<graphviz dot>
digraph swarm {
  start [shape=ellipse, label="run_swarm()"];
  split_comm[shape=box, label="Split MPI-communicator\ninto a master rank\nand multiple worker groups."];
  if_master [shape=diamond, label="Master rank?"];
  start -> split_comm -> if_master;

  node [shape=box, width=3.5];  
  if_master -> m1 [label="yes"];
  if_master -> w1 [label="no"];

subgraph cluster_m {
  color = blue;
  label = <<u>Master</u>>;
  m1[label="Receive report (blocking)"];
  m2[label="CALL steer(report, command)"];
  m3[label="Send new command as reply"];
  m1 -> m2 -> m3;
  m3:w -> m1:w [weight=0];
}

subgraph cluster_w {
  label = <<u>Worker</u>>;
  color = blue;
  w1[label="Send report"];
  w2[label="CALL execute(command, report)"];
  w3[label="Receive command (blocking)"];
  w1 -> w2 -> w3 [dir=back];
  w3:e -> w1:e [dir=back, weight=0];
}

w1 -> m1 [label="MPI", color=red,   minlen=3.5, weight=0];
m3 -> w3 [label="MPI", color=green, minlen=3.5, weight=0];

}
</graphviz>