# Why parfor loop memory keep increasing?

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Luqman Saleem on 22 Aug 2020
Commented: Raymond Norris on 23 Aug 2020
I am using R2018b on Windows 10. My code have 3 for loops and I want to use parfor above them. In each parfor loop, it is expected to save a 2D array of size N. I want to keep N around 500. The code is given below
clear; clc;
N = 500;
Ts = 0.1:0.1:10; %range of outter most "for" loop
xmin = -pi; xmax = pi; %range of first "for" loop
ymin = -pi; ymax = pi; %range of second "for" loop
wmin = 3; wmax = 15; %range of "parfor" loop
% building x,y and w arrays
dx = (xmax-xmin)/(N-1);
dy = (ymax-ymin)/(N-1);
dw = (wmax-wmin)/(2*N-1);
xs = xmin : dx : xmax;
ys = ymin : dy : ymax;
ws = wmin : dw : wmax;
L = length(ws); %length of parfor loop
%allocating memory for final results:
D3_I1 = zeros(1,length(Ts));
D3_I2 = zeros(1,length(Ts));
D3_I3 = zeros(1,length(Ts));
D3_I4 = zeros(1,length(Ts));
nT = 1;
for T = Ts
T
%allocating memory for parfor loop results:
D2_I1 = zeros(1,L);
D2_I2 = zeros(1,L);
D2_I3 = zeros(1,L);
D2_I4 = zeros(1,L);
parfor nw = 1:L
w = ws(nw);
% allocating memory for "for" loop results:
R1 = zeros(N,N);
R2 = zeros(N,N);
R3 = zeros(N,N);
R4 = zeros(N,N);
ny = 1;
for ky = ys
nx = 1;
for kx = xs
%following are the main calculations A1,2,3,4.
%in my original code these are actually
%some 3-by-3 matrices, but the final value of A1
%A2,A3 and A4 is just a complex number. this part is
%not very lengthy. it's around 12-15 expressions which
%include matrix multiplications, inverse and trace
%operations on (3by3) matrices.
A1 = x*y;
A2 = x*y;
A3 = x*y;
A4 = x*y;
R1(nx,ny) = A1;
R2(nx,ny) = A4;
R3(nx,ny) = A3;
R4(nx,ny) = A4;
nx = nx + 1;
end
ny = ny + 1;
end
D2_I1(nw) = sum(R1(:))*dx*dy;
D2_I2(nw) = sum(R2(:))*dx*dy;
D2_I3(nw) = sum(R3(:))*dx*dy;
D2_I4(nw) = sum(R4(:))*dx*dy;
WSaved(nw) = w;
end
D3_I1(nT) = sum(D2_I1)*dw;
D3_I2(nT) = sum(D2_I2)*dw;
D3_I3(nT) = sum(D2_I3)*dw;
D3_I4(nT) = sum(D2_I4)*dw;
nT = nT+1;
end
The memory just keep increasing. I have 128GB with 28 cores. I am using 22 cores for parfor. I was able to run it till T = 5. But then I get "memory error".
Why does parfor memory keep increasing? Should not it be resetted for every next T?

Raymond Norris on 22 Aug 2020
Hi Luqman,
Two suggestions:
1. Look at ticBytes/tocBytes to see how much data is being passed through to parfor
2. If you're able to upgrade to R2020a, consider using a threads pool, rather than a local pool. See the following for more information on threads vs local
Thanks,
Raymond
Raymond Norris on 23 Aug 2020
There are some limitations with threads pool (see the doc for more information), but you start and use a threads pool the same way as a local pool (see the doc for advantages of threads pool).
Good to hear you're in your good shape.

R2018b

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