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Here are some other ways of improving the execution speed of Octave programs.
a = zeros (1000); # create a 1000x1000 matrix b = a; # no copying done here b(1) = 1; # copying done here
Lazy copying applies to whole Octave objects such as matrices, cells, struct, and also individual cell or struct elements (not array elements).
Additionally, index expressions also use lazy copying when Octave can determine that the indexed portion is contiguous in memory. For example:
a = zeros (1000); # create a 1000x1000 matrix b = a(:,10:100); # no copying done here b = a(10:100,:); # copying done here
This applies to arrays (matrices), cell arrays, and structs indexed
using ‘()’. Index expressions generating comma-separated lists can also
benefit from shallow copying in some cases. In particular, when a is a
struct array, expressions like {a.x}, {a(:,2).x}
will use lazy
copying, so that data can be shared between a struct array and a cell array.
Most indexing expressions do not live longer than their parent objects. In rare cases, however, a lazily copied slice outlasts its parent, in which case it becomes orphaned, still occupying unnecessarily more memory than needed. To provide a remedy working in most real cases, Octave checks for orphaned lazy slices at certain situations, when a value is stored into a "permanent" location, such as a named variable or cell or struct element, and possibly economizes them. For example:
a = zeros (1000); # create a 1000x1000 matrix b = a(:,10:100); # lazy slice a = []; # the original "a" array is still allocated c{1} = b; # b is reallocated at this point
result = zeros (big_n, big_m) for i = over:and_over ridx = … cidx = … result(ridx, cidx) = new_value (); endfor
instead of
result = []; for i = ever:and_ever result = [ result, new_value() ]; endfor
Sometimes the number of items can not be computed in advance, and stack-like operations are needed. When elements are being repeatedly inserted or removed from the end of an array, Octave detects it as stack usage and attempts to use a smarter memory management strategy by pre-allocating the array in bigger chunks. This strategy is also applied to cell and struct arrays.
a = []; while (condition) … a(end+1) = value; # "push" operation … a(end) = []; # "pop" operation … endwhile
eval
or feval
excessively.
Parsing input or looking up the name of a function in the symbol table are
relatively expensive operations.
If you are using eval
merely as an exception handling mechanism, and not
because you need to execute some arbitrary text, use the try
statement instead. See The try Statement.
ignore_function_time_stamp
when appropriate.
If you are calling lots of functions, and none of them will need to change
during your run, set the variable ignore_function_time_stamp
to
"all"
. This will stop Octave from checking the time stamp of a
function file to see if it has been updated while the program is being run.
Next: Examples, Previous: Accumulation, Up: Vectorization and Faster Code Execution [Contents][Index]