Wednesday, June 24, 2009

Module Loader.2

Just did some testing and bug fixing. Module loader works as expected. Here's some technical info.

Each process has a list of loaded modules in it, including a reference count. When module A depends on module B, and A is loaded B is loaded automatically. If it was already a loaded (perhaps an already-loaded module C depended on it), its reference count gets incremented. This all assures that when all modules depending on a certain module are unloaded, it can as well be unloaded, so as to free up the address space it takes up. The per-process module list is just a linked list, containing base address, module name (not path), and a pointer to the system-global list.

The system-global module list is what enables memory sharing of code and read-only data between processes loading the same module. It is a hashtable, indexed by a hash of the PATH of the module. This means that process A can load library A version X from path Y ad process B can load library A version Z from path T and they will not collide. The global table entries also have reference count, but this is per-process. This means that if 2 processes load module A, it's ref count in global table will be 2. If one process loads the module 5 times, the global reference count will still be 1. In any case the point is that when no more processes have a module loaded, we can evict it from memory, and free it up.

1 comment:

Coffee-Turtle said...

Thank you for the explanation! That actually made sense, although, pretty complicated. We definitely take for granted all the things that are going on in the background when we tap the screen. :-)