Finding Your Cronies: Static Analysis for Dynamic Object Colocation Samuel Z. Guyer Kathryn S. McKinley T H E U N I V E R S I T Y O F T E X A S A T A U S T I N

OOPSLA, Oct 27, 2004Guyer, McKinley 2 Motivation Modern garbage collectors use multiple spaces Generational collectors: Allocate new objects in nursery space Many objects die young – collection is cheap Copy survivors into mature space – “promotion” Problem: longer-lived objects always copied javac : 25% (47 MB out of 185 MB) nurserymature space

OOPSLA, Oct 27, 2004Guyer, McKinley 3 Avoiding promotion Solution: skip nursery Allocate objects that will survive in mature space How do we predict nursery survivors? Look at why objects survive... Most objects survive because they are connected to older objects javac: 10% stack, 90% mature space nurserymature space p

OOPSLA, Oct 27, 2004Guyer, McKinley 4 Exploiting connectivity Pointers predict survival Create new object – point to it from mature space New object will be promoted Instead: colocate new object... Anticipate the pointer Allocate pointer target in same space as source nurserymature space

OOPSLA, Oct 27, 2004Guyer, McKinley 5 Dynamic Object Colocation Key: a cooperative approach... Run-time: new allocation routine coalloc() takes a Object argument – the colocator Allocates new object in same space as colocator Compile-time: static analysis Determine if a new object will be a pointer target At allocation site: pass pointer source as colocator Connected objects end up in the same space

OOPSLA, Oct 27, 2004Guyer, McKinley 6 Outline Motivation Dynamic object colocation Colocators Static analysis for finding colocators Run-time system Results Related work Conclusions

OOPSLA, Oct 27, 2004Guyer, McKinley 7 Simple Example The new B object will live as long as A * Colocate new B with A Run-time value of p determines behavior: void Simple(A p){ B newB = new B(); p.f = newB; } void Simple(A p){ B newB = coalloc B(p); p.f = newB; } A newB nurserymature space A newB A * Unless p.f is overwritten

OOPSLA, Oct 27, 2004Guyer, McKinley 8 Complex Example Problem: new C cannot be colocator for new B Solution: use p as colocator for both Connectivity is transitive – so is survival Simplifies task of finding colocators void BottomUp(A p){ B newB = new B(); C newC = new C(); newC.f = newB; p.f = newC; } void BottomUp(A p){ B newB = coalloc B(p); C newC = coalloc C(p); newC.f = newB; p.f = newC; } A newB newC

OOPSLA, Oct 27, 2004Guyer, McKinley 9 Finding colocators Use formal parameters Likely to refer to older objects Order of creation/connection doesn’t matter Goal: for each allocation site… Will the new object end up connected to a parameter? (Directly or indirectly) Find colocators using custom pointer analysis Determine connectivity using points-to graph

OOPSLA, Oct 27, 2004Guyer, McKinley 10 Analysis algorithm Points-to graph Node for each parameter, alloc site Edges represent “may point-to” Visit each instruction in IR Keep track of variable bindings Add edges at putfield and astore Find colocators using graph Test reachability of allocation nodes from parameters One method at a time newC newE newD newF void foo(A p, B q) {...} AB pq newG newE p

OOPSLA, Oct 27, 2004Guyer, McKinley 11 Interprocedural analysis Common case: local analysis good enough Problem: allocation, connection in different methods For example, container classes Connector methods Record how method connects arguments Apply summary at call sites Factory methods Treat calls to factory as allocations at call sites Helpful, but not required... p = new Element(); list.add(p);

OOPSLA, Oct 27, 2004Guyer, McKinley 12 Analysis features Observation: Allocation doesn’t affect correctness Colocation analysis can be unsound Simplify algorithm One pass – no fixed-point computation No conservative assumptions (parameter aliasing) No closed-world assumption Works with class loading, reflection, native methods Ignore some connections Heuristics to identify volatile pointers * Unless p.f is overwritten *

OOPSLA, Oct 27, 2004Guyer, McKinley 13 Volatility heuristics Some connections should not cause colocation Mistakes can fill the mature space with garbage Prune them out of the graph... void bar(Container c) { for (...) { c.add(new String(...)); } c.clear(); } New string is conditionally stored Heuristic: store must post- dominate allocation Container object is immediately cleared Heuristic: skip connections that have null assignments void foo(Container c, Value v) { String value_name = new String(v); if ( ! c.contains(value_name)) c.add(value_name); }

OOPSLA, Oct 27, 2004Guyer, McKinley 14 Run-time Coalloc routine – generational collector Factory methods At call site save the colocator At start of method retrieve the colocator Colocation depends on calling context VM_Address coalloc(int bytes, VM_Address colocator) { if (! colocator.isZero() && colocator.LT(NURSERY_START)) return matureAlloc(bytes); else return nursery.alloc(bytes); } If the colocator is non- null and resides in the mature space… … allocate the new object directly in the mature space. … otherwise, allocate the object in the nursery.

OOPSLA, Oct 27, 2004Guyer, McKinley 15 Methodology JikesRVM using MMTk 3.2 GHz Pentium 4, 1 GB memory, Linux Generational collectors – 4 MB bounded nursery GenCopy – copying older space GenMS – mark/sweep older space Benchmarks: SPECJVM98 + pseudojbb Compile all methods ahead of time (+ 5-10%) Goals: Reduce nursery promotion – GC time Avoid filling up the mature space with garbage

OOPSLA, Oct 27, 2004Guyer, McKinley 16 Nursery survival % 50% 150% jessraytracedbjavacmtrtjackpseudojbb Base Coloc Bytes in mature space (normalized) MB promotedMB mature space alloc BaseColocBaseColocBaseColocBaseColocBaseColocBaseColoc 13.8

OOPSLA, Oct 27, 2004Guyer, McKinley 17 GC Time: javac 62% 57%

OOPSLA, Oct 27, 2004Guyer, McKinley 18 GC Time: jbb -24% 40% 35% 55%

OOPSLA, Oct 27, 2004Guyer, McKinley 19 GC Time: average % speedup

OOPSLA, Oct 27, 2004Guyer, McKinley 20 Runtime: javac 4% 8%

OOPSLA, Oct 27, 2004Guyer, McKinley 21 Runtime: average % speedup

OOPSLA, Oct 27, 2004Guyer, McKinley 22 Runtime: db % speedup

OOPSLA, Oct 27, 2004Guyer, McKinley 23 Related Work Co-allocation for locality [Chilimbi 99] Manually add coalloc() calls Pretenuring [Blackburn 01] Static decision – needs alloc-site homogeneity Connectivity-based GC [Hirzel 03] Statically partitions objects Requires sound pointer analysis Our approach combines static and dynamic

OOPSLA, Oct 27, 2004Guyer, McKinley 24 Conclusions Dynamic object colocation overcomes limitations of static approaches Static analysis for garbage collection Complex property (object lifetime) predicted by simple information (local connectivity) Use aggressive, unconventional points-to analysis Cooperative approach Exploit different strengths of compiler and run-time

OOPSLA, Oct 27, 2004Guyer, McKinley 25 Thank You

OOPSLA, Oct 27, 2004Guyer, McKinley 26 GC Time: jbb % speedup

OOPSLA, Oct 27, 2004Guyer, McKinley 27 Conclusions Compiler can assist garbage collector Compiler can discover useful information Complex property (lifetime) predicted by simple information (connectivity) Cooperative approach Static analysis to identify opportunities Run-time system handles dynamic behavior Low cost and effective Generalizes to other multi-space collectors