Free-Me: A Static Analysis for Individual

Object Reclamation

Samuel Z. GuyerTufts University

Kathryn S. McKinleyUniversity of Texas at Austin

Daniel FramptonAustralian National University

T H E U N I V E R S I T Y O F

T E X A SA T A U S T I N

2

Motivation

Automatic memory reclamation (GC) No need for explicit “free” Garbage collector reclaims memory Eliminates many programming errors

Problem: when do we get memory back? Frequent GCs:

Reclaim memory quickly, with high overhead Infrequent GCs:

Lower overhead, but lots of garbage in memory

3

Example

Notice: String idName is often garbageMemory:

void parse(InputStream stream) { while (not_done) { String idName = stream.readToken(); Identifier id = symbolTable.lookup(idName); if (id == null) { id = new Identifier(idName); symbolTable.add(idName, id); } computeOn(id);}}

Read a token (new String)

Look up insymbol table

If not there, create new identifier, add

to symbol tableCompute on

identifier

4

Solution

Garbage does not accumulateMemory:

void parse(InputStream stream) { while (not_done) { String idName = stream.readToken(); Identifier id = symbolTable.lookup(idName) if (id == null) { id = new Identifier(idName); symbolTable.add(idName, id); } else free(idName); computeOn(id);}} String idName is garbage,

free immediately

5

Our approach

Adds free() automatically FreeMe compiler pass inserts calls to free() Preserve software engineering benefits

Can’t determine lifetimes for all objects Works with the garbage collector Implementation of free() depends on collector

Goal: Incremental, “eager” memory reclamation

Results: reduce GC load, improve performance

Potential: 1.7X performancemalloc/free vs GC

in tight heaps(Hertz & Berger, OOPSLA 2005)

6

Outline

Motivation Analysis Results Related work Conclusions

7

FreeMe Analysis Goal:

Determine when an object becomes unreachable

Not a whole-program analysis*

Idea: pointer analysis + liveness Pointer analysis for reachability Liveness analysis for when

Within a method, for allocation site “p = new A” where can we place a call to “free(p)”?

I’ll describe the interprocedural

parts later

8

Pointer Analysis

Connectivity graph Variables Allocation sites Globals (statics)

Analysis algorithm Flow-insensitive, field-insensitive

String idName = stream.readToken();Identifier id = symbolTable.lookup(idName);if (id == null) { id = new Identifier(idName); symbolTable.add(idName, id);}computeOn(id);

idName

symbolTable

readTokenString

Identifier

(global)

id

9

Adding liveness Key:

idName

readTokenString

Identifier(global)

An object is reachable only when all incoming pointers are live

From a variable: Live range of the variable

From a global: Live from the pointer store onward

Live from the pointer store until source object becomes unreachable

From other object:

Reachability is union of all these

live ranges

10

Computed as sets of edges Variables

Heappointers

Liveness Analysis

String idName = stream.readToken();

id = new Identifier(idName);

computeOn(id);

if (id == null)

Identifier id = symbolTable.lookup(idName);

symbolTable.add(idName, id);

idName

(global)

readTokenString

Identifier

11

Where can we free it?

Where object exists

-minus-

Where reachable

String idName = stream.readToken();

id = new Identifier(idName);

computeOn(id);

if (id == null)

Identifier id = symbolTable.lookup(idName);

symbolTable.add(idName, id);

readTokenString

Compiler inserts call to

free(idName)

12

Interprocedural component Detection of factory methods

Return value is a new object Can be freed by the caller

Effects of methods called

Describes how parameters are connected

Compilation strategy: Summaries pre-computed for all methods Free-me only applied to hot methods

String idName = stream.readToken();

symbolTable.add(idName, id);

Hashtable.add: (0 → 1) (0 → 2)

13

Implementation in JikesRVM FreeMe added to OPT compiler

Run-time: depends on collector Mark/sweep

Free-list: free() operation Generational mark/sweep

Unbump: move nursery “bump pointer” backward

Unreserve: reduce copy reserve Very low overhead Run longer without collecting

14

Volume freed – in MB

100%

50%

0% compress

105

jess

263

raytrace

91

mtrt

98

javac

183

jack

271

pseudojbb

180

xalan

8195antlr

1544716

bloat

fop

103

hsqldb

515

jython

348

pmd

822

ps

523

db

74

SPEC benchmarks DaCapo benchmarks

Increasing alloc size Increasing alloc size

15

Volume freed – in MB

100%

50%

0% compress

105

jess

263

raytrace

91

mtrt

98

javac

183

jack

271

pseudojbb

180

xalan

8195antlr

1544716

bloat

fop

103

hsqldb

515

jython

348

pmd

822

ps

523

db

74

016

7373

24

163

34 1607

673

22230

57

75

278

22

45

FreeMeMean: 32%

16

Compare to stack-like behavior

Notice: Stacks and regions won’t work for example idName escapes some of the time Not biased: 35% vs 65%

Comparison: restrict placement of free()Object must not escape No conditional free No factory methods

Other approaches: Optimistic, dynamic stack allocation [Azul, Corry 06] Scalar replacement

17

Volume freed – in MB

0% compress

105

jess

263

raytrace

91

mtrt

98

javac

183

jack

271

pseudojbb

180

xalan

8195antlr

1544716

bloat

fop

103

hsqldb

515

jython

348

pmd

822

ps

523

db

74

Stack-likeMean: 20%

100%

50%

016

73

73

15

103

1566

14635

21

283

5614

45

6

18

Mark/sweep – GC time

All benchmarks30%

9%

19

Mark/sweep – time

20%

15%

6%

All benchmarks

20

GenMS – time

All benchmarks

21

GenMS – GC time

Why doesn’t this help?

Note: the number of GCs is greatly reduced

FreeMe mostly finding short-lived objects

All benchmarks

Nursery reclaims dead objects for free

(cost ~ survivors)

22

Bloat – GC time

12%

23

Related work Compile-time memory management

Functional languages [Barth 77, Hughs 92, Mazur 01] Shape analysis [Shaham 03, Cherem 06]

Stack allocation [Gay 00, Blanchet 03, Choi 03] Tied to stack frames or other scopes Objects from a site must not escape

Region inference [Tofte 96, Hallenberg 02] Cheap reclamation – no scoping constraints Similar all-or-nothing limitation

24

Conclusions FreeMe analysis

Finds many objects to free: often 30% - 60% Most are short-lived objects

GC + explicit free() Advantage over stack/region allocation: no need to make

decision at allocation time

Generational collectors Nursery works very well Abandon techniques that replace nursery?

Mark-sweep collectors 50% to 200% speedup Works better as memory gets tighter

Embedded applications:Compile-ahead

Memory constrainedNon-moving collectors

25

Thank You