1 GC Advantage: Improving Program Locality Xianglong Huang, Zhenlin Wang, Stephen M Blackburn, Kathryn S McKinley, J Eliot B Moss, Perry Cheng

2 Motivation Memory gap How are Java programs affected?

3 Marksweep vs. Copying pseudojbb

4 Motivation Javac with perfect L1 and L2 cache. 16K L1 256K L2 Appel, GCTk. Breadth first

5 Motivation Copying collector can reorder objects Goal: take advantage of copying collectors reorder objects to improve locality

6 Exploring The Space Different policies for traversing roots Class-oblivious traversal orders  Which traversing order is the best? Class-based traversal orders  How to find the “important” data structure?

7 Different Root Traversal Policies Two different types of roots:  Stack, global variables  Remember sets (for generational) Different traversal orders  Copy all roots before traversing any children  Copy each root and its children (root-by-root)  Split roots Stack first and the children Remset first and the children

8 Experiment Setup JikesRVM, JMTk Generational copying collector with bounded nursery size of 4MB PseudoAdaptive 2 nd iteration

9 Different Root Traversal Policies RxR has the best mutator locality

10 Different Root Traversal Policies Total execution time

11 Exploring The Space Different policies for traversing roots Class-oblivious traversal orders  Which traversing order is the best? Class-based traversal orders  How to find the “important” data structure?

12 Different Traversal Orders Breadth first 1,2,3,4,5,6,7 Pure depth first 1,2,6,3,4,7,5 Pure depth first, LIFO 1,5,4,7,3,2,

13 Different Traversal Orders Breadth first 1,2,3,4,5,6,7 Pure depth first 1,2,6,3,4,7,5 Pure depth first, LIFO 1,5,4,7,3,2,6 Partial depth first, 2 children 1,2,6,3,4,5,

14 Class Oblivious Type Different traversal policies Partial DF is the best

15 Exploring The Space Different policies for traversing roots Class-oblivious traversal orders  Which traversing order is the best? Class-based traversal orders  How to find the “important” data structure?

16 Class-based Traversal Class-oblivious traversal orders inflexible Class-based object traversal  Static profiling  Dynamic sampling

17 Static Profiling Profile object accesses Find hot pairs with strong correlation Example  (1,4), (4,7) and (2,6) have strong correlation  Order: 1,4,7,2,6,3,

18 Online Profiling Use the adaptive compiler sampling  Hot method  Hot basic block Use field accesses to indicate hot fields Example: (In a hot method) { Class A a; a.b=…; … } A B b …..

19 Online Profiling Micro benchmark results

20 Online Profiling Geometric mean

21 Reasons No advice for most of the objects copied  For jess, db and raytrace, we only pick <<1% of the objects as hot objects  5% for javac The hot fields are within the first 2 pointers  90% of the advised objects for javac

22 Online Profiling PseudoJBB mutator results  Generate advice for 23% of the copied objects  75% of the objects have adviced hot fields other than first 2

23 Questions Have we found all the hot objects?  Not all hot objects are connected? Is class-base good enough?  For pseudojbb, we need instance-based? Locality for the nursery objects?

24 Future Work Sampling technique  Catch more hot objects access Lower the threshold Hot objects that are not connected  Dynamically change the advice for phase changing Nursery locality Different traversal orders for cold objects Instance-based

25 Conclusion Reorder objects during copying collection can improve locality In class-oblivious traversal orders partial depth first order is the best Online profiling, class-based traversal is  more flexible, up to 50% better.  very low overhead, ~0% Still mysteries

26 Questions?

27 Answers? Lower the threshold of the sampling, not only the hot methods For objects with only 1 or 2 pointers, it maybe easier just depth first Maybe the nursery locality is more important Instance-based advice

28 Online Profiling Execution overhead

29 Online Profiling Micro benchmark results for mutator time

30 Different Root Traversal Policies _227_mtrt

31 Static Profiling Results

32 Answers? Most objects have only one pointer Percentage of objects copied by advice (whether it is really hot?)  For pseudojbb ~50%, for jess <<1%, for our micro benchmark ~16% Change! Half of the pairs do not form chains longer than 2 Maybe the nursery locality is more important

33 Class Oblivious Orderings Different traversal policies Partial DF is better pseudoJBB

34 Motivation MarkSweep vs. Copying Collector Mutator time of _213_javac

35 Motivation Mutator L2 misses _213_javac