Developing parallel Computational Science and Engineering (CSE)applications is a complex task. One has to implement the right physics,develop or choose and code appropriate numerical methods, decide andimplement the proper input and output data formats, performvisualizations, and be concerned with correctness and efficiency of theprograms. It becomes even more complex for multi-physics coupledsimulations, many of which are dynamic and adaptively refined so thatload imbalance becomes a major challenge. In addition to imbalancecaused by dynamic program behavior, hardware factors such as latencies,variability, and failures must be tolerated by applications. Ourphilosophy is to lessen the burden of application developers byproviding advanced programming paradigms and versatile runtime systemsthat can handle many common programming and performance concernsautomatically and let application programmers focus on the actualapplication content.
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Charm++, which we use as a runtime system layer for the work describedhere, simplifies our approach. It embeds an elaborate performancetracing mechanism, a suite of plug-in load balancing strategies,infrastructure for defining and migrating computational load, and isinteroperable with other programming paradigms.
Since many CSE applications are originally written using MPI, one wouldhave to rewrite existing code if they were to be converted to Charm++ totake advantage of dynamic load balancing and other Charm++ features.This is indeed impractical. However, Converse - the runtime system ofCharm++ - supports interoperability between different parallelprogramming paradigms such as parallel objects and threads. Using thisfeature, we developed AMPI, which is described in more detail in thenext section.
AMPI provides a set of built-in attributes on all communicators andwindows to find the number of the worker thread, process, or host that arank is currently running on, as well as the total number of workerthreads, processes, and hosts in the job. We define a worker thread tobe a thread on which one of more AMPI ranks are scheduled. We define aprocess here as an operating system process, which may contain one ormore worker threads. The built-in attributes are AMPI_MY_WTH,AMPI_MY_PROCESS, AMPI_NUM_WTHS, and AMPI_NUM_PROCESSES.These attributes are accessible from any rank by callingMPI_Comm_get_attr, such as:
No further effort is needed. Global variables in example.cpp will beautomatically privatized when the program is run. Any libraries andshared objects compiled as PIE will also be privatized. However, ifthese objects call MPI functions, it will be necessary to build themwith the AMPI toolchain wrappers, -pipglobals, and potentially alsothe -standalone parameter in the case of shared objects. It isrecommended to do this in any case so that AMPI can ensure everything isbuilt as PIE.
There are several things to note in this example. First, the samesubroutine pup (declared in module pupmod) is called tosize/pack/unpack any type of data. This is possible because of procedureoverloading possible in Fortran90. Second is the integer argument p.It is this argument that specifies whether this invocation of subroutinechunkpup is sizing, packing or unpacking. Third, the integerparameters declared in the type chunk need not be packed or unpackedsince they are guaranteed to be constants and thus available on anyprocessor.
Interoperability between different modules is essential for codingcoupled simulations. In this extension to AMPI, each MPI applicationmodule runs within its own group of user-level threads distributed overthe physical parallel machine. In order to let AMPI know which ranks areto be created, and in what order, a top level registration routine needsto be written. A real-world example will make this clear. We have an MPIcode for fluids and another MPI code for solids, both with their mainprograms, then we first transform each individual code to run correctlyunder AMPI as standalone codes. Aside from the global and staticvariable privatization transformations needed, this also involves makingthe main program into a subroutine and naming it MPI_Main.
The feature is used in two phases: writing (logging) the environment andrepeating the run. The first logging phase is invoked by a parallel runof the AMPI program with some additional command line options.
The Charm++ distribution contains a script called charmrun thatmakes the job of running AMPI programs portable and easier across allparallel machines supported by Charm++. charmrun is copied to adirectory where an AMPI program is built using ampicc. It takes acommand line parameter specifying number of processors, and the name ofthe program followed by AMPI options (such as number of ranks to create,and the stack size of every user-level thread) and the programarguments. A typical invocation of an AMPI program pgm withcharmrun is:
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