Lecture 28: MPI Parallel Programming: MPI Collectives. Overview of topics covered in the class.
Lecture Summary
Last time
Wrap up p2p communication
Collective communication: Synchronizations, communications, operations
Today
Collective communication: Operations, data types
Talk about the final exam. Past exam
MPI Derived Types (did not have time to cover in class)
Previously we sent/received a contiguous buffer of identical elements of predefined data types
Now we want to send non-homogenous elements (structure) or chunks that are not contiguous in memory
MPI Datatypes
Primitive datatypes: MPI_CHAR, MPI_FLOAT, MPI_INTEGER, etc.
Derived datatypes: Can be constructed by four methods (contiguous, vector, indexed, struct)
Typemaps
Used to describe an MPI derived type
Specifies a sequence of primitive data types, and a sequence of integers that represent the byte displacements, measured from the beginning of the buffer
Typemap = {(type0, disp0), ..., (typen, dispn)}
Extent: distance, in bytes, from beginning to end of type
E.g., {(double,0),(char,8)} has extent 16
Type signature: The sequence of primitive data types
E.g., for a data type with typemap {(double,0), (int,8), (char, 12)}, its signature is {double, int, char}
Takeaways from ME 759
Know your hardware
Moving data around is expensive in energy and time
Seek solution approaches that expose concurrency/parallelism in your problem
Use the tools of the trade to work like a pro and then get paid like a pro
Debuggers, profilers, CMake, memory checkers, compiler flags, git, etc.
Do HPC
This has been such a fun ride.
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