Out of memory when assigning values to existing arrays?

Hi,

while dealing with several large arrays I'm facing some strange out of memory problems, which I don't understand. The problems occurred when I was trying to find a workaround for operations on large arrays...

1) It makes a difference if you create a new array (say by zeros(1e10,1), OOM Error) or if you copy an existing array of the same size to a new variable (new = old, no OOM Error). I thought this shouldn't make any difference in terms of memory usage?

2) Changing values in arrays causes an OOM Error, e.g. replace the first value in the array x = zeros(1e10,1) by x(1) = 1;. I would have expected that this operation does not influence the memory?

Any comments appreciated

Thanks, Christian

Answer 1

[Jan]

This is the expected behavior. Matlab uses a copy-on-write strategy:

a = rand(1e9, 1);
b = a;     % Shared data

Now the data pointer of b points to the same memory as a. But when any element of b is modified, the data are duplicated:

b(1) = 1;  % Data duplication

This strategy is useful when data are provided to functions. Then a shared data copy saves time and memory as long as no values are modified.

[Christian]

Cheers, this makes sense. Any ideas to the 2nd point? In your example the memory of a is allocated. Now change a(1) = 1; This shouldn't change the memory I thought, but it causes an OOM error...