7 Using Load Balancing Module

7.1 Coding

To activate load balancing module and prepare objects for migration, there are 3 things that needs to be added in Charisma code.

First, the programmer needs to inform Charisma about the load balancing with a "define ldb;" statement in the header section of the orchestration code. This will make Charisma generates extra Charm++ code to do load balancing such as PUP methods.

Second, the user has to provide a PUP function for each class with sequential data that needs to be moved when the object migrates. When choosing which data items to pup, the user has the flexibility to leave the dead data behind to save on communication overhead in migration. The syntax for the sequential PUP is similar to that in a Charm++ program. Please refer to the load balancing section in Charm++ manual for more information on PUP functions. A typical example would look like this in user's sequential .C file:

{foodecl}
  void JacobiWorker::sequentialPup(PUP::er& p){
    p|myLeft; p|myRight; p|myUpper; p|myLower;
    p|myIter;
    PUParray(p,(double *)localData,1000);
  }

Thirdly, the user will make the call to invoke load balancing session in the orchestration code. The call is AtSync(); and it is invoked on all elements in an object array. The following example shows how to invoke load balancing session every 4th iteration in a for-loop.

{foodecl}
  for iter = 1 to 100
    // work work
    if(iter % 4 == 0) then
      foreach i in workers
        workers[i].AtSync();
      end-foreach
    end-if
  end-for

If a while-loop is used instead of for-loop, then the test-condition in the if statement is a local variable in the program's MainChare. In the sequential code, the user can maintain a local variable called iter in MainChare and increment it every iteration.

7.2 Compiling and Running

Unless linked with load balancer modules, a Charisma program will not perform actual load balancing. The way to link in a load balancer module is adding -module EveryLB as a link-time option.

At run-time, the load balancer is specified in command line after the +balancer option. If the balancer name is incorrect, the job launcher will automatically print out all available load balancers. For instance, the following command uses RotateLB.

{foodecl}
    > ./charmrun ./pgm +p16 +balancer RotateLB