float> d_newData(b,b+n);
float c = 0.0;
thrust::for_each(
thrust::make_zip_iterator(thrust::make_tuple(d_oldData.begin(),d_newData.begin())),
thrust::make_zip_iterator(thrust::make_tuple(d_oldData.end(),d_newData.end())),
grid_functor(c));
}
The device_vector constructor will allocate device memory and then copy the contents of your host memory to the device. That should fix the error you are seeing, although I am not sure what you are trying to do with the for_each iterator and whether the functor you have wrttien is correct.
Edit:
Here is a complete, compilable, runnable version of your code:
#include <cstdlib>
#include <cstdio>
#include <thrust/device_vector.h>
#include <thrust/for_each.h>
#include <thrust/copy.h>
struct grid_functor
{
float a;
__host__ __device__
grid_functor(float grid_Info) : a(grid_Info) {}
template <typename Tuple>
__device__
void operator()(Tuple t)
{
volatile float data = thrust::get<0>(t);
float pos = data + 0.1f;
thrust::get<1>(t) = pos;
}
};
void setIndexedGridInfo(float* a, float*b, const int n)
{
thrust::device_vector<float> d_oldData(a,a+n);
thrust::device_vector<float> d_newData(b,b+n);
float c = 0.0;
thrust::for_each(
thrust::make_zip_iterator(thrust::make_tuple(d_oldData.begin(),d_newData.begin())),
thrust::make_zip_iterator(thrust::make_tuple(d_oldData.end(),d_newData.end())),
grid_functor(c));
thrust::copy(d_newData.begin(), d_newData.end(), b);
}
int main(void)
{
const int n = 8;
float* a= (float*)(malloc(n*sizeof(float)));
a[0]= 0; a[1]= 1; a[2]= 2; a[3]= 3; a[4]= 4; a[5]= 5; a[6]= 6; a[7]= 7;
float* b = (float*)(malloc(n*sizeof(float)));
setIndexedGridInfo(a,b,n);
for(int i=0; i<n; i++) {
fprintf(stdout, "%d (%f,%f)\n", i, a[i], b[i]);
}
return 0;
}
I can compile and run this code on an OS 10.6.8 host with CUDA 4.1 like this:
$ nvcc -Xptxas="-v" -arch=sm_12 -g -G thrustforeach.cu
./thrustforeach.cu(18): Warning: Cannot tell what pointer points to, assuming global memory space
./thrustforeach.cu(20): Warning: Cannot tell what pointer points to, assuming global memory space
./thrustforeach.cu(18): Warning: Cannot tell what pointer points to, assuming global memory space
./thrustforeach.cu(20): Warning: Cannot tell what pointer points to, assuming global memory space
ptxas info : Compiling entry function '_ZN6thrust6detail7backend4cuda6detail23launch_closure_by_valueINS2_18for_each_n_closureINS_12zip_iteratorINS_5tupleINS0_15normal_iteratorINS_10device_ptrIfEEEESB_NS_9null_typeESC_SC_SC_SC_SC_SC_SC_EEEEi12grid_functorEEEEvT_' for 'sm_12'
ptxas info : Used 14 registers, 160+0 bytes lmem, 16+16 bytes smem, 4 bytes cmem[1]
ptxas info : Compiling entry function '_ZN6thrust6detail7backend4cuda6detail23launch_closure_by_valueINS2_18for_each_n_closureINS_12zip_iteratorINS_5tupleINS0_15normal_iteratorINS_10device_ptrIfEEEESB_NS_9null_typeESC_SC_SC_SC_SC_SC_SC_EEEEj12grid_functorEEEEvT_' for 'sm_12'
ptxas info : Used 14 registers, 160+0 bytes lmem, 16+16 bytes smem, 4 bytes cmem[1]
$ ./a.out
0 (0.000000,0.100000)
1 (1.000000,1.100000)
2 (2.000000,2.100000)
3 (3.000000,3.100000)
4 (4.000000,4.100000)
5 (5.000000,5.100000)
6 (6.000000,6.100000)
7 (7.000000,7.100000)