Huge time difference if function is called from within another function

Function A, when called directly, runs in 10 minutes. However, if function A is called by function B, and I run function B, the time spent of running function A is 100 minutes or so. Is this is a feature of Matlab?

Also, if I run two instances of Matlab, and run the same function on both these instances, can it happen that the first instance takes less time compared to the other?

[James Tursa]

You need to give more information about what functions you are calling and how you are calling them. There is no generic 10x time penalty for calling a function from within another function.

[tirupam goel]

Thank you. Given the complexity of my code, I thought it would be better to simplify the question. I figured the issue. Suppose function A needs another functions C. It matters if function C is passed as a parameter to function B (which then passes it to function A), versus function C being constructed within function B. The former is much faster, and it worked for me.

Answer 1

[Walter Roberson]

What you describe could happen if function B restricts the number of threads to execute on, or opens a parpool that is smaller than the number of cores, and A is working with large matrices using data patterns that can normally be divided up into multiple cores automatically by BLAS or MLK or the like.

[tirupam goel]

Thank you. Parallel code was not the issue. I figured the issue. Suppose function A needs another functions C. It matters if function C is passed as a parameter to function B (which then passes it to function A), versus function C being constructed within function B. The former is much faster, and it worked for me.