RAM issues with Newton iteration

[orcahn]

I tried to replicate some results from the "Approximation of 2^d x 2^d matrices" paper. There were results given for the approximate inverse of a 2D Laplacian with the Newton method. However, with my code it already takes more than 20GB of RAM to calculate this Newton iteration for a 2D Laplacian with d=9. What are the reasons for this? Am I even supposed to minimize the absolute as opposed to the relative residual?
The code:

% Approximate inversion of 2D Laplace operator for varying n
d=[5,6,7,8,9,10,11,12];

max_rank=75;
eps=1e-5;

for i=1:8

A=tt_qlaplace_dd([d(i),d(i)]);
I = tt_eye(2,(d(i)*2)); I = tt_matrix(I);
X = 0.2*I;
err=2;
j = 0;

while norm(err,'F') > 1    
    Z = round(2*I-A*X,eps,max_rank);
    X = round(X*Z,eps,max_rank);
    err = I - A*X;
    j = j+1;
end
disp("n="+num2str(2^d(i))+"²" + " Iters: "+num2str(j) + " Error (Frob):" + num2str(norm(err,'F'),'%10.1e\n')); 
end

And the output:

n=32² Iters: 7 Error (Frob):8.0e-01
n=64² Iters: 9 Error (Frob):7.8e-01
n=128² Iters: 11 Error (Frob):7.7e-01
n=256² Iters: 13 Error (Frob):7.7e-01
Out of memory. Type "help memory" for your options.

Error in tt_tensor/round (line 55)
cr=cr(1:pos1); %Truncate storage if required

Error in tt_matrix/round (line 24)
tt.tt=round(tt.tt,eps,rmax);

Error in testinv (line 16)
    X = round(X*Z,eps,max_rank);

[dolgov]

Hi,

the devil is in the line X = round(XZ,eps,max_rank);
The TT ranks of A inverse are already pretty large, and they get squared in the exact * operation.
There is an iterative ALS-type method amen_mm that computes directly the approximate result of the matrix product (without constructing a TT matrix with squared ranks on the way).
Here is a code for e.g. d=10 that works fine on my laptop:
d = 10;
A = tt_qlaplace_dd([d d]);
I = tt_eye(2,2d);
X = 0.2I;
err = 2;
while (norm(err)>1)
X = amen_mm(X, 2I-AX, 1e-7);
err = I - AX;
fprintf('%g\n', norm(err));
end

[orcahn]

Thank you very much.

[oseledets]

@dolgov Maybe add this to examples/tutorial? This is a neat example.

[dolgov]

well, I can add this to tests, but not the quick_start, as I don't have a latex of it.