digraph {
    compound=true;
    subgraph cluster_00 {
      label="";
      AA [label="Allocate A"];
    }

    subgraph cluster_01 {
      label="";
      CA [label="Copy A H->D"];
    }

    subgraph cluster_10 {
      label="";
      AB [label="Allocate B"];
    }

    subgraph cluster_11 {
      label="";
      CB [label="Copy B H->D"];
    }

    subgraph cluster_0 {
      label="T1";
      K1 [label="K1"];
      K2 [label="K2"];
    }

    CA -> K1 [ltail=cluster_01,lhead=cluster_0,minlen=2];
    AA -> CA [ltail=cluster_00,lhead=cluster_01,minlen=2];

    CB -> K1 [ltail=cluster_11,lhead=cluster_0,minlen=2];
    AB -> CB [ltail=cluster_10,lhead=cluster_11,minlen=2];

    subgraph cluster_1 {
      label="T2";
      K3 [label="K3"];
    }
    subgraph cluster_2 {
      label="T3";
      K4 [label="K4"];
    }

    K1 -> K2;
    K2 -> K3 [ltail=cluster_0,lhead=cluster_1,minlen=2];
    K2 -> K4 [ltail=cluster_0,lhead=cluster_2,minlen=2];

    subgraph cluster_02 {
      label="";
      CA2 [label="Copy A D->A"];
    }

    subgraph cluster_12 {
      label="";
      CB2 [label="Copy B D->A"];
    }

    subgraph cluster_3 {
      label="T4";
      cb [label="callback"];
    }

    K3 -> CA2 [ltail=cluster_1,lhead=cluster_02,minlen=2];
    K4 -> CB2 [ltail=cluster_2,lhead=cluster_12,minlen=2];

    CA2 -> cb [ltail=cluster_02,lhead=cluster_3,minlen=2]
    CB2 -> cb [ltail=cluster_12,lhead=cluster_3,minlen=2]
}