Using GPU on multiple nested loops

Question

Jhinhwan Lee 2019-8-26

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/477542-using-gpu-on-multiple-nested-loops

回答： Raunak Gupta 2019-8-30

在 MATLAB Online 中打开

The following code is slow for large ncnt (typically >2000)

and I want to use my GPU for the outermost (iplane) loop.

Can you give me an hint? (I have an NVIDIA RTX 8000.)

nxs=101;
nys=101;
nzs=101;
ncnt=100;
xnmin=-1.0;
xnmax= 1.0;
ynmin=-1.0;
ynmax= 1.0;
znmin=-1.0;
znmax= 1.0;
coefftmp=complex(rand(1,ncnt));
igalltmp=rand(3,ncnt);
vktmp=rand(1,3);
wfrtmp=complex(zeros(nxs,nys,nzs));
tic
for iplane=1:ncnt % GPU loop
    ee=exp(2.*pi*complex(0.,1.));
    vkg=vktmp+double(igalltmp(:,iplane)');
    ekx=ee^vkg(1);
    eky=ee^vkg(2);
    ekz=ee^vkg(3);
    coefft=coefftmp(iplane);
    for iz=1:nzs
        z=znmin+(znmax-znmin)*double(iz-1)/double(nzs-1);
        ekzz=ekz^z;
        for iy=1:nys
            y=ynmin+(ynmax-ynmin)*double(iy-1)/double(nys-1);
            ekyy=eky^y;
            for ix=1:nxs
                x=xnmin+(xnmax-xnmin)*double(ix-1)/double(nxs-1);
                ekxx=ekx^x;
                wfrtmp(ix,iy,iz)=wfrtmp(ix,iy,iz)+coefft*ekxx*ekyy*ekzz;
            end
        end
    end
end
wfr(ispin,:,:,:)=wfrtmp/sqrt(Vcell);
toc

4 个评论
显示 2更早的评论隐藏 2更早的评论

Walter Roberson 2019-8-26

在 MATLAB Online 中打开

If you want to use GPU, you are going to have to rewrite your code to be vectorized.

I suggest you consider

zvec = linspace(znmin, znmax, nzs);
yvec = linspace(ynmin, ynmax, nys);
xvec = linspace(xnmin, xnmax, nxs);
[X, Y, Z] = ndgrid(xvec, yvec, zvec);

before any looping. After that you can do things like

coeff .* ekz.^Z .* eky.^Y .* ekx.^X

Jhinhwan Lee 2019-8-26

在 MATLAB Online 中打开

Thanks! I did something basically the same and it is more than ten times faster now.

I also found including the X, Y and Z in the argument of the exp function slightly better: In the cases of kx=1, 2, 3, ... ekx=exp(2.*pi*complex(0.,1.)*kx)=1 and ekx^x==1 no matter what x is, while exp(2.*pi*complex(0.,1.)*kx*x) depends on x (unless k=0) as expected.

coeff*exp(2.*pi*complex(0.,1.)*(kx*X+ky*Y+kz*Z))

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Raunak Gupta 2019-8-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/477542-using-gpu-on-multiple-nested-loops#answer_389714

Hi,

For speeding up the code you need to first vectorize the three loops inside the main loop as they are independent of each other. As mentioned in the comments you can use linspace and ndgrid for doing exponentiation for all three variables independently.

The above part only vectorizes the code but to actually use GPU you can create the initial arrays using gpuArray. This may also significantly fasten up the code. The function that you have used inside the code is supported for gpuArray but if you want to use any specific function you can check about all the supported function here.