Advanced Charm++ Tutorial

1. Advanced Charm++ Tutorial Charm Workshop Tutorial Sameer Kumar Orion Sky Lawlor charm.cs.uiuc.edu 2004/10/19

2. How to Become a Charm++ Hacker • Advanced Charm++ • Advanced Messaging • Writing system libraries • Groups • Delegation • Communication framework • Advanced load-balancing • Checkpointing • Threads • SDAG

3. Advanced Messaging

4. Prioritized Execution • If several messages available, Charm will process the message with highest priority • Otherwise, oldest message (FIFO) • Has no effect: • If only one message is available (common for network-bound applications!) • On outgoing messages • Very useful for speculative work, ordering timesteps, etc...

5. Priority Classes • Charm++ scheduler has three queues: high, default, and low • As signed integer priorities: • -MAXINT Highest priority -- -1 • 0 Default priority • 1 -- +MAXINT Lowest priority • As unsigned bitvector priorities: • 0x0000 Highest priority -- 0x7FFF • 0x8000 Default priority • 0x8001 -- 0xFFFF Lowest priority

6. Prioritized Marshalled Messages • Pass “CkEntryOptions” as last parameter • For signed integer priorities: CkEntryOptions opts; opts.setPriority(-1); fooProxy.bar(x,y,opts); • For bitvector priorities: CkEntryOptions opts; unsigned int prio[2]={0x7FFFFFFF,0xFFFFFFFF}; opts.setPriority(64,prio); fooProxy.bar(x,y,opts);

7. Prioritized Messages • Number of priority bits passed during message allocation FooMsg * msg = new (size, nbits) FooMsg; • Priorities stored at the end of messages • Signed integer priorities: *CkPriorityPtr(msg)=-1; CkSetQueueing(m, CK_QUEUEING_IFIFO); • Unsigned bitvector priorities CkPriorityPtr(msg)[0]=0x7fffffff; CkSetQueueing(m, CK_QUEUEING_BFIFO);

8. Advanced Message Features • Read-only messages • Entry method agrees not to modify or delete the message • Avoids message copy for broadcasts, saving time • Expedited messages • Message do not go through the charm++ scheduler (faster) • Immediate messages • Entries are executed in a interrupt or the communication thread • Very fast, but tough to get right

9. Read-Only, Expedited, Immediate • All declared in the .ci file { .. ... entry [nokeep] void foo_readonly(Msg *); entry [expedited] void foo_exp(Msg *); entry [immediate] void foo_imm(Msg *); .. .. .. }; // Immediate messages only currently work //for NodeGroups

10. Groups

11. Object Groups • A collection of objects (chares) • Also called branch office chares • Exactly one representative on each processor • Ideally suited for system libraries • A single proxy for the group as a whole • Similar to arrays: • Broadcasts, reductions, indexing • But not completely like arrays: • Non-migratable; one per processor

12. Declarations • .ci file group mygroup { entry mygroup(); //Constructor entry void foo(foomsg *); //Entry method }; • C++ file class mygroup : public Group { mygroup() {} void foo(foomsg *m) { CkPrintf(“Do Nothing”);} };

13. Creating and Calling Groups • Creation p = CProxy_mygroup::ckNew(); • Remote invocation p.foo(msg); //broadcast p[1].foo(msg); //asynchronous invocation • Direct local access mygroup *g=p.ckLocalBranch(); g->foo(….); //local invocation • Danger: if you migrate, the group stays behind!

14. Delegation

15. Delegation • Enables Charm++ proxy messages to be forwarded to a delegation manager group • Delegation manager can trap calls to proxy sends and apply optimizations • Delegation manager must inherit from CkDelegateMgr • User program must to call • proxy.ckDelegate(mgrID);

16. Delegation Interface • .ci file group MyDelegateMgr { entry MyDelegateMgr(); //Constructor }; • .h file class MyDelegateMgr : public CkDelegateMgr { MyDelegateMgr(); void ArraySend(...,int ep,void *m,const CkArrayIndexMax &idx,CkArrayID a); void ArrayBroadcast(..); void ArraySectionSend(.., CkSectionID &s); …………….. …………….. }

17. Communication Optimization

18. Automatic Communication Optimizations • The parallel-objects Runtime System can observe, instrument, and measure communication patterns • Communication libraries can optimize • By substituting most suitable algorithm for each operation • Learning at runtime • E.g. All to all communication • Performance depends on many runtime characteristics • Library switches between different algorithms • Communication is from/to objects, not processors • Streaming messages optimization

19. Managing Collective Communication • Communication operation where all (or most) the processors participate • For example broadcast, barrier, all reduce, all to all communication etc • Applications: NAMD multicast, NAMD PME, CPAIMD • Issues • Performance impediment • Naïve implementations often do not scale • Synchronous implementations do not utilize the co-processor effectively

20. All to All Communication • All processors send data to all other processors • All to all personalized communication (AAPC) • MPI_Alltoall • All to all multicast/broadcast (AAMC) • MPI_Allgather

21. Strategies For AAPC • Short message optimizations • High software over head (α) • Message combining • Large messages • Network contention

22. Short Message Optimizations • Direct all to all communication is α dominated • Message combining for small messages • Reduce the total number of messages • Multistage algorithm to send messages along a virtual topology • Group of messages combined and sent to an intermediate processor which then forwards them to their final destinations • AAPC strategy may send same message multiple times

23. Virtual Topology: Mesh Phase 1: Processors send messages to row neighbors Phase 2: Processors send messages to column neighbors 2* messages instead of P-1 Organizeprocessors in a 2D (virtual) Mesh Message from (x1,y1) to (x2,y2) goes via (x1,y2)

24. AAPC Performance

25. Large Message Issues • Network contention • Contention free schedules • Topology specific optimizations

26. Ring Strategy for Collective Multicast …… …….. 0 1 2 i i+1 P-1 • Performs all to all multicast by sending messages along a ring formed by the processors • Congestion free on most topologies

27. Streaming Messages • Programs often have streams of short messages • Streaming library combines a bunch of messages and sends them off • Stripping large charm++ header • Short array message packing • Effective message performance of about 3us

28. Using communication library • Communication optimizations embodied as strategies • EachToManyMulticastStrategy • RingMulticast • PipeBroadcast • Streaming • MeshStreaming

29. Bracketed vs. Non-bracketed • Bracketed Strategies • Require user to give specific end points for each iteration of message sends • Endpoints declared by calling ComlibBegin() and ComlibEnd() • Examples: EachToManyMulticast • Non bracketed strategies • No such end points necessary • Examples: Streaming, PipeBroadcast

30. Accessing the Communication Library • From mainchare::main • Creating a strategy Strategy *strat = new EachToManyMulticastStrategy(USE_MESH) Strat = new StreamingStrategy(); Strat->enableShortMessagePacking(); • Associating a proxy with a Strategy ComlibAssociateProxy(strat, myproxy); • myproxy should be passed to all array elements

31. Sending Messages ComlibBegin(myproxy);//Bracketed Strategies for(.. ..) { .. .. myproxy.foo(msg); .. .. } ComlibEnd(); //Bracketed strategies

32. Handling Migration • Migrating array element PUP’s the comlib associated proxy FooArray::pup(PUP::er &p) { .. .. .. p | myProxy; .. .. .. }

33. Compiling • You must include compile time option –module commlib

34. Advanced Load-balancersWriting a Load-balancing Strategy

35. Advanced load balancing: Writing a new strategy • Inherit from CentralLB and implement the work(…) function class foolb : public CentralLB { public: .. .. .. void work (CentralLB::LDStats* stats, int count); .. .. .. };

36. LB Database struct LDStats { ProcStats *procs; LDObjData* objData; LDCommData* commData; int *to_proc; //.. .. .. } //Dummy Work function which assigns all objects to //processor 0 //Don’t implement it! void fooLB::work(CentralLB::LDStats* stats,int count){ for(int count=0;count < nobjs; count++) stats.to_proc[count] = 0; }

37. Compiling and Integration • Edit and run Makefile_lb.sh • Creates Make.lb which is included by the LDB Makefile • Run make depends to correct dependencies • Rebuild charm++

38. Checkpoint Restart

39. Checkpoint/Restart • Any long running application must be able to save its state • When you checkpoint an application, it uses the pup routine to store the state of all objects • State information is saved in a directory of your choosing • Restore also uses pup, so no additional application code is needed (pup is all you need)

40. Checkpointing Job • In AMPI, use MPI_Checkpoint(<dir>); • Collective call; returns when checkpoint is complete • In Charm++, use CkCheckpoint(<dir>,<resume>); • Called on one processor; calls resume when checkpoint is complete

41. Restart Job from Checkpoint • The charmrun option ++restart <dir> is used to restart • Number of processors need not be the same • You can also restart groups by marking them migratable and writing a PUP routine – they still will not load balance, though

42. Threads

43. Why use Threads? • They provide one key feature: blocking • Suspend execution (e.g., at message receive) • Do something else • Resume later (e.g., after message arrives) • Example: MPI_Recv, MPI_Wait semantics • Function call interface more convenient than message-passing • Regular call/return structure (no CkCallbacks) • Allows blocking in middle of deeply nested communication subroutine

44. Why not use Threads? • Slower • Around 1us context-switching overhead unavoidable • Creation/deletion perhaps 10us • More complexity, more bugs • Breaks a lot of machines! (but we have workarounds) • Migration more difficult • State of thread is scattered through stack, which is maintained by compiler • By contrast, state of object is maintained by users • Thread disadvantages form the motivation to use SDAG (later)

45. What are (Charm) Threads? • One flow of control (instruction stream) • Machine Registers & program counter • Execution stack • Like pthreads (kernel threads) • Only different: • Implemented at user level (in Converse) • Scheduled at user level; non-preemptive • Migratable between nodes

46. How do I use Threads? • Many options: • AMPI • Always uses threads via TCharm library • Charm++ • [threaded] entry methods run in a thread • [sync] methods • Converse • C routines CthCreate/CthSuspend/CthAwaken • Everything else is built on these • Implemented using • SYSV makecontext/setcontext • POSIX setjmp/alloca/longjmp

47. How do I use Threads (example) • Blocking API routine: find array element int requestFoo(int src) { myObject *obj=...; return obj->fooRequest(src) } • Send request and suspend int myObject::fooRequest(int src) { proxy[dest].fooNetworkRequest(thisIndex); stashed_thread=CthSelf(); CthSuspend(); // -- blocks until awaken call -- return stashed_return; } • Awaken thread when data arrives void myObject::fooNetworkResponse(int ret) { stashed_return=ret; CthAwaken(stashed_thread); }

48. How do I use Threads (example) • Send request, suspend, recv, awaken, return int myObject::fooRequest(int src) { proxy[dest].fooNetworkRequest(thisIndex); stashed_thread=CthSelf(); CthSuspend(); return stashed_return; } void myObject::fooNetworkResponse(int ret) { stashed_return=ret; CthAwaken(stashed_thread); }

49. The Horror of Thread Migration

50. Stack Data • The stack is used by the compiler to track function calls and provide temporary storage • Local Variables • Subroutine Parameters • C “alloca” storage • Most of the variables in a typical application are stack data • Users have no control over how stack is laid out