Nested Functions vs Object-Oriented Programming: k-D Tree example and relative performance
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Jacob Lynch August on 13 Aug 2020
My colleagues tend to criticize my use of nested functions in MATLAB, likening that to the professional no-no of modifying global variables. To placate those peers, I'll recode those methods that use nested functions into a handle-based object-oriented methods. Now, they accept that the current properties are accessible to the object's methods.
However, I have a specific instance where the nested functions implementation outperforms the handle-based object-oriented programming style: k-D Trees.
Using nested subfunctions. https://www.mathworks.com/matlabcentral/fileexchange/79101-ikdtree
Using subclass of handle. https://www.mathworks.com/matlabcentral/fileexchange/79104-kdtree
Here is the benchmark code run from the command window.
pdat = randn(130001,3); % position data
tic;a_tree = kDTree(pdat); disp(toc) % 9.0822 seconds
tic;i_tree = ikdtree(pdat);disp(toc) % 1.0869 seconds
I'm seeking insights into what is causing these performance differences.
per isakson on 14 Aug 2020
Edited: per isakson on 14 Aug 2020
On my R2018b,Win10 the diffence is even bigger
% 18.585, 0.79143
% 18.937, 0.75923
The profiling result of get_root_index shows
that assigning values to the properties, LeftIndices and RightIndices, takes nearly all the time.
It's a known fact that assigning values to properties is slow in Matlab. See Is MATLAB OOP slow or am I doing something wrong? and Numerical operations are slow on class properties versus in workspace.
The execution engine (/JIT) is The Mathworks' trade secret.
More Answers (1)
Olaf Constantin Schmidtmann on 3 May 2022
Even in R2022a, the performance gap between these two is quite huge on my machine (MACI64). This contradicted my results of Andrew Janke’s matlab-bench and I wondered why.
Without digging too deep into the functions themselves, I found that the OOP version (subclass of handle) uses property validation, which is expensive at this point. Hence, the comparison is a bit unfair. ;)
If you remove the validation (lines 11-14), like so:
properties (SetAccess = protected)
LeftIndices %(:, 1)
RightIndices %(:, 1)
… the tables turned on my machine; the OOP version was even a bit faster:
OOP (kDTree, original): Elapsed time is 5.630146 seconds.
OOP (kDTree, no validation of indices): Elapsed time is 0.579906 seconds.
Nested (ikdtree): Elapsed time is 0.641538 seconds.
You may add a dependent variable for subclasses to keep the validation. Also you might compare a custom validation in the nested version…
Btw, I do not believe, that OOP is a silver bullet, but there is clearly more to the decision than performance issues.