4.3.2.3 Compilation with multiple threads

This section discusses compiling files for the first time. For reloading, see section 4.3.2.

Multiple threads can compile files concurrently. This requires special precautions only if multiple threads wish to load the same file at the same time. Therefore, load_files/2 checks whether some other thread is already loading the file. If not, it starts loading the file. If a thread detects that another thread is already loading the file the thread blocks until the other thread finishes loading the file. After waiting, and if the file is a module file, it imports the exported predicates and operators from the module.

Note that this schema does not prevent deadlocks under all situations. Consider two mutually dependent (see section 4.3.2.2) module files A and B, where thread 1 starts loading A and thread 2 starts loading B at the same time. Both threads will deadlock when trying to load the used module.

The current implementation does not detect such cases and the involved threads will freeze. This problem can be avoided if a mutually dependent collection of files is always loaded from the same start file.