1 Evaluating the Impact of Thread Escape Analysis on Memory Consistency Optimizations Chi-Leung Wong, Zehra Sura, Xing Fang, Kyungwoo Lee, Samuel P. Midkiff, Jaejin Lee and David Padua University of Illinois at Urbana-Champaign IBM T.J. Watson Research Center Purdue University Seoul National University

2 Outline Memory Models The Pensieve System Escape Analyses Qualitative Impact of Escape Analyses on Delay Set Analysis and Synchronization Analysis Experimental Results Conclusion

3 Memory Models Consider the following code segments: –Thread 1 : data = 100; data_ready = true; –Thread 2 : while (!data_ready); t = data; Can t == 0? –Yes if reordering happens Thread 1 : data_ready = true; data = 100; Can be done by compiler and hardware –Memory models tell us the answer Sequential Consistency says no

4 Objective of the Pensieve Project Sequential consistency (SC) on top of Intel x86 memory models –Implementation based on Jikes RVM All analyses done in JIT time Need to minimize both analysis and application execution time

5 Enforcing SC Done by enforcing memory accesses orders –not all orderings need to be enforced –only enforce orders really needed Delay Set Analysis (DSA) [SS88] computes such orders Our approach : Approximation of DSA –Orders enforced by inserting fences in generated code

6 Original DSA Program edge –x executes before y in the same thread Conflict edge –x and x’ conflict accesses Order of access affects program outcome In this paper: –to the same memory location –one of them is a write xx’xy y’ y x x’

7 Original DSA (Cont’d) Critical cycle –Minimal Cannot form smaller cycle using subset of nodes –Mixed Contains both edges Enforce program edges on a critical cycle y’ y x x’ Minimal Not minimal y’ y x x’ z Not mixed y x Mixed y x

8 Approximate DSA Approximate of critical cycle –x precedes y –Conflict accesses for x and x’ y and y’ –y’ precedes x’ Enforce program edges on approx critical cycle x yx’ y’

9 Source Program Code Optimizations Fence Insertion & Optimization Program Analyses Thread Escape Analysis Program Analyses The Pensieve System Target Program Orders to Enforce Synchronization Analysis Delay Set Analysis

10 Escape Analyses Identify objects which may be accessed by two or more threads Output: set of variables –{v | v points to an object may be accessed by >= 2 threads}

11 Impact on Delay Set Analysis x, y, y’, x’ must be escaping accesses –Cannot form a cycle if one of them is not escaping access Fewer escaping accesses implies fewer possible pairs of (x,y) –Fewer checks to be done –Fewer delays yx’ y’ x

12 Impact on Synchronization Analysis Synchronization analysis reduces number of conflict edges considered by DSA –Consider synchronized construct –Calls to start() and join() Our system only consider t1.join() –if it can match some t2.start() call –t1 and t2 are not escaping More precise escape info  more join() calls matched  more precise DSA result

13 Escape Analyses Comparison In this study, we compare 4 algorithms: –Connectivity Analysis (Pensieve) –Field Base Analysis (Pensieve) For comparison purposes –Bogda’s Analysis Removing Unnecessary Synchronization in Java. (OOPSLA 1999) –Ruf’s Analysis Effective Synchronization Removal for Java. (PLDI 2000)

14 Connectivity Escape Analysis An object is escaping if both –Reachable by more than one thread due to two possible cases: Reachable by a static field Passed from a thread constructor –Accessed by more than one thread Do not assume this escaping in run() by default Field insensitive for most memory accesses –I.e. do not distinguish x.f vs x.g –Except accesses to Runnable objects

15 Field Base Escape Analysis An object is escaping if –Reachable from a static field –Passed from a thread constructor Do not assume this escaping in run() by default –Similar to connectivity base analysis, Field sensitive –Suppose O 1, O 2 of same type O 1.f different from O 1.g O 1.f same as O 2.f

16 Bogda’s Escape Analysis An object is escaping if it is reachable: –By a static field –By a Runnable object –Via more than 1 field reference

17 Ruf’s Escape Analysis An object is escaping if both –Reachable from either A static field or A Runnable object –Synchronized by more than one thread Adapted for our own use –“synchronized”  “accessed”

18 Experimental Settings (Machine) Intel (Dell PowerEdge 6600 SMP) –4 Intel hyperthreaded 1.5Ghz Xeon processors –with 1MB cache each –6G system memory.

19 Experimental Settings (Software) Original –default Jikes RVM implementation –base case for performance comparison Enforcing SC –Empty –Arg Escaping –Connectivity analysis –Field-base analysis –Bogda’s analysis (bogda) –Ruf’s analysis

20 Measurements Escape Analysis Time Impact on Delay Set Analysis Time Impact on Synchronization Analysis Time Slowdown due to fence insertion –Delay Set Analysis only –Delay Set Analysis with Synchronization Analysis

21 Escape Analysis Time

22 Impact on Delay Set Analysis Time

23 Impact on Synchronization Analysis Time

24 Escape+DSA+ Synchronization Analysis Time / Compilation Time

25 Slowdown (DSA Only)

26 Slowdown (DSA+Sync Analysis)

27 Slowdown of connect (DSA+Sync Analysis)

28 Conclusions Evaluate interaction between escape analysis and synchronization/delay set analysis Montecarlo and jbb motivates enabling field sensitivity for connectivity base analysis

29 Backup Slides Follow

30 Number of Delay Checks Performed

31 Total Compilation Time

32 Number of Delays Found (DSA Only)

33 Number of Delays Found (DSA + Sync Analysis)