SubflowTasking Create a Subflow SubflowTasking_1CreateASubflow Join a Subflow SubflowTasking_1JoinASubflow Detach a Subflow SubflowTasking_1DetachASubflow Create a Nested Subflow SubflowTasking_1CreateANestedSubflow It is very common for a parallel program to spawn task dependency graphs at runtime. In Taskflow, we call this subflow tasking. Subflow tasks are those created during the execution of a graph. These tasks are spawned from a parent task and are grouped together to a subflow dependency graph. To create a subflow, emplace a callable that takes an argument of type tf::Subflow. A tf::Subflow object will be created and forwarded to the execution context of the task. All methods you find in tf::Taskflow are applicable for tf::Subflow. 1:tf::Taskflowtaskflow; 2:tf::Executorexecutor; 3: 4:tf::TaskA=taskflow.emplace([](){}).name("A");//statictaskA 5:tf::TaskC=taskflow.emplace([](){}).name("C");//statictaskC 6:tf::TaskD=taskflow.emplace([](){}).name("D");//statictaskD 7: 8:tf::TaskB=taskflow.emplace([](tf::Subflow&subflow){ 9:tf::TaskB1=subflow.emplace([](){}).name("B1");//subflowtaskB1 10:tf::TaskB2=subflow.emplace([](){}).name("B2");//subflowtaskB2 11:tf::TaskB3=subflow.emplace([](){}).name("B3");//subflowtaskB3 12:B1.precede(B3);//B1runsbeforeB3 13:B2.precede(B3);//B2runsbeforeB3 14:}).name("B"); 15: 16:A.precede(B);//BrunsafterA 17:A.precede(C);//CrunsafterA 18:B.precede(D);//DrunsafterB 19:C.precede(D);//DrunsafterC 20: 21:executor.run(taskflow).get();//executethegraphtospawnthesubflow 22:taskflow.dump(std::cout);//dumpthetaskflowtoaDOTformat Debrief: Lines 1-2 create a taskflow and an executor Lines 4-6 create three tasks, A, C, and D Lines 8-14 create a task B that spawns a task dependency graph of three tasks B1, B2, and B3 Lines 16-19 add dependencies among A, B, C, and D Line 21 submits the graph to an executor and waits until it finishes Line 22 dumps the entire task dependency graph Lines 8-14 are the main block to enable subflow tasking at task B. The runtime will create a tf::Subflow passing it to task B, and spawn a dependency graph as described by the associated callable. This new subflow graph will be added to the topology of its parent task B. Due to the property of subflow tasking, we cannot dump its structure before execution. We will need to run the graph first to spawn the graph and then call tf::Taskflow::dump. By default, a subflow joins its parent task when the program leaves its execution context. All nodes of zero outgoing edges in the subflow precede its parent task. You can explicitly join a subflow within its execution context to carry out recursive patterns. A famous implementation is fibonacci recursion. intspawn(intn,tf::Subflow&sbf){ if(n<2)returnn; intres1,res2; sbf.emplace([&res1,n](tf::Subflow&sbf){res1=spawn(n-1,sbf);}); sbf.emplace([&res2,n](tf::Subflow&sbf){res2=spawn(n-2,sbf);}); sbf.join();//jointomaterializethesubflowimmediately returnres1+res2; } taskflow.emplace([&res](tf::Subflow&sbf){ res=spawn(5,sbf); }); executor.run(taskflow).wait(); The code above computes the fifth fibonacci number using recursive subflow. Calling tf::Subflow::join immediately materializes the subflow by executing all associated tasks to recursively compute fibonacci numbers. The taskflow graph is shown below: Our implementation to join subflows is recursive in order to preserve the thread context in each subflow task. Having a deep recursion of subflows may cause stack overflow. In contract to joined subflow, you can detach a subflow from its parent task, allowing its execution to flow independently. 1:tf::Taskflowtaskflow; 2: 3:tf::TaskA=taskflow.emplace([](){}).name("A");//statictaskA 4:tf::TaskC=taskflow.emplace([](){}).name("C");//statictaskC 5:tf::TaskD=taskflow.emplace([](){}).name("D");//statictaskD 6: 7:tf::TaskB=taskflow.emplace([](tf::Subflow&subflow){ 8:tf::TaskB1=subflow.emplace([](){}).name("B1");//statictaskB1 9:tf::TaskB2=subflow.emplace([](){}).name("B2");//statictaskB2 10:tf::TaskB3=subflow.emplace([](){}).name("B3");//statictaskB3 11:B1.precede(B3);//B1runsbeforeB3 12:B2.precede(B3);//B2runsbeforeB3 13:subflow.detach();//detachthissubflow 14:}).name("B"); 15: 16:A.precede(B);//BrunsafterA 17:A.precede(C);//CrunsafterA 18:B.precede(D);//DrunsafterB 19:C.precede(D);//DrunsafterC 20: 21:tf::Executorexecutor; 22:executor.run(taskflow).wait();//executethegraphtospawnthesubflow 22:taskflow.dump(std::cout);//dumpthetaskflowtoDOTformat The figure below demonstrates a detached subflow based on the previous example. A detached subflow will eventually join the topology of its parent task. Detached subflow becomes an independent graph attached to the top-most taskflow. Running a taskflow multiple times will accumulate all detached tasks in the graph. For example, running the above taskflow 5 times results in a total of 19 tasks. executor.run_n(taskflow,5).wait(); assert(taskflow.num_tasks()==19); taskflow.dump(std::cout); The dumped graph is shown as follows: A subflow can be nested or recursive. You can create another subflow from the execution of a subflow and so on. 1:tf::Taskflowtaskflow; 2: 3:tf::TaskA=taskflow.emplace([](tf::Subflow&sbf){ 4:std::cout<<"AspawnsA1&subflowA2\n"; 5:tf::TaskA1=sbf.emplace([](){ 6:std::cout<<"subtaskA1\n"; 7:}).name("A1"); 8: 9:tf::TaskA2=sbf.emplace([](tf::Subflow&sbf2){ 10:std::cout<<"A2spawnsA2_1&A2_2\n"; 11:tf::TaskA2_1=sbf2.emplace([](){ 12:std::cout<<"subtaskA2_1\n"; 13:}).name("A2_1"); 14:tf::TaskA2_2=sbf2.emplace([](){ 15:std::cout<<"subtaskA2_2\n"; 16:}).name("A2_2"); 17:A2_1.precede(A2_2); 18:}).name("A2"); 19:A1.precede(A2); 20:}).name("A"); 21: 22://executethegraphtospawnthesubflow 23:tf::Executor().run(taskflow).get(); 24:taskflow.dump(std::cout); Debrief: Line 1 creates a taskflow object Lines 3-20 create a task to spawn a subflow of two tasks A1 and A2 Lines 9-18 spawn another subflow of two tasks A2_1 and A2_2 out of its parent task A2 Lines 23-24 runs the graph asynchronously and dump its structure when it finishes Similarly, you can detach a nested subflow from its parent subflow. A detached subflow will run independently and eventually join the topology of its parent subflow.