Why is the Kernel faster when my matrices are not initialized


I am trying to understand the basics of CUDA, and I am trying to measure the time execution of my programs.

I have a behavior that I found weird (but it is probably not). Here is my code.

#include <iostream>
#include <math.h>
#include <chrono>

// Kernel function to add the elements of two arrays

#define gpuErrchk(ans) { gpuAssert((ans), __FILE__, __LINE__); }
inline void gpuAssert(cudaError_t code, const char *file, int line, bool abort = true)
	if (code != cudaSuccess)
		fprintf(stderr, "GPUassert: %s %s %d\n", cudaGetErrorString(code), file, line);
		if (abort) exit(code);

__global__  void MatProd(float* C, float* A, float*B, int dimAx, int dimBx, int dimCx, int dimCy)
	int row = blockDim.y*blockIdx.y + threadIdx.y;
	int col = blockDim.x*blockIdx.x + threadIdx.x;

	//printf("oui! \n");
	//printf("%d \n", blockDim.y);

	double Result = 0;

	if (row <= dimCy - 1 && col <= dimCx - 1)
		for (int k = 0; k < dimAx; k++)
			Result += A[k + dimAx*row] * B[col + dimBx*k];
			//printf("(%d,%d) \n", row, col);

		C[col + row*dimCx] = Result;
int main(void)
	// Exemple de multiplication de matrice par un scalaire :
	std::chrono::high_resolution_clock::time_point programBegin = std::chrono::high_resolution_clock::now();
	int lambda = 1000;
	// tailles libres
	int dimAx = 100;
	int dimAy = pow(10, 5);
	int dimBx = 2;
	// tailles contraintes
	int dimBy = dimAx;
	int dimCx = dimBx;
	int dimCy = dimAy;

	float *mat, *res, *d_C, *A, *B;
	float millisecondsPureComputation = 0;
	float millisecondsMemoryTransfer = 0;

	size_t sizeA = sizeof(float)*dimAx*dimAy;
	size_t sizeB = sizeof(float)*dimBx*dimBy;

	cudaMallocManaged(&A, dimAx*dimAy*sizeof(float));
	cudaMallocManaged(&B, dimBx*dimBy*sizeof(float));

	// BUT WHY ???
	for (int i = 0; i < dimAy; i++)
	for (int j = 0; j < dimAx; j++)
	A[j + dimAx*i] = j + 10 * i;
	for (int i = 0; i < dimBy; i++)
	for (int j = 0; j < dimBx; j++)
	B[j + dimBx*i] = (j + 1)*pow(i, 2);
	// --------------------END OF COMMENTED SECTION --------------------------------------------------------------------------

	// Allocating memory for the result matric C.
	gpuErrchk(cudaMallocManaged(&d_C, dimCx*dimCy*sizeof(float)));

	// Computation part :

	int threadPerBlockx = 32;
	int threadPerBlocky = 32;
	int BlockPerGridx = 1 + (dimCx - 1) / threadPerBlockx;
	int BlockPerGridy = 1 + (dimCy - 1) / threadPerBlockx;

	dim3 BlockPerGrid(BlockPerGridx, BlockPerGridy, 1);
	dim3 ThreadPerBlock(threadPerBlockx, threadPerBlocky, 1);

	gpuErrchk(cudaDeviceSynchronize()); // I wait that the GPU & the CPU start at the same time
	std::chrono::high_resolution_clock::time_point ComputationBegin = std::chrono::high_resolution_clock::now();

	MatProd << <BlockPerGrid, ThreadPerBlock >> >(d_C, A, B, dimAx, dimBx, dimCx, dimCy);

	gpuErrchk(cudaDeviceSynchronize()); // I wait that the Kernel finishes its execution
	std::chrono::high_resolution_clock::time_point ComputationEnd = std::chrono::high_resolution_clock::now();

	auto durationComputation = std::chrono::duration_cast<std::chrono::milliseconds>(ComputationEnd - ComputationBegin).count();


	std::chrono::high_resolution_clock::time_point programEnd = std::chrono::high_resolution_clock::now();
	auto durationProgram = std::chrono::duration_cast<std::chrono::milliseconds>(programEnd - programBegin).count();

	std::cout << "Milliseconds CUDA computation : " << durationComputation << " ms" << std::endl;
	std::cout << "Total elapsed time : " << durationProgram << " ms" << std::endl;

	return 0;

In fact my kernel computes in 79ms (global execution : 484ms) when the section from lines 63 to 79 of my code is commented. But when it is not the kernel last for 276ms (global execution : 719ms).

But for me the kernel execution that occurs at line 99 shouldn’t be different ? Indeed when I don’t initialise the variables should just have some random values. And as the initialisation part is not in the kernel, the kernel execution shouldn’t change. It is probably more a C++ issue than a CUDA one, I am not sure.

(Remark : I measure this time of execution using C++ functions at lines 97 & 102)

The point of my post is also to check if I understood well how to measure time using CPU functions (I
think I got how to use the CUDA event functions now). Around the Kernel I synchronised before and after the CPU and GPU.

After the kernel to be sure that the CPU wait that the kernel indeed finished its process before measuring the time.

And before the kernel to be sure that the GPU is not late before starting measuring the time. Indeed if I don’t put this one, I would probably start measuring the time before the GPU is ready to start the computation Kernel.

Thanks a lot.

It’s important to state things like what GPU you are using, and also what platform (windows/linux) and what command line you are using to compile, when asking performance related questions.

You are using managed memory. The following comments apply for pascal and newer architectures on linux.

When you perform a managed allocation, the allocation is initially done on the device.

If you then “touch” that allocation in host code, the necessary pages will be allocated in host memory and the “location” of the allocation is shifted there.

Then when you touch them on the device, the demand-paging system must move the pages from host to device as you touch them. This will slow the kernel down, but you can adjust this behavior:

[url]c++ - Why is NVIDIA Pascal GPUs slow on running CUDA Kernels when using cudaMallocManaged - Stack Overflow

On the other hand, if you don’t touch the pages after the managed allocation, they are resident on the device. When the kernel starts executing, it can access the data without generating any page faults. So it runs faster, generally.

Yes sorry I forgot.

I have GTX 970M on an MSI GT72.
I work on Windows 10.
I am using CUDA 8

About the compiling options : I didn’t changed anything from the default configuration of Visual Studio, I am running in Debug x64 in the option of the IDE.

I think this is what you ask (I’m not sure)

# (Approximate command-line.  Settings inherited from host are not visible below.)
# (Please see the output window after a build for the full command-line)

# Driver API (NVCC Compilation Type is .cubin, .gpu, or .ptx)
set CUDAFE_FLAGS=--sdk_dir "C:\Program Files (x86)\Windows Kits.1\"
"C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0\bin\nvcc.exe" --use-local-env --cl-version 2013 -ccbin "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64"     -G   --keep-dir x64\Debug -maxrregcount=0  --machine 64 --compile -cudart static  -o x64\Debug\%(Filename)%(Extension).obj "%(FullPath)"

# Runtime API (NVCC Compilation Type is hybrid object or .c file)
set CUDAFE_FLAGS=--sdk_dir "C:\Program Files (x86)\Windows Kits.1\"
"C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v8.0\bin\nvcc.exe" --use-local-env --cl-version 2013 -ccbin "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64"     -G   --keep-dir x64\Debug -maxrregcount=0  --machine 64 --compile -cudart static  -g    -Xcompiler "/EHsc  /nologo  /FS /Zi   " -o x64\Debug\%(Filename)%(Extension).obj "%(FullPath)"

Ok, the fact that I use managed memories here makes the things slow down. Because I am changing the value on the host the variable goes from device to host memory. And then I work in a Kernel so the GPU has to take the variable back on the GPU memory.

In summary, managed memory is nice for practical purpose but if I badly use it like here it can slow down my calculations.

And finally just to check : you are ok with my time measurement and the synchronisation I used ? I’m almost sure it’s ok but as I am used to a lot of hidden behavior for a beginner in parallel programming I prefer to check.