Book keeping. Will post HW5 tonight. OK to work in pairs. Midterm review next Wednesday

Transcription

1 Garbage Collection

2 Book keeping Will post HW5 tonight Will test file reading and writing Also parsing the stuff you reads and looking for some patterns On the long side But you ll have two weeks to do it (due Thu. Oct 19) OK to work in pairs List team members in your header file and in the canvas submission Midterm review next Wednesday Come with questions

3 What do references really do? When you initialize a new collection/class, you get a reference to that object s representation on the heap What really happens is the OS gives you permission to use the memory where the object is stored The programming language takes care of interpreting the memory and figuring how out how to use it for different objects In most modern languages (basically every language after C), you won t be able to access memory outside of what the OS allows Special care needs to be taken to return memory back to the OS when it is not used anymore If you don t return memory that you re not using, you have a memory leak Firefox used to have a huge memory leak and would eventually use up all of your memory

4 How to get a (hypothetical) memory leak If your program never returned memory to the OS, it would be very simple to artificially create a leak Just keep allocating new memory without returning it Example: i = while True: mylist = list(range(1,i)) i += 1 Here, mylist points to a new object in every loop iteration But my computer somehow still works if I run this program!

5 Enter the garbage collector Most modern languages take care of memory management for you If you lost a pointer to memory, the programming language will return it to the OS This is the job of a garbage collector Keeps your program from going rogue The garbage collector scans all pointers and objects and frees memory to the OS if you can no longer access that memory

6 Garbage collection C-style In C, you have to manage your memory usage on your own When you create a new list (known as an array in C), you have to tell the OS exactly how much memory you need The OS then allocates that much memory space (if your machine has that much) When you are done using the list, you have to explicitly tell the OS that you are freeing that memory That memory is allocated to your program until you free it Could potentially be a big problem if your program runs for a long time Manually managing memory may be very challenging if you have a large program with a complex structure

7 How the garbage collector works Each object x has a reference count, i.e., the number of references to x The garbage collector keeps track of that reference count at all times When the reference count drops to 0, the memory x was stored in is returned to the OS Your program no longer has access to that memory or to x But if the count is at 0 you lost all your references to x anyway, so you had no access to it anyway The garbage collector runs periodically in the background

8 What about circular references The vanilla garbage collector cannot deal with circular references class Person: def init (self, name): self.name = name def setbrother(self, brother): self.brother = brother b = Person( Bob ) m = Person( Mike ) b.setbrother(m) m.setbrother(b) b = Person( Ben ) m = Person( Mitch )

9 Detector for circular references Ideally, start from the root of the program and trace all references from there Not so easy in Python because of imported modules, so it becomes difficult to say where the root is The following procedure is implemented that will never free used memory 1. For each object, set gc_refs equal to the object s reference count 2. For each object, find which object it references and decrement the referenced object s gc_refs field 3. All objects that now have a gc_refs field greater than one are referenced from outside the set of objects. We cannot free these. 4. Any objects referenced from the objects in 3 also cannot be freed. 5. Any remaining objects can be freed Does not detect absolutely all circular references due to some Python technicalities

10 Can you control the garbage collector? In principle, yes For example, you might want to disable the collector during some crucial execution so as to do it faster Python has a module gc that you can use to change the garbage collector s settings It has a lot of functions such as collect, disable, enable, get_objects, get_referrers Depending on the implementation, these functions are more like recommendations and may be ignored by the program Bottom line: using the gc functions correctly and in a way that will improve the performance of your program is tricky! Tip: let the garbage collector be!

11 Garbage collector cost vs. benefits The obvious benefit is that you don t have to worry about the memory usage of your program Makes it easier to program Also easier to avoid memory bugs The obvious cost is overhead Some estimates show that the garbage collector can slow your program by as much as 4% Also requires more memory to store all references info Overall it is worth it Virtually all modern languages have a garbage collector