1 Garbage Collection Advantage: Improving Program Locality Xianglong Huang (UT) Stephen M Blackburn (ANU), Kathryn S McKinley (UT) J Eliot B Moss (UMass), Zhenlin Wang (MTU), Perry Cheng (IBM)

2 Motivation Memory gap problem OO programs become more popular OO programs exacerbates memory gap problem –Automatic memory management –Pointer data structures –Many small methods Goal: improve OO program locality

3 Cache Performance Matters

4 Opportunity Generational copying garbage collector reorders objects at runtime

Copying of Linked Objects Breadth First

Copying of Linked Objects Breadth First Depth First

Copying of Linked Objects Depth First Online Object Reordering 1 4 Breadth First

8 Outline Motivation Online Object Reordering (OOR) Methodology Experimental Results Conclusion

9 Online Object Reordering Where are the cache misses? How to identify hot field accesses at runtime? How to reorder the objects?

10 Where Are The Cache Misses? VM ObjectsStack Older Generation Heap structure: Nursery Not to scale

11 Where Are The Cache Misses?

12 Where Are The Cache Misses? Two opportunities to reorder objects in the older generation –Promote nursery objects –Full heap collection

13 How to Find Hot Fields? Runtime info (intercept every read)? Compiler analysis? Runtime information + compiler analysis Key: Low overhead estimation

14 Which Classes Need Reordering? Step 1: Compiler analysis –Excludes cold basic blocks –Identifies field accesses Step 2: JIT adaptive sampling identifies hot methods –Mark as hot field accesses in hot methods Key: Low overhead estimation

15 Example: Compiler Analysis Compiler Hot BB Collect access info Cold BB Ignore Compiler Access List: 1. A.b 2. …. …. Method Foo { Class A a; try { …=a.b; … } catch(Exception e){ …a.c }

16 Example: Adaptive Sampling Method Foo { Class A a; try { …=a.b; … } catch(Exception e){ …a.c } Adaptive Sampling Foo is hot Foo Accesses: 1. A.b 2. …. …. A.b is hot A B b ….. c A’s type information cb

Copying of Linked Objects Online Object Reordering Type Information Hot space Cold space

18 OOR System Overview Baseline Compiler Source Code Executing Code Adaptive Sampling Optimizing Compiler Hot Methods Access Info Database Register Hot Field Accesses Look Up Adds Entries GC: Copies Objects Affects Locality Advice GC: Copies Objects OOR addition JikesRVM componentInput/Output Optimizing Compiler Adaptive Sampling Improves Locality

19 Outline Motivation Online Object Reordering Methodology Experimental Results Conclusion

20 Methodology: Virtual Machine Jikes RVM –VM written in Java –High performance –Timer based adaptive sampling –Dynamic optimization Experiment setup –Pseudo-adaptive –2 nd iteration [Eeckhout et al.]

21 Methodology: Memory Management Memory Management Toolkit (MMTk): –Allocators and garbage collectors –Multi-space heap Boot image Large object space (LOS) Immortal space Experiment setup –Generational copying GC with 4M bounded nursery

22 Overhead: OOR Analysis Only BenchmarkBase Execution Time (sec) w/ only OOR Analysis (sec) Overhead jess % jack % raytrace % mtrt % javac % compress % pseudojbb % db % antlr % hsqldb % ipsixql % jython % ps-fun % Mean -0.19%

23 Detailed Experiments Separate application and GC time Vary thresholds for method heat Vary thresholds for cold basic blocks Three architectures –x86, AMD, PowerPC x86 Performance counter: –DL1, trace cache, L2, DTLB, ITLB

24 Performance javac

25 Performance db

26 Performance jython Any static ordering leaves you vulnerable to pathological cases.

27 Phase Changes

28 Related Work Evaluate static orderings [Wilson et al.] –Large performance variation Static profiling [Chilimbi et al., and others] –Lack of flexibility Instance-based object reordering [Chilimbi et al.] –Too expensive

29 Conclusion Static traversal orders have up to 25% variation OOR improves or matches best static ordering OOR has very low overhead Past predicts future

30 Questions? Thank you!

31 OOR System Overview Records object accesses in each method (excludes cold basic blocks) Finds hot methods by adaptive sampling Reorders objects with hot fields in older generation during GC Copies hot objects into separate region