Mersenne Twister Weird Error

Hi,

We have recently got the new conformant OpenCL SDK and we have tried to convert the CUDA SDK Mersenne Twister algorithm and we have encountered a weird error. Where screen is garbled and cannot be taken back, the only solution is to restart the Windows Vista x64. (BTW : We are not registered users :) )

We are giving the following “problematic” code.

#define   MT_RNG_COUNT 4096

#define		  MT_MM 9

#define		  MT_NN 19

#define	   MT_WMASK 0xFFFFFFFFU

#define	   MT_UMASK 0xFFFFFFFEU

#define	   MT_LMASK 0x1U

#define	  MT_SHIFT0 12

#define	  MT_SHIFTB 7

#define	  MT_SHIFTC 15

#define	  MT_SHIFT1 18

typedef struct

{

	unsigned int matrix_a;

	unsigned int mask_b;

	unsigned int mask_c;

	unsigned int seed;

} mt_struct_stripped;

// OpenCL Kernel Function for element by element vector addition

__kernel void Rand(__global float *d_Random,  __constant mt_struct_stripped* ds_MT, __global int NPerRng)

{

	const int	  tid = get_global_id(0);

	const int THREAD_N = get_global_size(0);

	unsigned int mt[MT_NN];

	int iState, iState1, iStateM, iOut, iRng;

	unsigned int mti, mti1, mtiM, x;

	//Initialize current state

	for(iRng = tid; iRng < MT_RNG_COUNT; iRng += THREAD_N)

	{

		mt_struct_stripped config = ds_MT[iRng];

		mt[0] = config.seed;

		for(iState = 1; iState < MT_NN; iState++)

			mt[iState] = (1812433253U * (mt[iState - 1] ^ (mt[iState - 1] >> 30)) + iState) & MT_WMASK;

		iState = 0;

		mti1 = mt[0];

		for(int iOut = 0; iOut < NPerRng; iOut++)

		{

			iState1 = iState + 1;

			iStateM = iState + MT_MM;

			if(iState1 >= MT_NN) iState1 -= MT_NN;

			if(iStateM >= MT_NN) iStateM -= MT_NN;

			mti  = mti1;

			mti1 = mt[iState1];

			mtiM = mt[iStateM];

			x	= (mti & MT_UMASK) | (mti1 & MT_LMASK);

			x	=  mtiM ^ (x >> 1) ^ ((x & 1) ? config.matrix_a : 0);

			mt[iState] = x;

			iState = iState1;

			//Tempering transformation

			x ^= (x >> MT_SHIFT0);

			x ^= (x << MT_SHIFTB) & config.mask_b;

			x ^= (x << MT_SHIFTC) & config.mask_c;

			x ^= (x >> MT_SHIFT1);

			//Convert to (0, 1] float and write to global memory

			d_Random[iRng + iOut * MT_RNG_COUNT] = ((float)x + 1.0f) / 4294967296.0f;

		}

	}

}

We have found that this line of code causes the error (we have a workaround for it)

x	=  mtiM ^ (x >> 1) ^ ((x & 1) ? config.matrix_a : 0);

The following code is safe but “a lot” slower.

x	=  mtiM ^ (x >> 1) ^ ((x & 1) ? ds_MT[iRng].matrix_a : 0);

Any comments on this subject may be useful.

Thanks in advance.

Hi,

It’s strange because mersenne Twister works fine for me…

The only difference it’s i don’t use .cl and i put my kernel in a char * with a macro :

#ifndef STRINGIFY

#define STRINGIFY(x) #x “\n”

#endif

So, it’s impossible to me to give structure as paramaters so i pass every paramaters of the structure

An finally it works for me because i make

matrix_a = pmatrix_a[iRng]
x	=  mtiM ^ (x >> 1) ^ ((x & 1) ? matrix_a : 0);

Thx

J

When I’m using the mt_struct_stripped struct, it results with the error described above. Using the work around described above works but a lot slower.
I’ve tried replacing the struct with a uint4. no error, but the results are wrong.
Finally I’ve splitted all of the variables of the struct to different buffers. (like jonathan81 suggested) This works, correct results and less slow down.

But, we couldnt find why the struct. version of the uint4 version does not work.

I don’t have an answer to your question but I have just coincidentally ported my CUDA-based Mersenne Twister generator to OpenCL. It’s a variant of the CUDA SDK example which, rather than generating a block of random numbers, provides a function which any thread can use to pull a new random number - like rand().

I’ve tested it on NVIDIA’s drivers and it isn’t astoundingly fast - about 1.7B PRNs/second into a GPU register on a GTX 260 - but it should improve as the drivers catch up. It’s also a cute demonstration of using Python to control OpenCL. Here’s the repository if you want to have a look:

http://git.jcornwall.me.uk/MersenneTwisterOCL/

Grab the source with:

git clone git://git.jcornwall.me.uk/MersenneTwisterOCL

We found out why the error happens. We don’t know why but we cannot assign a struct with “mt_struct_stripped config = ds_MT[iRng];”. assigning all the variables separately works though.