Why is the MEX file I generated from my MATLAB code slower than the MATLAB code?

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I expect that creating a MEX file from my MATLAB code should always provide a speed improvement. Why is this not the case?

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 24 Nov 2020
It is important to remember that under the hood, a MEX file is simply a function that calls a C/C++ (or sometimes Fortran) subroutine. So when we are comparing the execution time of a MATLAB script to that of a MEX file, we are comparing the amount of time it takes the MATLAB script to be interpreted to the amount of time it takes the generated C/C++ code to execute.
Now it is reasonable to think that compiled C code should execute faster than M code is interpreted. However, it has been many years since MATLAB has been a true interpreted language. Today it is a just-in-time (JIT) compiled language with a large library of pre-compiled routines that are optimized for the target that MATLAB is running on. With MEX code generation, we are generating portable C code. Sometimes the MATLAB libraries are even multi-threaded while the generated code is not. Generally speaking, when an application mostly exercises pre-compiled binaries in MATLAB, or things that the JIT compiler handles well, the more realistic expectation is for MATLAB to be faster than the generated C code. The times when we see large speed-ups tend to correspond to those functions which are still implemented as complicated MATLAB functions in MATLAB. One might expect, therefore, to see speedups with QUADGK or QUAD2D, for example, but not with FFT.
With a simple script, we are really just comparing the C compiler to the MATLAB JIT compiler. For instance, the "mod" function is executing the exact same binary code in MATLAB as it is in a MEX file generated with MATLAB Coder. There just is not any opportunity for speedup here.
  1 Comment
cui,xingxing
cui,xingxing on 3 Nov 2022
Edited: cui,xingxing on 3 Nov 2022
@MathWorks Support Team Thanks for the explanation,then the question arises, if my generated C code is executing slower, how do I debug which matlab statements/functions are indirectly causing the C code to execute more slowly? As far as I am not fully aware, matlab already provides a profiler to monitor the efficiency of matlab code execution, so how do I locate the corresponding generated C code/mex?

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