OntheUsefulnessofLiveness forGarbageCollection andLeakDetection - PowerPoint PPT Presentation

On�the�Usefulness�of�Liveness for�Garbage�Collection and�Leak�Detection Martin�Hirzel,�Amer�Diwan,�and Antony�Hosking {hirzel,diwan}@cs.colorado.edu�����hosking@cs.purdue.edu ECOOP�June�2001�Budapest,�Hungary 1

What�is�Liveness? • A�variable�is� live if�its�value�will�be�used�in�the�future. ast … Tree�*ast�=�parse(); Cfg *cfg =�translate(ast); 〈 code�that�does�not�use�the�value of� ast� 〉 … 2

Accuracy�and�Reachability Unreachable p B A x C ast E D 3

Accuracy�and�Reachability Unreachable p B A x C ast [not�live] E D • Type�accuracy: which�variables�hold�pointers? • Liveness accuracy: which�variables�are�live? 4

Accuracy�and�Reachability Unreachable p B A x [not�a�pointer] C ast E D • Type�accuracy: which�variables�hold�pointers? 5

Accuracy�and�Reachability Unreachable p B A x [not�a�pointer] C ast [not�live] E D • Type�accuracy: which�variables�hold�pointers? • Liveness accuracy: which�variables�are�live? 6

The�Questions • Can�liveness accuracy�benefit�reachability� traversals? – Useful�for�garbage�collection? – Useful�for�leak�detection? • What�kind�of�analysis�is�necessary? 7

Motivation conservative partial full Type Accuracy none weak strong Liveness investigated�in�literature Accuracy unexplored 8

Results�in�a�Nutshell conservative partial full Type Accuracy none weak strong Liveness investigated�in�literature Accuracy unexplored 9

Outline Introduction • Motivation • Preview�of�results Methodology • Obtaining�liveness�information • Metrics Results • Reachable�heap�given�various� accuracy�schemes Conclusion • Related�work • Summary 10

Liveness Approaches • Static�analysis – Compiler-analysis�of�source�code – Disadvantage:�difficult,�cost�+�benefits�unclear • Dynamic�analysis – Trace-based�analysis – Disadvantage:�two�runs�needed,�limit�study 11

Infrastructure�for�Experiments Analysis Accuracy library selection Trace C�or�Eiffel Instrument Link Run-1 Liveness program analysis Type�Information Link Run-2 Liveness�Information Stubs�+ BDW�gc 12

Infrastructure�for�Experiments Type-analysis Accuracy library selection Trace C�or�Eiffel Instrument Link Run-1 Liveness program analysis Type�Information Link Run-2 Liveness�Information Stubs�+ BDW�gc 13

Generating�the�Trace Original�Code Instrumented�Code Trace x�=�malloc(4*N); x�= malloc(4*N); … note_allocation(); allocation(91) note_assign(&x); assign(x) i�=�0; i�=�0; assign(i) note_assign(&i); assign(y,�x,�i) while(i�<�N){ while(i�<�N){ use(y) y�=�x�+�4*i; y�=�x�+�4*i; assign(91.0,�i) note_assign(&y,&x,&i); assign(i,�i) *y�=�i; *y�=�i; assign(y,�x,�i) note_use(&y); use(y) note_assign(y,&i); assign(91.4,�i) i++; i++; … note_assign(&i,&i); } } 14

Analyzing�the�Trace Trace Simulated�Liveness�State Resulting�Information x y i … allocation(91) assign(x) assign(i) T1:�{ x,�i } assign(y,�x,�i) use(y) assign(91.0,�i) assign(i,�i) T1:�{ x,�i } assign(y,�x,�i) use(y) assign(91.4,�i) T2:�{} … “A�variable�is� live if�its�value�will�be�used�in�the�future.” 15

� Usefulness�Metric�for�Accuracy Bytes reachable Conservative Accurate Time Reachability�traversal:�����1���������������������2�������������������3 Reachability reduction:��10%���������������20%����������������0% The�accurate scheme�reduced�reachability by�10%�on�average. 16

Outline Introduction • Motivation • Preview�of�results Methodology • Obtaining�liveness�information • Metrics Results • Reachable�heap�given�various�levels of�accuracy Conclusion • Related�work • Summary 17

Benchmarks Name Language Lines�of�Code Total�allocation�[Bytes] Author/Source Programs�written�with�GC�in�mind: gctest3 C 85 2�200�004 Bartlett gctest C 196 1�123�180 Bartlett bshift Eiffel 350 28�700 Hirzel erbt Eiffel 927 222�300 Durian ebignum Eiffel 3�137 109�548 Hillion li C 7�597 9�030�872 Spec95 gegrep Eiffel 17�185 106�392 Bezault Programs�with�explicit�deallocation: anagram C 647 259�512 Austin ks C 782 7�920 Austin ft C 2�156 166�832 Austin yacr2 C 3�979 41�380 Austin bc C 7�308 12�382�400 Austin gzip C 8�163 14�180 GNU 18 ijpeg C 31�211 148�664 Spec95

Usefulness�of�Liveness Reachability�reduction�w ith�strongest�liveness % �Bytes 60 average 50 at�peak 40 30 20 10 0 m m 3 t p t 2 p g t s f t c i s t b e k f r e i u l a i b e s h z r r c p n r e t g g a s e c g t j g b e y c g i a i g g b n e a 19

Usefulness�of�Liveness Reachability�reduction�w ith�strongest�liveness % �Bytes 98 60 average 50 94 at�peak 40 30 90 98 87 50 20 10 98 20 79 0 2 0 0 0 0 m m 3 t p t 2 p g t s f t c i s t b e k f r e i u l a i b e s h z r r c p n r e t g g a s e c g t j g b e y c g i a i g g b n e a Traversals � different % Num �traversal s 20

Different�Levels�of�Liveness Reachability�reduction % �Bytes stack� intra�scalars stack�� inter scalars 60 stack�� inter�all 50 s.+glob intra�scalars s.+glob inter�scalars 40 s.+glob inter�all 30 20 10 0 m 3 t p 2 p g t f c i t b u e r e i l i b s h z r r c p n e g g s a e j t g b e y c i i g g b e 21

Type�versus�Liveness�Accuracy Reachability�reduction % �Bytes Type�(Pentium ) 60 Liveness 50 Type+Liveness 40 30 20 10 0 ebignum gegrep gctest3 yacr2 ijpeg bshift gzip erbt bc li 22

Validation • Comparing�liveness�information�found�in� different�runs – For�how�many�locations�did�the�obtained� liveness�information�differ? Benchmark Stack Global %�different %�different gegrep 0.7 0.0 yacr2 2.7 0.0 gzip 1.3 2.2 23

Outline Introduction • Motivation • Preview�of�results Methodology • Obtaining�liveness�information • Metrics Results • Reachable�heap�given�various�levels of�accuracy Conclusion • Related�work • Summary 24

Related�Work • Evaluating�Accuracy – Hirzel,�Diwan:�On�the�type�accuracy�of�garbage� collection.�ISMM�2000. – Shaham,�Kolodner,�Sagiv:�On�the�effectiveness� of�GC�in�Java.�ISMM�2000. • Implementing�Accuracy – [Bartlett1988]�[DiwanMossHudson1992]� [SmithMorrisett1998]�[Zorn1993]� [AgesenDetlefsMoss1998]�… 25

Summary • Liveness�accuracy�can�be�very�useful�for� reachability traversals. • Strong�analyses�are�necessary�to�reach� significantly�fewer�Bytes. • Type�accuracy�was�not�very�useful�in�these� experiments. 26