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Modern languages can allocate memory for arrays in the "heap" and "stack". Global arrays (or arrays with the "save" attribute or arrays defined in a module outside of a subroutine) are allocated in the heap when the model initializes and these exist (taking up memory) for the entire simulation. Arrays can be allocated while the code is running with the allocate() and deallocate() statements - these arrays are also placed in the heap, but the allocate() statement requires a system call and takes significant time (tens of thousands of instructions) as the system looks for available memory in the heap. Allocate can be even slower when running in a threaded region - as it usually requires thread synchronization. Allocate statement should never be used inside frequently called subroutines.
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