Task-aware Garbage Collection in a Multi-Tasking Virtual Machine Sunil Soman Laurent Daynès Chandra Krintz RACE Lab, UC Santa Barbara Sun Microsystems Labs

2 Outline  Introduction  MVM overview  Task-aware collection Performance isolation Fast, immediate resource reclamation Independent allocation & young gen collection  Evaluation  Conclusion

3 Introduction  Multi-tasking VMs Multiple users/applications Improved startup Reuse & sharing Performance isolation Accounting & timely resource reclamation  Sharing/performance isolation conflict Loss of scalability Bottleneck: memory management system

4 MVM overview  Multiple isolated instances in same VM Implements Isolates API (JSR 121) Safety & isolation guaranteed  Task re-entrance Class sharing Separate task-dependent parts, rest shared Accessed thru table indexed by unique task id  Based on Hotspot VM Generational mark-compact GC

5 MVM Garbage Collection  Shared generational heap Scavenged young gen (eden & survivor spaces) 4 phase mark-compact collected old generation Card table for tracking mature to young pointers edenoldpermanentfromtoyoung

6 MVM Garbage Collection  Lack of “task awareness” Scalability & performance isolation Space occupied by dead tasks reclaimed on GC Parameter virtualization

7 Task-aware collection  Performance isolation Hybrid heap layout Task-concurrent scavenging  Fast, immediate resource reclamation Without requiring full or partial GC

8 Hybrid heap layout edenoldpermanenttofrom eden from to.... eden from to Per-task

9 Hybrid heap layout: young gen

10 Hybrid heap layout: Old gen  Promotion Area Buffers Per-task old generation regions Allocated on demand Adjusted on full heap GC Used for scavenging & pretenuring  Provide Performance isolation GC-less reclamation on task termination Per-task card table scanning Enabled due to precise accounting of old gen

11 Resource Reclamation Allocation top Free PABsTask 2 Allocation top Task 3Task Allocation top

12 Task-concurrent Scavenging  Leverages hybrid heap design  Mutators can execute during scavenge  Concurrent old gen direct allocation  Synchronization Threads belonging to GC initiator stopped Threads poll on protected page Suspended on access fault Threads belonging to other tasks not suspended

13 Evaluation time long app short app …  Experimental setup Dual UltraSPARC 1.5GHz, Solaris 10 Dacapo & Spec98 benchmarks  Throughput & Response time # short apps & average pause time Hybrid heap layout vs Shared heap

14 Results: Thruput & resp. time Short (javac w/small input) + long running program (jess, jack, ps)

15 Evaluation  Concurrent execution performance Hybrid heap + task-conc. scavenging vs shared Instances of same task run concurrently 1 thru 5 instances  GC time & # GCs  End-to-end execution time

16 Concurrent Exec: GC Time 62%54%100%

17 Concurrent Exec: Change in GCs

18 Concurrent Exec: Exec. Time

19 Related Work  Prior MVM  Thread-local heaps [ Domani ’02, Steensgaard ’00 ]  Isolation [ Back ’05 ]  Card table/remset opt [ Azagury ’98, Detlefs ’02 ]

20 Summary  Task-aware collection Performance isolation Immediate, GC-free reclamation Heap layout, mechanisms & perf. evaluation Significantly improves overall execution, throughput & response time

21 Future Work  Better multi-tasking workloads  Old generation collection Independent collection of PABs Different collection policies  Automatic performance tuning Per-application tuning of GC parameters  Global management