Lecture 15: Charm++ Abhinav Bhatele, Department of Computer Science - - PowerPoint PPT Presentation

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Lecture 15: Charm++ Abhinav Bhatele, Department of Computer Science - - PowerPoint PPT Presentation

Feb 23, 2023 638 likes •792 views

Introduction to Parallel Computing (CMSC498X / CMSC818X) Lecture 15: Charm++ Abhinav Bhatele, Department of Computer Science Task-based programming models Describe program / computation in terms of tasks Tasks might be short-lived or

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Lecture 15: Charm++

Abhinav Bhatele, Department of Computer Science

Introduction to Parallel Computing (CMSC498X / CMSC818X)

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Task-based programming models

  • Describe program / computation in terms of tasks
  • Tasks might be short-lived or persistent throughout program execution
  • Notable examples: Charm++, StarPU, HPX, Legion
  • Attempt at classification: https://link.springer.com/article/10.1007/s11227-018-2238-4

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Charm++: Key principles

  • Programmer decomposes data and work into objects (called chares)
  • Decoupled from number of processes or cores
  • Runtime assigns objects to physical resources (cores and nodes)
  • Each object can only access its own data
  • Request data from other objects via remote method invocation: foo.get_data()
  • Asynchronous message-driven execution

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Hello World in Charm++

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Charm++ Tutorial: http://charmplusplus.org/tutorial/ArrayHelloWorld.html

mainmodule hello { array [1D] Hello { entry Hello(); entry void sayHi(); }; };

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Hello World in Charm++

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Charm++ Tutorial: http://charmplusplus.org/tutorial/ArrayHelloWorld.html

mainmodule hello { array [1D] Hello { entry Hello(); entry void sayHi(); }; }; void Hello ::sayHi() { CkPrintf("Hello from chare %d on processor %d.\n”, thisIndex, CkMyPe()); }

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Hello World in Charm++

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Charm++ Tutorial: http://charmplusplus.org/tutorial/ArrayHelloWorld.html

mainmodule hello { array [1D] Hello { entry Hello(); entry void sayHi(); }; }; void Hello ::sayHi() { CkPrintf("Hello from chare %d on processor %d.\n”, thisIndex, CkMyPe()); } Main::Main(CkArgMsg* msg) { numObjects = 5; // number of objects CProxy_Hello helloArray = CProxy_Hello::ckNew(numObjects); helloArray.sayHi(); }

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Compiling a charm program

  • Charm translator for .ci file
  • Generates charm_hello.decl.h and charm_hello.def.h
  • C++ code:

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charmc hello.ci charmc -c hello.C charmc -o hello hello.o

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Chare arrays

  • User can create indexed collection of data-driven objects
  • Different kinds: 1D, 2D, 3D, …
  • Mapping of array elements (objects) to hardware resources handled by the runtime

system (RTS)

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CProxy_Hello helloArray = CProxy_Hello::ckNew(numElements);

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Object-based virtualization

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  • User programs in terms of chares or objects

User View System View

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Over-decomposition

  • Create lots of “small” objects per physical core
  • Objects grouped into arrays: 1D, 2D, …
  • System assigns objects to processors and can migrate objects between physical

resources

  • Facilitates automatic load balancing

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Message-driven execution

  • An object is scheduled by the runtime scheduler only when a message for it is

received

  • Facilitates adaptive overlap of computation and communication

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Abhinav Bhatele (CMSC498X/CMSC818X) LIVE RECORDING

Cost of creating more objects?

  • Context switch overhead
  • Cache performance
  • Memory overhead
  • Fine-grained messages

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Abhinav Bhatele 5218 Brendan Iribe Center (IRB) / College Park, MD 20742 phone: 301.405.4507 / e-mail: bhatele@cs.umd.edu