estimate memory size in matlab

Hi,

    Is there anyway of estimating memory before execution?So that i can cut down my problem size.(out of memory error)
if i try to give   x = ones(10^6);
  size of x is out of memory.
Please help in this regards.

Thanks, Sita

[James VanderVeen]

The memory size in Bytes required by an rectangular array is (# of rows)*(# of cols)*(size of one element).

E.g. Double precision variables are by default 64-bits or 8-bytes

For a double precision array with 25850 rows and colums the memory size is:

format longG
arraysize = 25850*25850*8;

fprintf("The array will need %.0f bytes in memory", arraysize)

The array will need 5345780000 bytes in memory

1 GB is 1024 MB

1 MB is 1024 KB

1 KB is 1024 B

Therefore;

inKB = arraysize/1024;
inMB = arraysize/1024^2;
inGB = arraysize/1024^3;

fprintf("The array will need %.5f KB or %.5f MB or %.5f GB of memory", inKB, inMB, inGB)

The array will need 5220488.28125 KB or 5098.13309 MB or 4.97865 GB of memory

The MATLAB online version I'm using has a maximum array size of 5.0 GB, but if I increase the array to 25851 by 25851, it says I exceed the 5.0 GB limit even though the new array is only 4.9797 GB in size. This appears to mean that there is about 22 MB of data to store information about the array.

[Stephen23]

See also: https://en.wikipedia.org/wiki/Binary_prefix

Answer 1

[Jan]

Notice that ones(10^6) creates a square matrix with 1e6*1e6 = 1e12 elements, which needs a free memory block of 8 TB. So with less than 16 TB free RAM I would not expect this to run reliably.

Answer 2

[Chetan Aswathanarayana]

I' am not sure if we can do this currently without creating a variable. However if the variable is already in the workspace, then the below command would give the bytes allocated.

>>whos x

However you could type the below command, you would then know the maximum memory avaliable in your system

>>memory

This is what I got on my PC:

Maximum possible array:     18848 MB (1.976e+10 bytes) *
Memory available for all arrays:     18848 MB (1.976e+10 bytes) *
Memory used by MATLAB:      1680 MB (1.761e+09 bytes)
Physical Memory (RAM):     12279 MB (1.288e+10 bytes)

Answer 3

[Iain]

The generic size of any variable is:

sum of (each element of the array * bytes in that element) + some overhead.

Normal arrays: Double, uint64 & int64 formats have 8 bytes per element. single, uint32 & int32 formats have 4 bytes per element. some character types, uint16 & int16 formats have 2 bytes per element. logical, most character types, uint8 & int8 formats have 1 byte per element. Overheads are "small" - I think this would be a few 10s of bytes for normal arrays.

Cell arrays and structures are much more detailed and have higher overheads.

[Jan]

The overhead is about 100 bytes per array.

[Haiyin Amania]

where do you find this info from ? because I'm still trying to figure out how I can get the data overheads recorded in matlab. thanks.

[Bruno Luong]

Using C Mex you can find the size by sizeof(MxArray)

It changes from version to version.

Answer 4

[Tom]

Use the MEMORY function.