Very low speed

Please test the following code sample:

#include <cutil.h>

#define ARRAY_SIZE 10000000
#define SHARED_BLOCK_SIZE 1000

global void Calc_GPU();

extern “C” void CUDA_Calculate(unsigned long* timeDelta)
unsigned int timer = 0;

Calc_GPU<<<1, 1>>>();

*timeDelta = cutGetTimerValue(timer);


global void Calc_GPU()
shared float sharedArrayOne[SHARED_BLOCK_SIZE];
shared float sharedArrayTwo[SHARED_BLOCK_SIZE];
shared float sharedArrayThree[SHARED_BLOCK_SIZE];

for (int i = 0 ; i < numCycles ; i++)
	for (int j = 0 ; j < SHARED_BLOCK_SIZE - 1 ; j++)
		sharedArrayThree[j] = sharedArrayOne[j] * sharedArrayTwo[j];


On my 280 GTX I get *timeDelta = 1100 ms, or little more then one second. So caculation speed is about 10 million operations per second. And it is from the shared memory.
I think it is too slow even for one GPU kernel.

What is wrong?

Using just one warp will cause latency problems because of register stalls. You need at least 192 threads in a block to guarantee that those stalls don’t happen. It has to do with the pipeline depth of the ALU.

This is hidden in the CUDA programming guide, search for “192” to find the reference.

Not only pipeline and register stalls, but you can run at least 30 blocks or 32 threads in exactly the same amount of time as your one block/one thread example. That would give you 900x more work being done.