Garbage Collection Jan Midtgaard Michael I. Schwartzbach Aarhus - PowerPoint PPT Presentation

Compilation 2010 Garbage Collection Jan Midtgaard Michael I. Schwartzbach Aarhus University

The Garbage Collector  A garbage collector is part of the runtime system  It reclaims heap-allocated records (objects) that are no longer in use  A garbage collector should : • reclaim all unused records • spend very little time per record • not cause significant delays • allow all of memory to be used  These are difficult and conflicting requirements Garbage Collection 2

Life Without Garbage Collection  Unused records must be explicitly deallocated  This is superior if done correctly  But it is easy to miss some records  And it is dangerous to handle pointers  Memory leaks in real life ( ical v.2.1 ): 35 MB 30 25 20 15 10 5 hours 0 Garbage Collection 3

Record Liveness  Which records are still in use?  Ideally, those that will be accessed in the future execution of the program  But that is of course undecidable...  Basic conservative approximation: A record is live if it is reachable from a stack location (local variable or local stack)  Dead records may still point to each other Garbage Collection 4

A Heap With Live and Dead Records 00017 00008 15 12 p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 5

The Mark-and-Sweep Algorithm  Explore pointers starting from all stack locations and mark all the records encountered  Sweep through all records in the heap and reclaim the unmarked ones  Unmark all marked records  Assumptions: • we know the start and size of each record in memory • we know which record fields are pointers • reclaimed records are kept in a freelist Garbage Collection 6

Pseudo Code for Mark-and-Sweep function DFS(x) { if (x is a heap pointer) if (x is not marked) { mark x; for (i=1; i<=|x|; i++) function Sweep() { DFS(x.f i ) p = first address in heap; } while (p<last address in heap) { } if (p is marked) unmark p; else { function Mark() { p.f 1 = freelist; foreach (v in a stack frame) freelist = p; DFS(v); } } p = next object pointer after p } } Garbage Collection 7

Marking and Sweeping (1/11) 00017 00008 15 12 p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 8

Marking and Sweeping (2/11) 00017 00008 15 12 p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 9

Marking and Sweeping (3/11) 00017 00008 15 12 p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 10

Marking and Sweeping (4/11) 00017 00008 15 12 p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 11

Marking and Sweeping (5/11) 00017 00008 15 12 p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 12

Marking and Sweeping (6/11) 00017 00008 15 12 freelist p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 13

Marking and Sweeping (6/11) 00017 00008 15 12 freelist p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 14

Marking and Sweeping (6/11) 00017 00008 15 12 freelist p 37 q 00042 00113 9 r 7 00249 20 37 00738 59 00371 Garbage Collection 15

Marking and Sweeping (7/11) 00017 00008 15 12 freelist p 37 q 00042 00113 r 7 00249 20 37 00738 59 00371 Garbage Collection 16

Marking and Sweeping (8/11) 00017 00008 15 12 freelist p 37 q 00042 00113 r 7 00249 20 37 00738 59 00371 Garbage Collection 17

Marking and Sweeping (9/11) 00017 00008 15 12 freelist p 37 q 00042 00113 r 7 00249 20 37 00738 59 00371 Garbage Collection 18

Marking and Sweeping (10/11) 00017 00008 15 12 freelist p 37 q 00042 00113 r 7 00249 20 37 00738 59 00371 Garbage Collection 19

Marking and Sweeping (11/11) 00017 00008 15 12 freelist p 37 q 00042 00113 r 7 00249 20 37 00738 59 00371 Garbage Collection 20

Analysis of Mark-and-Sweep  Assume the heap has H words  Assume that R words are reachable  The cost of garbage collection is: c 1 R + c 2 H  The cost per reclaimed word is: (c 1 R + c 2 H)/(H - R)  If R is close to H, then this is expensive Garbage Collection 21

Allocation  The freelist must be searched for a record that is large enough to provide the requested memory  Free records may be sorted by size  The freelist may become fragmented: containing many small free records but none that is large enough  Defragmentation joins adjacent free records Garbage Collection 22

Pointer Reversal  The DFS recursion stack could have size H  It has at least size log (H)  This may be too much (after all, memory is low)  The recursion stack may be cleverly embedded in the fields of the marked records  This technique makes mark-and-sweep practical Garbage Collection 23

The Reference Counting Algorithm  Maintain a counter of the total number of references to each record  For each assignment, update the counters  A record is dead when its counter is zero  Advantages: • catches dead records immediately • does not cause long pauses  Disadvantages: • cannot detect cycles of dead records • is rather expensive Garbage Collection 24

Pseudo Code for Reference Counting function Increment(x) { x.count++; } function Decrement(x) { x.count--; if (x.count==0) PutOnFreeList(x); function PutOnFreelist(x) { } Decrement(x.f 1 ); x.f 1 = freelist; freelist = x; } function RemoveFromFreelist(x) { for (i=2; i<=|x|; i++) Decrement(x.f i ); } Garbage Collection 25

The Stop-and-Copy Algorithm  Divide the heap space into two parts  Only use one part at a time  When it runs full, copy live records to the other part of the heap space  Then switch the roles of the two parts  Advantages: • fast allocation (no freelist) • avoids fragmentation  Disadvantage: • wastes half your memory Garbage Collection 26

Before and After Stop-and-Copy 8 8 7 7 6 6 5 4 3 4 3 next 5 limit from-space to-space to-space from-space next limit Garbage Collection 27

Pseudo Code for Stop-and-Copy function Forward(x) { if (x  from-space) { if (x.f 1  to-space) return x.f 1 ; else for (i=1; i<|x|; i++) next.f i = x.f i ; x.f 1 = next; next = next + sizeof(x); function Copy() { return x.f 1 ; scan = next = start of to-space; } else return x; foreach (v in a stack frame) } v = Forward(v); while (scan < next) { for (i=1; i<=|scan|; i++) scan.f i = Forward(scan.f i ); scan = scan + sizeof(scan); } } Garbage Collection 28

Stopping and Copying (1/13) 00017 00017 00008 15 15 12 p 37 00042 q 00113 9 r 7 00249 20 00738 37 59 00371 to-space from-space Garbage Collection 29

Stopping and Copying (2/13) 09000 00017 00017 00008 15 15 15 12 p 37 00042 q 00113 9 r 7 00249 20 00738 37 59 00371 to-space from-space Garbage Collection 30

Stopping and Copying (3/13) 09000 00017 00017 00008 15 15 15 12 p 37 00042 q 00113 9 r 7 00249 20 00738 37 59 00371 to-space from-space Garbage Collection 31

Stopping and Copying (4/13) 09000 00017 00017 00008 15 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 20 00738 37 59 00371 to-space from-space Garbage Collection 32

Stopping and Copying (5/13) 09000 00017 00017 00008 15 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 20 00738 37 59 00371 to-space from-space Garbage Collection 33

Stopping and Copying (6/13) 09000 09024 00017 00017 00008 15 12 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 20 00738 37 59 00371 to-space from-space Garbage Collection 34

Stopping and Copying (7/13) 09000 09024 00017 00017 00008 15 12 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 20 00738 37 59 00371 to-space from-space Garbage Collection 35

Stopping and Copying (8/13) 09000 09024 00017 00017 00008 15 12 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 20 00738 37 59 00371 to-space from-space Garbage Collection 36

Stopping and Copying (9/13) 09000 09024 00017 00017 00008 15 12 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 00249 20 20 00738 37 59 00371 to-space from-space Garbage Collection 37

Stopping and Copying (10/13) 09000 09024 00017 00017 00008 15 12 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 00936 20 20 00738 37 59 00371 to-space from-space Garbage Collection 38

Stopping and Copying (11/13) 09000 09024 00017 00017 00008 15 12 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 00936 20 20 00738 37 00948 59 59 00371 to-space from-space Garbage Collection 39

Stopping and Copying (12/13) 09000 09024 00017 00017 00008 15 12 15 15 12 p 37 00042 q 00113 9 r 09012 7 00249 37 00936 20 20 00738 37 00948 59 59 00371 to-space from-space Garbage Collection 40

Stopping and Copying (13/13) 09000 09024 15 12 p 37 q r 09012 37 00936 20 00948 59 from-space to-space Garbage Collection 41