1. Implementation and Performance of Munin (Distributed Shared Memory System) Dongying Li Department of Electrical and Computer Engineering University of Toronto (Original Authors: J. B. Carter, et al.) ECE 1147, Parallel Computation Oct. 30, 2006

2. 2 Distributed Shared Memory Shared address space spanning the processors of a distributed memory multiprocessor proc1proc3 X=0 proc2 X=0

3. 3 Distributed Shared Memory mem0 proc0 mem1 proc1 mem2 proc2 memN procN network... shared memory

4. 4 Distributed Shared Memory Challenges –Good performance comparable to shared memory programs –No significant deviation from shared memory coding model –Low communication and message passing overheads

5. 5 Munin System Characterized features –Software released consistency –Multiple consistency protocols Deviations from shared memory model –Annotated shared memory variable pattern –All Synchronization visible to system

6. 6 Contents Basic concepts –Shared object –Software release consistency –Multiple consistency protocols Software implementation –Prototype overview –Execution process –Advanced programming features –Data object directory and delayed update queue –Synchronization Performance Overview of other DSM systems Conclusion

7. 7 Basic Concepts Basic concepts –Shared object –Software release consistency –Multiple consistency protocols Software implementation –Prototype overview –Execution process –Advanced programming features –Data object directory and delayed update queue –Synchronization Performance Overview of other DSM systems Conclusion

8. 8 Shared Object x y x x 8-kilo

9. 9 Software Release Consistency Sequential Consistency –All processors observe the same order –Must correspond to some serial order –Only ordering constraint is that reads/writes of P1 appear in the same order, but no restrictions on relative ordering between processors. Synchronous read/write –Writes must be propagated before moving on to the next operation

10. 10 Software consistency Problems –Message passing overhead –False sharing w(x) r(y) r(x) w(x)

11. 11 Weak Consistency Data modifications only propagated at synchronization. Works fine if program properly synchronized through system primitives. w(x) r(y) r(x) synch w(x)

12. 12 Weak Consistency w(x) r(y) r(x) synch

13. 13 Software Release Consistency Special weak consistency protocol Reduction of message passing overhead Two categories of shared variable operations –Ordinary access Read Write –Synchronization access (lock, semaphore, barrier) Acquire Release

14. 14 Software Release Consistency Before ordinary access (read, write) allowed, all previous acquire performed Before release allowed, all previous ordinary access performed Before acquire allowed, all previous release performed Before release allowed, all previous acquire performed In a word, results of writes prior to a release propagated before next processor acquiring this released lock

15. 15 Eager Release Consistency Write propagating at release

16. 16 Lazy Release Consistency Write propagating at acquire

17. 17 Multiple Consistency Protocols No single consistency protocol suitable for all parallelization purpose Shared variables accessed in different ways within single program Variable access pattern changes during execution Multiple protocols allow access pattern-oriented tuning for different shared variables

18. 18 Multiple Consistency Protocols High-level sharing pattern annotation –Specified in shared variable declaration –Combinations of low-level protocol parameters Low-level protocol parameter –Specified in shared variable directory –Specific aspect of protocol

19. 19 Protocol Parameters I:invalidate or update? R:Replicas allowed? D:Delayed operation allowed? FO:Having fixed owner? M:Multiple writers allowed? S:Stable access pattern? FL:Flushing changes to owner? W:Writable? (write protected?)

20. 20 Sharing annotations Read only –Simplest pattern: once initialized, no further access –Suitable for constant etc. Migratory –Only one thread can access at one period of time –Suitable for variables accessed only in critical session Write-shared –Can be written concurrently by multiple threads –Different threads update different words of variable Producer-consumer –Written only by one threads and read by others –Replicate and update the object, not invalidate

21. 21 Sharing annotations Example: producer-consumer for some number of timesteps/iterations { for (i=0; i<n; i++ ) for( j=1, j<n, j++ ) temp[i][j] = 0.25 * ( grid[i-1][j] + grid[i+1][j] grid[i][j-1] + grid[i][j+1] ); for( i=0; i<n; i++ ) for( j=1; j<n; j++ ) grid[i][j] = temp[i][j]; } back

22. 22 Sharing annotations Reduction –Accessed by fetching and operation (read, write then release) –Example: min(), a++ Result –Phase 1: multiple write allowed –Phase 2: one thread (the result) access exclusively Conventional –Conventional update protocol for shared variables

23. 23 Sharing annotations Sharing Annotations Protocol Parameters IRDFOMSFLW Read-onlyNY-----N MigratoryYN-NN-NY Write-sharedNYYNYNNY Producer- Consumer NYYNYYNY ReductionNYNYN-NY ResultNYYYY-YY ConventionalYYNNN-NY

24. 24 Software Implementation Basic concepts –Shared object –Software release consistency –Multiple consistency protocols Software implementation –Prototype overview –Execution process –Advanced programming features –Data object directory and delayed update queue –Synchronization Performance Overview of other DSM systems Conclusion

25. 25 Prototype Overview A simple processor converting annotations to suitable format A linker creating the shared memory segment Library routines linked into program Operating system support for fault handling and page table manipulation

26. 26 Execution Process Compiling Sharing annotations Munin processor Auxiliary file Linker Shared data segment Shared data description table

27. 27 Execution Process Initialization P1 P2 Pn.... Munin root thread Munin worker thread User_init() Code copy Data segment Code copy Data segment user root thread

28. 28 Execution Process Synchronization P1 P2 Pn.... Munin root thread Munin worker thread Synchronization operation User thread

29. 29 Advanced Programming Features Associate data & Synch backback msg acq(m) r(x) rel(m) msg acq(m) r(x) rel(m) w(x)

30. 30 Advanced Programming Features PhaseChange() –Change the producer consumer relationship –Example: adaptive mesh sorsor ChangeAnnotation() –Change the access pattern in execution Invalidate() Flush() SingleObject() PreAcquire()

31. 31 Data Object Directory Start Address and Size Protocol parameters Object state (valid, writable, invalid) Copyset (which remote has copies) Synchq (corresponding synchronization object)Synchq Probable owner Home node Access control semaphore Links

32. 32 Delayed Update Queue acq(m) w(x) w(y) rel(m) x x y

33. 33 Multiple Writer Handling

34. 34 Multiple Writer Handling

35. 35 Synchronization Queue based synchronization Request – reply – lock forward mechanism AcquireLock(), Unlock(), WaitAtBarrier()

36. 36 Performance Basic concepts –Shared object –Software release consistency –Multiple consistency protocols Software implementation –Prototype overview –Execution process –Advanced programming features –Data object directory and delayed update queue –Synchronization Performance Overview of other DSM systems Conclusion

37. 37 Matrix Multiply

38. 38 Matrix Multiply Optimized

39. 39 SOR

40. 40 Effect of Multiple Protocols ProtocolMatrix MultiplySOR Multiple72.4127.64 Write-shared75.5964.48 Conventional75.8567.64

41. 41 Overview of Other DSM System Basic concepts –Shared object –Software release consistency –Multiple consistency protocols Software implementation –Prototype overview –Execution process –Advanced programming features –Data object directory and delayed update queue –Synchronization Performance Overview of other DSM systems Conclusion

42. 42 Overview of Other DSM System Clouds:per-segment (object) based consistency protocol Mirage: per-page based Orca: reliable ordered broadcast protocol Amber:user responsible for the data distribution among processors Linda:shared variable in tuple space, atomic operation: insertion, removal, reading Midway:using entry consistency (weaker consistency than release consistency) DASH:hardware DSM

43. 43 Conclusion Objective: efficient DSM system with similar protocol to shared memory programming and small message passing overhead Special feature: multiple protocols, software release consistency Implementation: synchronization realized by Munin root thread and Munin worker threads

44. 44 Thank you