Efficient Home Migration Protocol in Distributed Java Virtual Machine

1. A Novel Home Migration Protocol in Home-based DSM W.J. Fang C.L. Wang W.Z. Zhu F.C.M. Lau System Research Group Department of Computer Science The University of Hong Kong

2. Outline • Background • Problem Statement • Our Approach • Performance Evaluation • Conclusion wjfang@csis.hku.hk

3. Distributed Java Virtual Machine (DJVM) import java.util.*; class worker extends Thread{ private long n; public worker(long N){ n=N; } public void run(){ long sum=0; for(long i=0; i<n; i++) sum+=i; System.out.println(“N=“+n+” Sum="+sum);} } public class test { static final int N=100; public static void main(String args[]){ worker [] w= new worker[N]; Random r = new Random(); for (int i=0; i<N; i++) w[i] = new worker(r.nextLong()); for (int i=0; i<N; i++) w[i].start(); try{ for (int i=0; i<N; i++) w[i].join();} catch (Exception e){}} } • A distributed Java Virtual Machine (DJVM) consists of a group of extended JVMs running on a distributed environment to support true parallel execution of a multithreaded Java application. • A DJVM provides all the JVM services, that are compliant with the Java language specification. • Presents a Single System Image (SSI) of clusters to Java programs– anillusion that the program is running on a single machine, yet more powerful. (Single System Image) Global Object Space DJVM Thread Exec Class JVM JVM JVM JVM Java thread HKU JESSICA2 project: http://www.srg.cs.hku.hk/ wjfang@csis.hku.hk

4. T1 T2 Before lock: flush cache Before unlock: write back Access fault Global Object Space • Virtualizes a single object heap spanning on the whole cluster. • A DSM service in the distributed JVM. • Java Memory Model: Resembles lazy release consistency. wjfang@csis.hku.hk

5. LRC - Homeless Multiple-Writer Protocol • False Sharing • Two processes simultaneously write on the different parts of the same shared object. • Ping-pong effect if using single-writer protocol. • Twin and Diff • Twin: the copy before writes. • Diff: the difference between the current copy and the twin. • Detect and batch the writes ever performed. • Homeless • The diff is saved and managed at each process. wjfang@csis.hku.hk

6. Drawbacks of Homeless Protocol • Communication overhead • Fetch diff from each previous writer. • Redundant operations • Fetch and apply diffs at each faulting node. • Memory consumption of twin and diff • Requires a global garbage collection. • Not scalable! wjfang@csis.hku.hk

7. LRC - Home-based Protocol • Each shared object has a home to which all writes (diffs) are propagated and from which all copies are derived. • Demonstrated to be more scalable than the homeless protocol. • Less communication, lower diff overhead, less memory consumption. • Access asymmetry • The access overhead at the home node is more lightweight than that at non-home nodes. wjfang@csis.hku.hk

8. Outline • Background • Problem Statement • Our Approach • Performance Evaluation • Conclusion wjfang@csis.hku.hk

9. P0 P1 (Home of X) (Home) Home Migration Lock Fault-in Write(X) Diff propagation Home Assignment Problem A home-based multiple-writer protocol imple-menting LRC Create twin Create diff Access symmetry → Home assignment problem → Find the optimal home Unlock Apply diff wjfang@csis.hku.hk

10. Related Work • JiaJia • Barrier manager detects single-writer pattern between two barriers. • JUMP • The process requiring the page may become the new home. • Blind to access pattern. • Jackal’s Lazy Flushing • Home is fixed. • A process may become the exclusive owner. • Blind to access pattern. • Some homeless DSMs (ADSM, Adaptive TreadMarks) can switch between single-writer protocol and multiple-writer protocol. wjfang@csis.hku.hk

11. Home Migration • A must in home-based protocols. • Adapts to memory access pattern. • Targets at the single-writer pattern. • Home migration makes little difference in the multiple-writer situation as long as the home node is one of the writers. • Negative effect • Home migration notices • We need a timely and precise home migration protocol. wjfang@csis.hku.hk

12. Outline • Background • Problem Statement • Our Approach • Performance Evaluation • Conclusion wjfang@csis.hku.hk

13. P0 P1 (Home) The less sensitive the protocol is Sensitivity of Home Migration Protocol remote read (fault-in) remote write (diff) !Migrate home !Migrate home !Migrate home wjfang@csis.hku.hk

14. Home Home Home Robustness of Home Migration Protocol remote read remote write HM notice P0 P1 P2 P3 A home migration protocol should be robust enough to prevent unnecessary home migrations. wjfang@csis.hku.hk

15. Sensitivity vs. Robustness • At a first glance • Sensitivity , robustness , and vice versa. • Can we achieve both sensitivity and robustness? • Single-writer pattern • Lasting single-writer pattern • Transient single-writer pattern • A good home migration protocol should demonstrate • sensitivity to the lasting single-writer pattern • robustness against the transient single-writer pattern • home migration is prohibited. wjfang@csis.hku.hk

16. Our Previous Work • A distributed JVM with GOS support • A home-based, object-based, multiple-writer protocol • Runtime detection of distributed-shared object • Adaptive optimizations • Object home migration (fixed threshold) • Synchronized method migration • Connectivity-based object pushing Weijian Fang, Cho-Li Wang and Francis C.M. Lau, ``On the Design of Global Object Space for Efficient Multi-threading Java Computing on Clusters’’, Parallel Computing Journal, Vol.29, pp. 1563-1587, November-December 2003. wjfang@csis.hku.hk

17. An Adaptive Threshold HM Protocol • An adaptive per-object home migration threshold. • decreases with increased likelihood that a particular object exhibits a lasting single-writer pattern. • A lower threshold  more likely a lasting single-writer patter  quicker home migration. • A higher threshold  less likely a lasting single-writer patter (probably a transient single-writer pattern)  more conservative home migration • Continuously adjusted at runtime according to the feedback of previous home migration decisions for each object. wjfang@csis.hku.hk

18. Runtime Feedback • Consecutive remote writes • Issued from the same remote node and not interleaved with the writes from either the home node or other remote nodes. • To detect single-writer pattern. • Exclusive home writes • There is no remote write between an exclusive home write and an earlier home write. • Positive feedback of previous home migrations. • Redirected object requests • Home migration notices. • Negative feedback of previous home migrations. • Home access coefficient • The overhead ratio of one eliminated pair of object fault-in and diff propagation to one home migration notice. wjfang@csis.hku.hk

19. Formalization • Ci: the number of consecutive remote writes since the (i - 1)th home migration. • Ti: the value of the adaptive home migration threshold since the (i - 1)th home migration. • Tinit: the initial threshold, which is equal to 1. • Ri: the number of redirected object requests since the (i - 1)th home migration. • Ei: the number of exclusive home writes since the (i - 1)th home migration. • : the home access coefficient. • : the feedback coefficient. It is set to 1. wjfang@csis.hku.hk

20. Formalization (contd.) Home migration is taken if Ci= Ti where Ti= max{(Ti-1 + (Ri - Ei)), Tinit} where T0 = Tinit= 1 wjfang@csis.hku.hk

21. Outline • Background • Problem Statement • Our Approach • Performance Evaluation • Conclusion wjfang@csis.hku.hk

22. Effects of Home Migration wjfang@csis.hku.hk

23. Adaptive Threshold vs. Fixed Threshold wjfang@csis.hku.hk

24. Sensitivity and Robustness Analysis wjfang@csis.hku.hk

25. Contributions • This is the first home migration protocol taking home migration notice overhead into account. • Our protocol is both sensitive to the lasting single-writer pattern and robust against the transient single-writer pattern. • We formulate the protocol. • The protocol uses a per-object adaptive migration threshold, which decreases with increased likelihood that the object presents the lasting single-writer pattern. • The threshold is continuously adjusted according to the feedback of previous home migration decisions at runtime. • We implement and evaluate this protocol in a distributed JVM. wjfang@csis.hku.hk

26. Questions? wjfang@csis.hku.hk