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Methods of a chare that can be remotely invoked are called entrymethods. Entry methods may take serializable parameters, or a pointer toa message object. Since chares can be created on remote processors,obviously some constructor of a chare needs to be an entry method.Ordinary entry methods 1 are completely non-preemptive- Charm++ willnot interrupt an executing method to start any other work, and all callsmade are asynchronous.

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Charm++ program execution is terminated by the CkExit call. Like theexit system call, CkExit never returns, and it optionally accepts aninteger value to specify the exit code that is returned to the callingshell. If no exit code is specified, a value of zero (indicatingsuccessful execution) is returned. The Charm RTS ensures that no moremessages are processed and no entry methods are called after a CkExit.CkExit need not be called on all processors; it is enough to call itfrom just one processor at the end of the computation.

The top-level construct in a ci file is a named container forinterface declarations called a module. Modules allow relateddeclarations to be grouped together, and cause generated code for thesedeclarations to be grouped into files named after the module. Modulescannot be nested, but each ci file can have several modules. Modulesare specified using the keyword module. A module name must be a validC++ identifier.

The extern keyword places an include statement for the decl.h file ofthe extern ed module in the generated code of the current module. Hence,decl.h files generated from extern ed modules are required during thecompilation of the source code for the current module. This is usuallyrequired anyway because of the dependencies between user programentities across the two modules.

The Charm++ framework implements its own main function andretains control until the parallel execution environment is initializedand ready for executing user code. Hence, the user program must notdefine a main() function. Control enters the user code via themainchare of the mainmodule. This will be discussed in further detailin 2.1.2.

Charm++ provides a compiler-wrapper called charmc that handles all ci,C, C++ and Fortran source files that are part of a user program. Users caninvoke charmc to parse their interface descriptions, compile source codeand link objects into binaries. It also links against the appropriateset of charm framework objects and libraries while producing a binary.charmc and its functionality is described in 2.6.2.

void CkExit()This call informs the Charm RTS that computation on allprocessors should terminate. This routine never returns, so any codeafter the call to CkExit() inside the function that calls it will notexecute. Other processors will continue executing until they receivenotification to stop, so it is a good idea to ensure throughsynchronization that all useful work has finished before callingCkExit().

Typically the client entry method of a reduction takes a single argumentof type CkReductionMsg (see Section, by giving an entry method the reductiontarget attribute in file, you can instead use entry methods that take arguments ofthe same type as specified by the contribute call. When creating acallback to the reduction target, the entry method index is generated byCkReductionTarget(ChareClass, method_name) instead ofCkIndex_ChareClass::method_name(...). For example, the code for atyped reduction that yields an int, would look like this:

The reduction set order is undefined. You should add a source field tothe contributed elements if you need to know which array element gave aparticular contribution. Additionally, if the contributed elements areof a complex data type, you will likely have to supply code forserializing/deserializing them. Consider using the PUP interface(Section 2.2.5) to simplify your object serializationneeds.

In each step, this object expects pairs of messages, and waits toprocess the incoming data until it has both of them. This sequencing isencoded across 4 different functions, which in real code could be muchlarger and more numerous, resulting in a spaghetti-code mess.

The most basic construct in SDAG is the serial block (previously alsodenoted by atomic, this usage is now deprecated). Serial blocks containsequential C++ code, and the code within them executes to completion withoutreturning control to the Charm++ runtime scheduler, thus avoiding interruptionfrom incoming messages. Typically, serial blocks hold the code that actuallydeals with incoming messages in a when statement or performs localoperations before a message is sent or after it is received. The earlier examplecan be adapted to use serial blocks as follows:

Note that chare classes containing SDAG code must include a fewadditional declarations in addition to inheriting from theirCBase_Foo class, by incorporating the Foo_SDAG_CODEgenerated-code macro in the class.

Entry methods defined by a when are not executed immediately when amessage targeting them is delivered, but instead are held until controlflow in the chare reaches a corresponding when clause. Conversely,when control flow reaches a when clause, the generated code checkswhether a corresponding message has arrived: if one has arrived, it isprocessed; otherwise, control is returned to the Charm++ scheduler.

Like an if or while in C code, each when clause has a bodymade up of the statement or block following it. The variables declaredas arguments to the entry method triggering the when are available inthe scope of the body. By using the sequenced execution of SDAG code andthe availability of parameters to when-defined entry methods in theirbodies, the counter expectedMessageCount and the intermediate copiesof the received input are eliminated. Note that the entry methodsfirstInput and secondInput are still declared in the .cifile, but their definition is in the SDAG code. The interface translatorgenerates code to handle buffering and triggering them appropriately.

SDAG supports the for and while loop constructs mostly as ifthey appeared in plain C or C++ code. In the running example,computeInteractions() calls startStep() when it is finished tostart the next step. Instead of this arrangement, the loop structure canbe made explicit:

Another construct offered by SDAG is the forall loop. These loopsare used when the iterations of a loop can be performed independentlyand in any order. This is in contrast to a regular for loop, inwhich each iteration is executed sequentially. The loop iterations areexecuted entirely on the calling PE, so they do not run in parallel.However, they are executed concurrently, in that execution of differentiterations can interleave at when statements, like any other SDAGcode. SDAG statements following the forall loop will not executeuntil all iterations have completed. The forall loop can be seen asan overlap with an indexed set of otherwise identical statements inthe body.

The range from MIN to MAX is inclusive. In each iteration instance ofBODY, the IDENT variable will take on one of the values in thespecified range. The IDENT variable must be declared in theapplication C++ code as a member of the enclosing chare class.

The PUP (Pack/Unpack) framework is a generic way to describe the data inan object and to use that description for serialization. TheCharm++ system can use this description to pack the object into amessage and unpack the message into a new object on another processor,to pack and unpack migratable objects for load balancing orcheckpoint/restart-based fault tolerance. The PUP framework alsoincludes support special for STL containers to ease development in C++.

Array objects can migrate from one PE to another. For example, the loadbalancer (see section might migrate arrayelements to better balance the load between processors. For an arrayelement to be migratable, it must implement a pup method. The standardPUP contract (see section and constraintswrt to serializing data apply. The one exception for chare , group andnode group types is that since the runtime system will be the one toinvoke their PUP routines, the runtime will automatically call PUP onthe generated CBase_ superclasses so users do not need to call PUP ongenerated superclasses.

Parameter marshalling requires serialization and is thereforeimplemented using the PUP framework. User defined data types passed asparameters must abide by the standard PUP contract (see section2.2.5.1).

There are two different ways to use load balancing for chare arrays tomeet different needs of the applications. These methods are differentin how and when a load balancing phase starts. The two methods are:AtSync mode and periodic load balancing mode. In AtSyncmode, the application invokes the load balancer explicitly at anappropriate location (generally at a pre-existing synchronizationboundary) to trigger load balancing by inserting a function call(AtSync()) in the application source code. In periodic loadbalancing mode, a user specifies only how often load balancing is tooccur, using the +LBPeriod runtime parameter or theLBManager::SetLBPeriod(double period) call to specify the timeinterval.

A CkGroupID is useful to specify dependence in group creations usingCkEntryOptions. For example, in the following code, the creation ofgroup GroupB on each PE depends on the creation of GroupA onthat PE.

This code will execute the routines fooNodeInit and staticbar::barNodeInit once on every logical node and fooProcInit andbar::barProcInit on every PE before the main computation starts.Initnode calls are always executed before initproc calls. Both initcalls (declared as static member functions) can be used in chares, charearrays and groups.

Managing the memory buffer associated with a message. As suggestedin the example above, the biggest difference between marshalledparameters and messages is that an entry method invocation is assumed tokeep the message that it is passed. That is, the Charm++ runtimesystem assumes that code in the body of the invoked entry method willexplicitly manage the memory associated with the message that it ispassed. Therefore, in order to avoid leaking memory, the body of anentry method must either delete the message that it is receives, or savea pointer to it, and delete it a later point in the execution of thecode.


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