# Adding Large Matrices

Hi,

I’m trying to add two large matrices but am getting weird results…

Here is my kernel:

``````__global__ void matAdd_kernel(float* result,float* A,float* B,int size)

{

int x=blockIdx.x*blockDim.x+threadIdx.x;

int y=blockIdx.y*blockDim.y+threadIdx.y;

int idx=x*y+x;

if(idx<size)

{

result[idx]=A[idx]+B[idx];

}

}
``````

And here’s the code:

``````void matAdd(Matrix C,Matrix A,Matrix B)

{

int N=A.w*A.h;

dim3 dimBlock=dim3(22,22);

int yBlocks=A.w/dimBlock.y+((A.w%dimBlock.y)==0?0:1);   //yBlocks is 219 for dimBlock(22,22) and 9600x4800

int xBlocks=A.h/dimBlock.x+((A.h%dimBlock.x)==0?0:1);   //xBlocks is 437 for dimBlock(22,22) and 9600x4800

dim3 dimGrid=dim3(xBlocks,yBlocks);

matAdd_kernel<<<dimGrid,dimBlock>>>(C.data,A.data,B.data,N);

cutilCheckMsg("kernel launch failure");

}
``````

Matrix is defined as follows:

``````typedef struct{

int w;

int h;

float* data;

}Matrix;
``````

Matrix A, B and C are 4800x9600.

I have a function to print off the top-10 and bottom-10 rows, here are the results:

top-10 rows A:

-0.023930 0.047744 -0.074694 0.053555 -0.032298 0.038762 -0.068890 0.088894 -0.044989 0.005679 -0.054846 0.064743

-0.070026 0.059445 -0.078712 0.001957 -0.050910 0.067603 -0.089646 0.076562 -0.039840 0.052980 -0.074809 0.037390

-0.042785 0.087303 -0.005369 0.017769 -0.075572 0.075981 -0.064457 0.067737 -0.045192 0.046887 -0.030999 0.006888

-0.040708 0.020566 -0.089926 0.082820 -0.010478 0.021086 -0.086581 0.095966 -0.054339 0.068906 -0.060855 0.087460

-0.059717 0.038708 -0.026613 0.053984 -0.088490 0.066764 -0.005617 0.091969 -0.018239 0.097972 -0.073692 0.010064

-0.052374 0.048555 -0.037706 0.043377 -0.071556 0.075888 -0.002523 0.037950 -0.065693 0.078094 -0.011694 0.039196

-0.092799 0.011099 -0.056766 0.091866 -0.059577 0.029236 -0.063502 0.091717 -0.030844 0.079273 -0.087244 0.048310

-0.089582 0.004614 -0.002560 0.058306 -0.006922 0.097391 -0.099892 0.039699 -0.036129 0.038520 -0.084387 0.012408

-0.054143 0.048351 -0.006309 0.002902 -0.073858 0.012903 -0.089030 0.041077 -0.034445 0.030259 -0.071056 0.002762

-0.048605 0.047165 -0.082960 0.096326 -0.066084 0.029297 -0.070599 0.034394 -0.044475 0.075287 -0.063274 0.023137

-0.033560 0.030527 -0.019907 0.078961 -0.052821 0.088959 -0.043210 0.061808 -0.020862 0.058320 -0.028586 0.079149

-0.087878 0.034127 -0.040097 0.092205 -0.033817 0.099641 -0.002590 0.012473 -0.050764 0.093213 -0.065811 0.075233

bottom-10 rows A:

0.006791 -0.015538 0.072625 -0.049761 0.029860 -0.093147 0.023192 -0.031281 0.030545 -0.068470 0.020244

0.014509 -0.057081 0.049269 -0.047556 0.022443 -0.092672 0.065184 -0.030968 0.097352 -0.052493 0.062981

0.004188 -0.028991 0.063084 -0.082578 0.005537 -0.030271 0.038801 -0.043018 0.066686 -0.004677 0.054946

0.013995 -0.011381 0.075888 -0.069206 0.012784 -0.009126 0.068735 -0.066544 0.070738 -0.055201 0.097867

0.083719 -0.007838 0.018854 -0.098974 0.023769 -0.044483 0.028541 -0.032198 0.047691 -0.005788 0.039455

0.066290 -0.033136 0.097825 -0.051469 0.012732 -0.038881 0.076786 -0.069891 0.084848 -0.050189 0.017055

0.077407 -0.088394 0.006851 -0.047383 0.081140 -0.094065 0.002880 -0.072353 0.095627 -0.096577 0.025683

0.023140 -0.008283 0.096901 -0.011595 0.031076 -0.079637 0.050198 -0.014112 0.027430 -0.012270 0.054234

0.011981 -0.053835 0.076015 -0.062570 0.082806 -0.040616 0.030618 -0.003141 0.031599 -0.093869 0.048415

0.065879 -0.060177 0.085832 -0.000699 0.038540 -0.014198 0.018127 -0.013525 0.094031 -0.072898 0.083781

0.056596 -0.090405 0.092818 -0.013577 0.078385 -0.061543 0.053441 -0.092938 0.032074 -0.017903 0.051810

top-10 rows B:

-0.023930 0.047744 -0.074694 0.053555 -0.032298 0.038762 -0.068890 0.088894 -0.044989 0.005679 -0.054846 0.064743

-0.070026 0.059445 -0.078712 0.001957 -0.050910 0.067603 -0.089646 0.076562 -0.039840 0.052980 -0.074809 0.037390

-0.042785 0.087303 -0.005369 0.017769 -0.075572 0.075981 -0.064457 0.067737 -0.045192 0.046887 -0.030999 0.006888

-0.040708 0.020566 -0.089926 0.082820 -0.010478 0.021086 -0.086581 0.095966 -0.054339 0.068906 -0.060855 0.087460

-0.059717 0.038708 -0.026613 0.053984 -0.088490 0.066764 -0.005617 0.091969 -0.018239 0.097972 -0.073692 0.010064

-0.052374 0.048555 -0.037706 0.043377 -0.071556 0.075888 -0.002523 0.037950 -0.065693 0.078094 -0.011694 0.039196

-0.092799 0.011099 -0.056766 0.091866 -0.059577 0.029236 -0.063502 0.091717 -0.030844 0.079273 -0.087244 0.048310

-0.089582 0.004614 -0.002560 0.058306 -0.006922 0.097391 -0.099892 0.039699 -0.036129 0.038520 -0.084387 0.012408

-0.054143 0.048351 -0.006309 0.002902 -0.073858 0.012903 -0.089030 0.041077 -0.034445 0.030259 -0.071056 0.002762

-0.048605 0.047165 -0.082960 0.096326 -0.066084 0.029297 -0.070599 0.034394 -0.044475 0.075287 -0.063274 0.023137

-0.033560 0.030527 -0.019907 0.078961 -0.052821 0.088959 -0.043210 0.061808 -0.020862 0.058320 -0.028586 0.079149

-0.087878 0.034127 -0.040097 0.092205 -0.033817 0.099641 -0.002590 0.012473 -0.050764 0.093213 -0.065811 0.075233

bottom-10 rows B:

0.006791 -0.015538 0.072625 -0.049761 0.029860 -0.093147 0.023192 -0.031281 0.030545 -0.068470 0.020244

0.014509 -0.057081 0.049269 -0.047556 0.022443 -0.092672 0.065184 -0.030968 0.097352 -0.052493 0.062981

0.004188 -0.028991 0.063084 -0.082578 0.005537 -0.030271 0.038801 -0.043018 0.066686 -0.004677 0.054946

0.013995 -0.011381 0.075888 -0.069206 0.012784 -0.009126 0.068735 -0.066544 0.070738 -0.055201 0.097867

0.083719 -0.007838 0.018854 -0.098974 0.023769 -0.044483 0.028541 -0.032198 0.047691 -0.005788 0.039455

0.066290 -0.033136 0.097825 -0.051469 0.012732 -0.038881 0.076786 -0.069891 0.084848 -0.050189 0.017055

0.077407 -0.088394 0.006851 -0.047383 0.081140 -0.094065 0.002880 -0.072353 0.095627 -0.096577 0.025683

0.023140 -0.008283 0.096901 -0.011595 0.031076 -0.079637 0.050198 -0.014112 0.027430 -0.012270 0.054234

0.011981 -0.053835 0.076015 -0.062570 0.082806 -0.040616 0.030618 -0.003141 0.031599 -0.093869 0.048415

0.065879 -0.060177 0.085832 -0.000699 0.038540 -0.014198 0.018127 -0.013525 0.094031 -0.072898 0.083781

0.056596 -0.090405 0.092818 -0.013577 0.078385 -0.061543 0.053441 -0.092938 0.032074 -0.017903 0.051810

top-10 rows C:

-0.047860 0.095488 -0.149388 0.107110 -0.064596 0.077524 -0.137780 0.177788 -0.089978 0.011358 -0.109692 0.129486

-0.140052 0.118890 -0.157424 0.003913 -0.101820 0.135206 -0.179292 0.153124 -0.079680 0.105960 -0.149618 0.074780

-0.085570 0.174606 -0.010738 0.035538 -0.151144 0.151962 -0.128914 0.135474 -0.090384 0.093774 -0.061998 0.013776

-0.081416 0.000000 -0.179852 0.165640 -0.020956 0.042172 -0.173162 0.000000 -0.108678 0.137812 -0.121710 0.000000

-0.119434 0.077416 -0.053226 0.107968 -0.176980 0.133528 -0.011234 0.000000 -0.036478 0.195944 -0.147384 0.000000

-0.104748 0.000000 -0.075412 0.086754 -0.143112 0.151776 -0.005047 0.000000 -0.131386 0.156188 -0.023388 0.078392

-0.185598 0.022198 0.000000 0.183732 -0.119154 0.058472 -0.127004 0.000000 -0.061688 0.158546 -0.174488 0.096620

-0.179164 0.000000 -0.005121 0.116612 -0.013844 0.194782 -0.199784 0.079398 -0.072258 0.077040 -0.168774 0.024816

-0.108286 0.096702 -0.012618 0.005803 -0.147716 0.025806 -0.178060 0.082154 -0.068890 0.060518 -0.142112 0.005524

-0.097210 0.000000 0.000000 0.000000 -0.132168 0.058594 -0.141198 0.000000 -0.088950 0.150574 -0.126548 0.046274

-0.067120 0.061054 0.000000 0.000000 -0.105642 0.177918 -0.086420 0.123616 -0.041724 0.116640 -0.057172 0.158298

-0.175756 0.068254 -0.080194 0.184410 -0.067634 0.199282 -0.005179 0.000000 -0.101528 0.186426 -0.131622 0.150466

bottom-10 rows C:

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.029018 0.000000 0.000000 0.000000 0.000000 0.000000 0.130368 0.000000 0.000000 0.000000 0.000000

0.000000 -0.057982 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.000000 0.000000 0.151776 0.000000 0.000000 0.000000 0.000000 -0.133088 0.000000 0.000000 0.000000

0.000000 0.000000 0.000000 -0.197948 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.000000 0.000000 0.000000 0.000000 0.025464 0.000000 0.000000 0.000000 0.169696 0.000000 0.000000

0.000000 0.000000 0.000000 0.000000 0.000000 -0.188130 0.000000 0.000000 0.000000 0.000000 0.000000

0.000000 0.000000 0.193802 0.000000 0.000000 0.000000 0.100396 0.000000 0.000000 -0.024540 0.000000

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 -0.006282 0.000000 0.000000 0.000000

0.000000 -0.120354 0.000000 0.000000 0.000000 -0.028396 0.000000 0.000000 0.188062 0.000000 0.167562

0.000000 0.000000 0.000000 -0.027154 0.000000 0.000000 0.000000 0.000000 0.000000 -0.035806 0.000000

As you can see, C=A+B just isn’t working…

It’s ok for the top part of the matrix, but goes hay-wire in the bottom half. I have no idea where all those zeros are coming from…

Am I doing something wrong?

Shouldn’t this be

``````int idx=width*y+x;
``````

where width is an extra parameter to your kernel and contains the width of the matrix.

Also, i don’t think it’s sufficient nor necessary to print out the bottom/top 10 rows as a sanity check. Why don’t you also add the two matrices in the CPU and compare the result you get in the CPU vs the GPU (e.g. summing all the elements of the matrix)?

Hi,

Many thanks for that. I changed that to ‘width’.

The top-half correct and is as before, however the bottom half is now:

``````0.050416 -0.125518 0.013534 -0.112098 0.061948 -0.174712 0.143346 -0.002243 0.087270 -0.114012 0.036482

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.029018 -0.114162 0.098538 -0.095112 0.044886 -0.185344 0.130368 -0.061936 0.194704 -0.104986 0.125962

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.027990 -0.022762 0.151776 -0.138412 0.025568 -0.018252 0.137470 -0.133088 0.141476 -0.110402 0.195734

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.132580 -0.066272 0.195650 -0.102938 0.025464 -0.077762 0.153572 -0.139782 0.169696 -0.100378 0.034110

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.046280 -0.016566 0.193802 -0.023190 0.062152 -0.159274 0.100396 -0.028224 0.054860 -0.024540 0.108468

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

0.131758 -0.120354 0.171664 -0.001398 0.077080 -0.028396 0.036254 -0.027050 0.188062 -0.145796 0.167562

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
``````

It’s a big improvement for sure, but every second row is printing zeros… ;(

Yes, that is a good idea… Thanks for that… Good tip. I’ll remember in future.

Ok, I passed A.h as the width instead of A.w and it’s working now!

Will do a checksum next – just to make sure. Gosh, this has been really wrecking my head for the last couple of days…