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MATLAB array size limit error - why?

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Andres
Andres on 18 Oct 2015
Commented: Mishary Alfarah on 13 Apr 2019
I am receiving an unexpected error because of an array allegedly exceeding the maximum array size limit with a non-double type variable.
Here is a short example to reproduce the error:
First, to keep the example variables small, go to preferences -> MATLAB -> workspace and set the limit of the maximum array size temporarily to 1% of the RAM.
You may provoke the correct error with a double array demanding too much memory:
[~,sys] = memory;
testLen = round(sys.PhysicalMemory.Total/100/2);
N = zeros(testLen,1);
Error using zeros
Requested 62859407x1 (0.5GB) array exceeds maximum array size preference. Creation of arrays greater than this limit may take a long time and cause MATLAB to become unresponsive. See array size limit or preference panel for more information.
(for 12GB of RAM)
A uint8 array of the same length is ok:
n = zeros(testLen,1,'uint8');
returns no error. You can manipulate it by
n(end) = uint8(1);
as expected.
N(end-5:end).'
gives
0 0 0 0 0 1
But why do
n(end-1:end) = uint8(1);
% or
n(end-1:end) = uint8([1;1]);
and similar manipulations error:
Requested 62859405x1 (0.5GB) array exceeds maximum array size preference. Creation of arrays greater than this limit may take a long time and cause MATLAB to become unresponsive. See array size limit or preference panel for more information.
?
Is MATLAB (R2015b) simply miscalculating the array memory size for non-doubles here or am I doing something wrong?
  2 Comments
Andres
Andres on 19 Oct 2015
Thanks for your comment, dpb. Meanwhile I'm convinced this is a bug or at least an unintended behaviour, too. I have sent a bug report.

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Answers (2)

Walter Roberson
Walter Roberson on 18 Oct 2015
n(end-1:end) = uint8(1) may require that n be duplicated if n is an array that is currently shared. For example if n is a parameter passed in, and is not being used in the restricted context
n = SomeFunction(n)
whose header is
function n = SomeFunction(n)
then MATLAB may well need to duplicate the array.

Andres
Andres on 19 Oct 2015
Edited: Andres on 19 Oct 2015
Thanks for your answer ( Link), Walter. The error first occured inside a function, but my example works in plain matlab workspace, too.
Based on your idea, I did some more analysis with different data types, trying out different test lengths. If the manipulation should temporarily duplicate the memory demand of a variable, the error occurence would depend on the data type size (8/16/32/64 bit).
clear k l m n p q
k = zeros(testLen,1,'uint8');
l = false(testLen,1);
m = zeros(testLen,1,'uint16');
n = zeros(testLen,1,'uint32');
p = zeros(testLen,1,'single');
q = zeros(testLen,1);
k(end-1:end) = uint8(1);
l(end-1:end) = true;
m(end-1:end) = uint16(1);
n(end-1:end) = uint32(1);
p(end-1:end) = single(1);
q(end-1:end) = 1;
Near the maximum length that will allow double (64bit) type creation, the allowableness of the manipulation of all smaller data types changes, too:
[~,sys] = memory;
testLen = floor(sys.PhysicalMemory.Total/100/8);
% maximum length that will work with all types (k l m n p q)
% x(end-1:end) = .. will work on all variables
testLen = floor(sys.PhysicalMemory.Total/100/8)+1;
% creates variables only up to 32bit types (k l m n p)
% (i.e. smallest length that will error on double creation)
% x(end-1:end) = .. will work on all variables created
testLen = floor(sys.PhysicalMemory.Total/100/8)+2;
% creates variables only up to 32bit (k l m n p)
% does NOT allow x(end-1:end) = .. on ANY of the variables created
(maximum array size was limited to 1% of RAM, as above)
I think this is not a reasonable or intended behaviour, so I tend to send a bug report.
Btw, is there a way to get or set the maximum array size including the percentage value found in the preferences tab programmatically? The preferences tab only shows a slider without displaying a value.
  6 Comments
Mishary Alfarah
Mishary Alfarah on 13 Apr 2019
I uchecked that limit and on my activity monitor it showed that it took 63 GB of RAM! then it immediately quit. I think keeping it on maximum maybe is better than uncheck it.

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