Difference between revisions of "Enable large arrays: Build octave such that it can use arrays larger than 2Gb."

From Octave
Jump to navigation Jump to search
(Add an example to see the effect of using BLAS libraries with either 32 or 64 bit indices.)
m (Improve text and example.)
Line 14: Line 14:
 
* Qhull
 
* Qhull
  
To determine the integer size of the BLAS library used by Octave run one of the following lines:
+
To determine the integer size of the BLAS library used by Octave, the following code can be executed:
  
 
<syntaxhighlight lang="Octave">
 
<syntaxhighlight lang="Octave">
clear all; N = 2^31; a = b = ones (N, 1, "single"); c = a' * b  # ~ 8 GB RAM required!
+
clear all;
clear all; N = 2^31; a = b = ones (N, 1);           c = a' * b # ~16 GB RAM required!
+
N = 2^31;
 +
## The following line requires about 8 GB of RAM!
 +
a = b = ones (N, 1, "single");
 +
c = a' * b
 
</syntaxhighlight>
 
</syntaxhighlight>
  
If the BLAS library uses '''32-bit integers''', an error will be thrown in both cases:
+
If the BLAS library uses '''32-bit integers''', an error will be thrown:
  
 
  error: integer dimension or index out of range for Fortran INTEGER type
 
  error: integer dimension or index out of range for Fortran INTEGER type
Line 29: Line 32:
 
  c = 2^31 = 2147483648
 
  c = 2^31 = 2147483648
  
Note that the two test cases above would usually require twice the memory, if <code>a</code> and <code>b</code> differ and are not assigned by <code>a = b = ...</code>.
+
Note that the test case above usually requires twice the memory, if <code>a</code> and <code>b</code> are not assigned by <code>a = b = ...</code>.
Note further, that the single precision data type has a precision of about 23 binary bits.
+
Note further, that the data type "single" has a precision of about 23 binary bits.
 
In this particular example no rounding errors occur.
 
In this particular example no rounding errors occur.
  

Revision as of 23:38, 28 August 2019

Info icon.svg
The following only applies to systems that have 64-bit pointers (64bit architecture).

Starting with Octave 4.4.0, 64-bit indexing is the default for targets with 64-bit pointers. You can override that default by specifying --disable-64 when configuring Octave.

However, if the configure script determines that the BLAS library uses 32-bit integers, then operations using the following libraries are limited to arrays with dimensions that are smaller than 2^31 elements:

  • BLAS
  • LAPACK
  • QRUPDATE
  • SuiteSparse
  • ARPACK

Additionally, the following libraries use "int" internally, so maximum problem sizes are always limited:

  • glpk
  • Qhull

To determine the integer size of the BLAS library used by Octave, the following code can be executed:

clear all;
N = 2^31;
## The following line requires about 8 GB of RAM!
a = b = ones (N, 1, "single");
c = a' * b

If the BLAS library uses 32-bit integers, an error will be thrown:

error: integer dimension or index out of range for Fortran INTEGER type

Otherwise, if the BLAS library uses 64-bit integers, the result is:

c = 2^31 = 2147483648

Note that the test case above usually requires twice the memory, if a and b are not assigned by a = b = .... Note further, that the data type "single" has a precision of about 23 binary bits. In this particular example no rounding errors occur.

Versions prior to Octave 4.4

On previous versions of Octave, the default is that the size of a single Octave array cannot exceed 2 GB of memory. Trying to create one will produce the following error:

>> a = zeros (1024*1024*1024*3, 1, 'int8');
error: out of memory or dimension too large for Octave's index type

You will obtain this error even if your system has enough RAM to create this array (4 GB in the above case).

To use arrays with more than (approximately) elements, Octave has to be configured with the option --enable-64. This option is experimental and you are (as always) encouraged to submit bug reports if you find a problem. With this option, Octave will use internally 64-bit integers for array dimensions and indexing. However, all numerical libraries used by Octave will need to use also 64-bit integers for array dimensions and indexing, and in most cases they need to be compiled from source.

For details about how to compile these libraries please read the GNU Octave manual, or alternatively you can use MXE (M Cross Environment) which takes care of the different packages automatically (especially the configure flags --enable-64 and --enable-fortran-int64).