MATLAB 'parfor' Loops Very Slow When Compared With 'for' loop - matlab

Question

I have a script that I'm running, and at one point I have a loop over n objects, where I want n to be fairly large.
I have access to a server, so I put in a parfor loop. However, this is incredibly slow compared with a standard for loops.
For example, running a certain configuration ( the one below ) with the parfor loop on 35 workers took 68 seconds, whereas the for loop took 2.3 seconds.
I know there's stuff to do with array-broadcasting that can cause issues, but I don't know a lot about this.
n = 20;
r = 1/30;
tic
X = rand([2,n-1]);
X = [X,[0.5;0.5]];
D = sq_distance(X,X);
A = sparse((D < r) - eye(n));
% Infected set I
I = n;
[S,C] = graphconncomp(A);
compnum = C(I);
I_new = find(C == compnum);
I = I_new;
figure%('visible','off')
gplot(A,X')
hold on
plot(X(1,I),X(2,I),'r.')
hold off
title('time = 0')
axis([0,1,0,1])
time = 0;
t_max = 10; t_int = 1/100;
TIME = 1; T_plot = t_int^(-1) /100;
loops = t_max / T_plot;
F(loops) = struct('cdata',[],'colormap',[]);
F(1) = getframe;
% Probability of healing in interval of length t_int
heal_rate = 1/3; % (higher number is faster heal)
p_heal = t_int * heal_rate;
numhealed = 0;
while time < t_max
time = time+t_int;
steps = poissrnd(t_int,[n,1]);
parfor k = 1:n
for s = 1:steps(k)
unit_vec = unif_unitvector;
X_new = X(:,k) + unit_vec*t_int;
if ( X_new < 1 == ones(2,1) ) ...
& ( X_new > 0 == ones(2,1) )
X(:,k) = X_new;
end
end
end
D = sq_distance(X,X);
A = sparse((D < r) - eye(n));
[S,C] = graphconncomp(A);
particles_healed = binornd(ones(length(I),1),p_heal);
still_infected = find(particles_healed == 0);
I = I(still_infected);
numhealed = numhealed + sum(particles_healed);
I_new = I;
% compnum = zeros(length(I),1);
for i = 1:length(I)
compnum = C(I(i));
I_new = union(I_new,find(C == compnum));
end
I = I_new;
if time >= T_plot*TIME
gplot(A,X')
hold on
plot(X(1,I),X(2,I),'r.')
hold off
title(sprintf('time = %1g',time))
axis([0,1,0,1])
% fprintf('number healed = %1g\n',numhealed)
numhealed = 0;
F(TIME) = getframe;
TIME = TIME + 1;
end
end
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