Garbage Collection Introduction and Overview Christian Schulte Programming Systems Lab Universität des Saarlandes, Germany

Purpose of Talk Explaining basic concepts terminology Garbage collection… …is simple …can be explained at a high-level Organization

Purpose of Talk Explaining basic concepts terminology (never to be explained again) Garbage collection… …is simple …can be explained at a high-level Organization

Overview What is garbage collection objects of interest principal notions classic examples with assumptions and properties Discussion software engineering issues typical cost areas of usage why knowledge is profitable Organizational Material Requirements

Overview What is garbage collection objects of interest principal notions classic examples with assumptions and properties Discussion software engineering issues typical cost areas of usage why knowledge is profitable Organizational Material Requirements

Garbage Collection… …is concerned with the automatic reclamation of dynamically allocated memory after its last use by a program

Garbage Collection… dynamically allocated memory …is concerned with the automatic reclamation of dynamically allocated memory after its last use by a program

Garbage Collection… dynamically allocated memory last use by a program …is concerned with the automatic reclamation of dynamically allocated memory after its last use by a program

Garbage Collection… dynamically allocated memory last use by a program automatic reclamation …is concerned with the automatic reclamation of dynamically allocated memory after its last use by a program

Garbage collection… Dynamically allocated memory Last use by a program Examples for automatic reclamation

Kinds of Memory Allocation static int i; void foo(void) { int j; int* p = (int*) malloc(…); }

Static Allocation By compiler (in text area) Available through entire runtime Fixed size static int i; void foo(void) { int j; int* p = (int*) malloc(…); }

Automatic Allocation Upon procedure call (on stack) Available during execution of call Fixed size static int i; void foo(void) { int j; int* p = (int*) malloc(…); }

Dynamic Allocation Dynamically allocated at runtime (on heap) Available until explicitly deallocated Dynamically varying size static int i; void foo(void) { int j; int* p = (int*) malloc(…); }

Dynamically Allocated Memory Also: heap-allocated memory Allocation: malloc, new, … before first usage Deallocation: free, delete, dispose, … after last usage Needed for C++, Java: objects SML:datatypes, procedures anything that outlives procedure call

Getting it Wrong Forget to free (memory leak) program eventually runs out of memory long running programs: OSs. servers, … Free to early (dangling pointer) lucky: illegal access detected by OS horror: memory reused, in simultaneous use programs can behave arbitrarily crashes might happen much later Estimates of effort Up to 40%! [Rovner, 1985]

Nodes and Pointers Node n Memory block, cell Pointer p Link to node Node access: *p Children children(n) set of pointers to nodes referred by n n p

Mutator Abstraction of program introduces new nodes with pointer redirects pointers, creating garbage

Nodes referred to by several pointers Makes manual deallocation hard local decision impossible respect other pointers to node Cycles instance of sharing Shared Nodes

Garbage collection… Dynamically allocated memory Last use by a program Examples for automatic reclamation

Last Use by a Program Question: When is node M not any longer used by program? Let P be any program not using M New program sketch: Execute P; Use M; Hence: M used P terminates We are doomed: halting problem! So last use undecidable!

Safe Approximation Decidable and also simple What means safe? only unused nodes freed What means approximation? some unused nodes might not be freed Idea nodes that can be accessed by mutator

Reachable Nodes Reachable from root set processor registers static variables automatic variables (stack) Reachable from reachable nodes root

Summary: Reachable Nodes A node n is reachable, iff n is element of the root set, or n is element of children(m) and m is reachable Reachable node also called live

MyGarbageCollector Compute set of reachable nodes Free nodes known to be not reachable Known as mark-sweep in a second…

Reachability: Safe Approximation Safe access to not reachable node impossible depends on language semantics but C/C++? later… Approximation reachable node might never be accessed programmer must know about this! have you been aware of this?

Garbage collection… Dynamically allocated memory Last use by a program Examples for automatic reclamation

Example Garbage Collectors Mark-Sweep Others Mark-Compact Reference Counting Copying skipped here read Chapter 1&2 of [Lins&Jones,96]

The Mark-Sweep Collector Compute reachable nodes: Mark tracing garbage collector Free not reachable nodes: Sweep Run when out of memory: Allocation First used with LISP [McCarthy, 1960]

Allocation node* new() { if (free_pool is empty) mark_sweep(); …

Allocation node* new() { if (free_pool is empty) mark_sweep(); return allocate(); }

The Garbage Collector void mark_sweep() { for (r in roots) mark(r); …

The Garbage Collector void mark_sweep() { for (r in roots) mark(r); … all live nodes marked

Recursive Marking void mark(node* n) { if (!is_marked(n)) { set_mark(n); … }

Recursive Marking void mark(node* n) { if (!is_marked(n)) { set_mark(n); … } nodes reachable from n marked

Recursive Marking void mark(node* n) { if (!is_marked(n)) { set_mark(n); for (m in children(n)) mark(m); } i-th recursion: nodes on path with length i marked

The Garbage Collector void mark_sweep() { for (r in roots) mark(r); sweep(); …

The Garbage Collector void mark_sweep() { for (r in roots) mark(r); sweep(); … all nodes on heap live

The Garbage Collector void mark_sweep() { for (r in roots) mark(r); sweep(); … all nodes on heap live and not marked

Eager Sweep void sweep() { node* n = heap_bottom; while (n < heap_top) { … }

Eager Sweep void sweep() { node* n = heap_bottom; while (n < heap_top) { if (is_marked(n)) clear_mark(n); else free(n); n += sizeof(*n); }

The Garbage Collector void mark_sweep() { for (r in roots) mark(r); sweep(); if (free_pool is empty) abort(Memory exhausted); }

Assumptions Nodes can be marked Size of nodes known Heap contiguous Memory for recursion available Child fields known!

Assumptions: Realistic Nodes can be marked Size of nodes known Heap contiguous Memory for recursion available Child fields known

Assumptions: Conservative Nodes can be marked Size of nodes known Heap contiguous Memory for recursion available Child fields known

Mark-Sweep Properties Covers cycles and sharing Time depends on live nodes (mark) live and garbage nodes (sweep) Computation must be stopped non-interruptible stop/start collector long pause Nodes remain unchanged (as not moved) Heap remains fragmented

Variations of Mark-Sweep In your talk…

Implementation In your talk…

Efficiency Analysis In your talk…

Comparison In your talk…

Application In your talk…

Overview What is garbage collection objects of interest principal invariant classic examples with assumptions and properties Discussion software engineering issues typical cost areas of usage why knowledge is profitable Organizational Material Requirements

Software Engineering Issues Design goal in SE: decompose systems in orthogonal components Clashes with letting each component do its memory management liveness is global property leads to local leaks lacking power of modern gc methods

Typical Cost Early systems (LISP) up to 40% [Steele,75] [Gabriel,85] garbage collection is expensive myth Well engineered system of today 10% of entire runtime [Wilson, 94]

Areas of Usage Programming languages and systems Java, C#, Smalltalk, … SML, Lisp, Scheme, Prolog, … Modula 3, Microsoft.NET Extensions C, C++ (Conservative) Other systems Adobe Photoshop Unix filesystem Many others in [Wilson, 1996]

Understanding Garbage Collection: Benefits Programming garbage collection programming systems operating systems Understand systems with garbage collection (e.g. Java) memory requirements of programs performance aspects of programs interfacing with garbage collection (finalization)

Overview What is garbage collection objects of interest principal invariant classic examples with assumptions and properties Discussion software engineering issues typical cost areas of usage why knowledge is profitable Organizational Material Requirements

Material Garbage Collection. Richard Jones and Rafael Lins, John Wiley & Sons, Uniprocessor garbage collection techniques. Paul R. Wilson, ACM Computing Surveys. To appear. Extended version of IWMM 92, St. Malo.

Organization Requirements Talk duration 45 min (excluding discussion) Attendance including discussion Written summary 10 pages to be submitted in PDF until Mar 31st, 2002 Schedule weekly starting Nov 14th, 2001 next on Dec 5th, 2001

Topics For You! The classical methods Copying1.[Brunklaus, Guido Tack] Mark-Sweep2.[Schulte, Hagen Böhm] Mark-Compact3.[Schulte, Jens Regenberg] Reference Counting6.[Brunklaus, Regis Newo] Advanced Generational4.[Brunklaus, Mirko Jerrentrup] Conservative (C/C++)5.[Schulte, Stephan Lesch] Incremental & Concurrent7.[Brunklaus, Uwe Kern]

Invariants Only nodes with rc zero are freed RC always positive