JNI/the new Foreign FFI communicate with CPython via CPython's C API. The primary issue is getting the garbage collectors to work with each other. The Java solver works by repeatedly calling user defined functions when calculating the score. As a result:
- The Java side needs to store opaque Python pointers which may have no references on the CPython side.
- The CPython side need to store generated proxies for some Java objects (the result of constraint collectors, which are basically aggregations of a solution's data).
Solving runs a long time, typically at least a hour (although you can modify how long it runs for). If we don't free memory (by releasing the opaque Python Pointer return values), we will quickly run out of memory after a couple of minutes. The only way to free memory on the Java side is to close the arena holding the opaque Python pointer. However, when that arena is closed, its memory is zeroed out to prevent use-after-free. As a result, if CPython haven't garbage collected that pointer yet, it will cause a segmentation fault on the next CPython garbage collection cycle.
JPype (a CPython -> Java bridge) does dark magic to link the JVM's and CPython's garbage collector, but has performance issues when calling a CPython function inside a Java function, since its proxies have to do a lot of work. Even GraalPy, where Python is ran inside a JVM, has performance issues when Python calls Java code which calls Python code.
- The Java side needs to store opaque Python pointers which may have no references on the CPython side.
- The CPython side need to store generated proxies for some Java objects (the result of constraint collectors, which are basically aggregations of a solution's data).
Solving runs a long time, typically at least a hour (although you can modify how long it runs for). If we don't free memory (by releasing the opaque Python Pointer return values), we will quickly run out of memory after a couple of minutes. The only way to free memory on the Java side is to close the arena holding the opaque Python pointer. However, when that arena is closed, its memory is zeroed out to prevent use-after-free. As a result, if CPython haven't garbage collected that pointer yet, it will cause a segmentation fault on the next CPython garbage collection cycle.
JPype (a CPython -> Java bridge) does dark magic to link the JVM's and CPython's garbage collector, but has performance issues when calling a CPython function inside a Java function, since its proxies have to do a lot of work. Even GraalPy, where Python is ran inside a JVM, has performance issues when Python calls Java code which calls Python code.