1. David Evans http://www.cs.virginia.edu/evans CS201j: Engineering Software University of Virginia Computer Science Lecture 17: Garbage Collection

2. 5 November 2002CS 201J Fall 20022 Menu Stack and Heap Mark and Sweep Stop and Copy Reference Counting Java’s Garbage Collector

3. 5 November 2002CS 201J Fall 20023 Stack and Heap Review public class Strings { public static void test () { StringBuffer sb = new StringBuffer ("hello"); } static public void main (String args[]) { test (); } sb java.lang.StringBuffer “hello” Stack Heap When do the stack and heap look like this? A B 1 2 3

4. 5 November 2002CS 201J Fall 20024 Stack and Heap Review public class Strings { public static void test () { StringBuffer sb = new StringBuffer ("hello"); } static public void main (String args[]) { test (); } sb java.lang.StringBuffer “hello” Stack Heap B 2

5. 5 November 2002CS 201J Fall 20025 Garbage Heap public class Strings { public static void test () { StringBuffer sb = new StringBuffer ("hello"); } static public void main (String args[]) { while (true) test (); } “hello” Stack Heap “hello”

6. 5 November 2002CS 201J Fall 20026 Garbage Collection System needs to reclaim storage on the heap used by garbage objects How can it identify garbage objects? How come we don’t need to garbage collect the stack?

7. 5 November 2002CS 201J Fall 20027 Mark and Sweep

8. 5 November 2002CS 201J Fall 20028 Mark and Sweep John McCarthy, 1960 (first LISP implementation) Start with a set of root references Mark every object you can reach from those references Sweep up the unmarked objects What are the root references? References on the stack.

9. 5 November 2002CS 201J Fall 20029 public class Phylogeny { static public void main (String args[]) { SpeciesSet ss = new SpeciesSet (); … (open file for reading) while (…not end of file…) { Species current = new Species (…name from file…, …genome from file…); ss.insert (current); } public class SpeciesSet { private Vector els; public void insert (/*@non_null@*/ Species s) { if (getIndex (s) < 0) els.add (s); }

10. 5 November 2002CS 201J Fall 200210 Stack Top of Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: current: Species “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: SpeciesSet.inser t this: SpeciesSet s: Species public class Phylogeny { static public void main (String args[]) { SpeciesSet ss = new SpeciesSet (); while (…not end of file…) { Species current = new Species (…name…, …genome…); ss.insert (current); } } public class SpeciesSet { private Vector els; public void insert (/*@non_null@*/ Species s) { if (getIndex (s) < 0) els.add (s); }

11. 5 November 2002CS 201J Fall 200211 Stack Top of Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: current: Species “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: SpeciesSet.inser t this: SpeciesSet s: Species public class Phylogeny { static public void main (String args[]) { SpeciesSet ss = new SpeciesSet (); while (…not end of file…) { Species current = new Species (…name…, …genome…); ss.insert (current); } } public class SpeciesSet { private Vector els; public void insert (/*@non_null@*/ Species s) { if (getIndex (s) < 0) els.add (s); } After els.add (s)…

12. 5 November 2002CS 201J Fall 200212 Stack Top of Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: current: Species “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: SpeciesSet.inser t this: SpeciesSet s: Species public class Phylogeny { static public void main (String args[]) { SpeciesSet ss = new SpeciesSet (); while (…not end of file…) { Species current = new Species (…name…, …genome…); ss.insert (current); } } public class SpeciesSet { private Vector els; public void insert (/*@non_null@*/ Species s) { if (getIndex (s) < 0) els.add (s); } SpeciesSet.insert returns…

13. 5 November 2002CS 201J Fall 200213 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: current: Species “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: public class Phylogeny { static public void main (String args[]) { SpeciesSet ss = new SpeciesSet (); while (…not end of file…) { Species current = new Species (…name…, …genome…); ss.insert (current); } } public class SpeciesSet { private Vector els; public void insert (/*@non_null@*/ Species s) { if (getIndex (s) < 0) els.add (s); } Finish while loop… Top of Stack

14. 5 November 2002CS 201J Fall 200214 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: public class Phylogeny { static public void main (String args[]) { SpeciesSet ss = new SpeciesSet (); while (…not end of file…) { Species current = new Species (…name…, …genome…); ss.insert (current); } } public class SpeciesSet { private Vector els; public void insert (/*@non_null@*/ Species s) { if (getIndex (s) < 0) els.add (s); } Garbage Collection Top of Stack

15. 5 November 2002CS 201J Fall 200215 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack Initialize Mark and Sweeper: active = all objects on stack

16. 5 November 2002CS 201J Fall 200216 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable (non-garbage) foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive

17. 5 November 2002CS 201J Fall 200217 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable (non-garbage) foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive

18. 5 November 2002CS 201J Fall 200218 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable (non-garbage) foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive

19. 5 November 2002CS 201J Fall 200219 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable (non-garbage) foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive

20. 5 November 2002CS 201J Fall 200220 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable (non-garbage) foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive

21. 5 November 2002CS 201J Fall 200221 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive sweep () // remove unmarked objects on heap

22. 5 November 2002CS 201J Fall 200222 Stack Bottom of Stack Phylogeny.main “Duck” “CATAG” root: Species name: genome: String[]: args ss: SpeciesSet “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: After main returns… Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive sweep () // remove unmarked objects on heap

23. 5 November 2002CS 201J Fall 200223 Stack Bottom of Stack “Duck” “CATAG” name: genome: “in.spc” els: “Goat” “CAGTG” name: genome: “Elf” “CGGTG” name: genome: “Frog” “CGATG” name: genome: Garbage Collection Top of Stack active = all objects on stack while (!active.isEmpty ()) newactive = { } foreach (Object a in active) mark a as reachable foreach (Object o that a points to) if o is not marked newactive = newactive U { o } active = newactive sweep () // remove unmarked objects on heap

24. 5 November 2002CS 201J Fall 200224 Problems with Mark and Sweep Fragmentation: free space and alive objects will be mixed –Harder to allocate space for new objects –Poor locality means bad memory performance Caches make it quick to load nearby memory Multiple Threads –One stack per thread, one heap shared by all threads –All threads must stop for garbage collection

25. 5 November 2002CS 201J Fall 200225 Stop and Copy Solves fragmentation problem Copy all reachable objects to a new memory area After copying, reclaim the whole old heap Disadvantages: –More complicated: need to change stack and internal object pointers to new heap –Need to save enough memory to copy –Expensive if most objects are not garbage

26. 5 November 2002CS 201J Fall 200226 Generational Collectors Observation: –Many objects are short-lived Temporary objects that get garbage collected right away –Other objects are long-lived Data that lives for the duration of execution Separate storage into regions –Short term: collect frequently –Long term: collect infrequently Stop and copy, but move copies into longer-lived areas

27. 5 November 2002CS 201J Fall 200227 Reference Counting What if each object kept track of the number of references to it? If the object has zero references, it is garbage!

28. 5 November 2002CS 201J Fall 200228 Referencing Counting T x = new T (); The x object has one reference y = x; The object x references has 1 more ref The object y pre references has 1 less ref }Leave scope where x is declared The object x references has 1 less ref

29. 5 November 2002CS 201J Fall 200229 Reference Counting class Recycle { private String name; private Vector pals; public Recycle (String name) { this.name = name; pals = new Vector (); } public void addPal (Recycle r) { pals.addElement (r); } } public class Garbage { static public void main (String args[]) { Recycle alice = new Recycle ("alice"); Recycle bob = new Recycle ("bob"); bob.addPal (alice); alice = new Recycle ("coleen"); bob = new Recycle ("dave"); } “Alice” name: pals: refs: 1 “Bob” name: pals: refs: 1 2

30. 5 November 2002CS 201J Fall 200230 Reference Counting class Recycle { private String name; private Vector pals; public Recycle (String name) { this.name = name; pals = new Vector (); } public void addPal (Recycle r) { pals.addElement (r); } } public class Garbage { static public void main (String args[]) { Recycle alice = new Recycle ("alice"); Recycle bob = new Recycle ("bob"); bob.addPal (alice); alice = new Recycle ("coleen"); bob = new Recycle ("dave"); } “Alice” name: pals: refs: 1 “Bob” name: pals: refs: 1 2 “Coleen” name: pals: refs: 1

31. 5 November 2002CS 201J Fall 200231 Reference Counting class Recycle { private String name; private Vector pals; public Recycle (String name) { this.name = name; pals = new Vector (); } public void addPal (Recycle r) { pals.addElement (r); } } public class Garbage { static public void main (String args[]) { Recycle alice = new Recycle ("alice"); Recycle bob = new Recycle ("bob"); bob.addPal (alice); alice = new Recycle ("coleen"); bob = new Recycle ("dave"); } “Alice” name: pals: refs: 1 “Bob” name: pals: refs: 1 0 0

32. 5 November 2002CS 201J Fall 200232 Circular References class Recycle { private String name; private Vector pals; public Recycle (String name) { this.name = name; pals = new Vector (); } public void addPal (Recycle r) { pals.addElement (r); } } public class Garbage { static public void main (String args[]) { Recycle alice = new Recycle ("alice"); Recycle bob = new Recycle ("bob"); bob.addPal (alice); alice.addPal (bob); alice = null; bob = null; } “Alice” name: pals: refs: 1 “Bob” name: pals: refs: 1 2 2

33. 5 November 2002CS 201J Fall 200233 Reference Counting Summary Advantages –Can clean up garbage right away when the last reference is lost –No need to stop other threads! Disadvantages –Need to store and maintain reference count –Some garbage is left to fester (circular references) –Memory fragmentation

34. 5 November 2002CS 201J Fall 200234 Java’s Garbage Collector Mark and Sweep collector Before collecting an object, it will call finalize Can call garbage collector directly: System.gc () protected void finalize() throws Throwable

35. 5 November 2002CS 201J Fall 200235 Garbage in, Garbage out? class Recycle { private String name; private Vector pals; public Recycle (String name) { this.name = name; pals = new Vector (); } public void addPal (Recycle r) { pals.addElement (r); } protected void finalize () { System.err.println (name + " is garbage!"); } } public class Garbage { static public void main (String args[]) { Recycle alice = new Recycle ("alice"); Recycle bob = new Recycle ("bob"); bob.addPal (alice); alice = new Recycle ("coleen"); System.gc (); bob = new Recycle ("dave"); System.gc (); } > java Garbage First collection: Second collection: alice is garbage! bob is garbage!

36. 5 November 2002CS 201J Fall 200236 Testing Strategy The applicant's source code and documentation shall be reviewed to verify that the software conforms to the documentation and that the documentation is sufficient to enable the Division to design and conduct all tests at any level of the software structure to verify that the software meets the requirements and objectives of its design specification. Either the division staff or the ITA, which will conduct software qualification tests, shall witness compilation of the source code into baseline object code. The baseline object code and the object code tested must be identical. The object code tested and certified must be identical to the object code released. A copy of the baseline object code will be retained by the Division and used to verify that the baseline code, tested code, and the release code are identical. Once the Division has conducted a review of system software and documentation, test plans shall be designed to exercise all system functions controlled by software under nominal load and data conditions and throughout the range of conditions for which performance is claimed. The first phase of Florida Software Qualification Testing is designed to evaluate the system software’s functionality, and to establish baselines for the system being tested. … Florida Voting Systems Standard, http://election.dos.state.fl.us/laws/pdf/FVSS402.pdf

37. 5 November 2002CS 201J Fall 200237 Election Day How much do you trust the programs in the voting machines to record your vote correctly? How much do you trust the programs that tabulate the votes to count them correctly? Hint #1: These programs were probably written in C. Hint #2: These programs were probably not written by Libertarians. Hint #3: The authors of these programs didn’t pass CS201J and weren’t able to get more interesting, high-paying programming jobs.

38. 5 November 2002CS 201J Fall 200238 Charge In Java: be happy you have a garbage collector to clean up for you In C: need to deallocate storage explicitly –Why is it hard to write a garbage collector for C? (next class) In the real world: clean up after yourself and others! Garbage Collectors (COAX, Seoul, 18 June 2002)