Composable Tasking

Composition is a key to improve the programmability of a complex workflow. This chapter describes how to create a large parallel graph through composition of modular and reusable blocks that are easier to optimize.

Compose a Taskflow

A powerful feature of tf::Taskflow is its composable interface. You can break down a large parallel workload into smaller pieces each designed to run a specific task dependency graph. This largely facilitates the modularity of writing a parallel task program.

 1: // f1 has three independent tasks
 2: tf::Taskflow f1;
 3: f1.name("F1");
 4: tf::Task f1A = f1.emplace([&](){ std::cout << "F1 TaskA\n"; });
 5: tf::Task f1B = f1.emplace([&](){ std::cout << "F1 TaskB\n"; });
 6: tf::Task f1C = f1.emplace([&](){ std::cout << "F1 TaskC\n"; });
 7: 
 8: f1A.name("f1A");
 9: f1B.name("f1B");
10: f1C.name("f1C");
11: f1A.precede(f1C);
12: f1B.precede(f1C);
13:
14: // f2A ---
15: //        |----> f2C ----> f1_module_task ----> f2D
16: // f2B --- 
17: tf::Taskflow f2;
18: f2.name("F2");
19: tf::Task f2A = f2.emplace([&](){ std::cout << "  F2 TaskA\n"; });
20: tf::Task f2B = f2.emplace([&](){ std::cout << "  F2 TaskB\n"; });
21: tf::Task f2C = f2.emplace([&](){ std::cout << "  F2 TaskC\n"; });
22: tf::Task f2D = f2.emplace([&](){ std::cout << "  F2 TaskD\n"; });
23: 
24: f2A.name("f2A");
25: f2B.name("f2B");
26: f2C.name("f2C");
27: f2D.name("f2D");
28:
29: f2A.precede(f2C);
30: f2B.precede(f2C);
31:
32: tf::Task f1_module_task = f2.composed_of(f1).name("module");
33: f2C.precede(f1_module_task);
34: f1_module_task.precede(f2D);
35:
36: f2.dump(std::cout);
Taskflow cluster_p0x7ffee9223970 Taskflow: F2 cluster_p0x7ffee92238d0 Taskflow: F1 p0x7f816f402b60 f2A p0x7f816f402d80 f2C p0x7f816f402b60->p0x7f816f402d80 p0x7f816f402fa0 module [Taskflow: F1] p0x7f816f402d80->p0x7f816f402fa0 p0x7f816f402c70 f2B p0x7f816f402c70->p0x7f816f402d80 p0x7f816f402e90 f2D p0x7f816f402fa0->p0x7f816f402e90 p0x7f816f402830 f1A p0x7f816f402a50 f1C p0x7f816f402830->p0x7f816f402a50 p0x7f816f402940 f1B p0x7f816f402940->p0x7f816f402a50

Debrief:

  • Lines 1-12 create a taskflow of three tasks f1A, f1B, and f1C with f1A and f1B preceding f1C
  • Lines 17-30 create a taskflow of four tasks f2A, f2B, f2C, and f2D
  • Line 32 creates a module task from taskflow f1 through the method Taskflow::composed_of
  • Line 33 enforces task f2C to run before the module task
  • Line 34 enforces the module task to run before task f2D

Create a Module Task

The task created from Taskflow::composed_of is a module task that runs on a pre-defined taskflow. A module task does not own the taskflow but maintains a soft mapping to the taskflow. You can create multiple module tasks from the same taskflow but only one module task can run at one time. For example, the following composition is valid. Even though the two module tasks module1 and module2 refer to the same taskflow F1, the dependency link prevents F1 from multiple executions at the same time.

Taskflow cluster_p0x7ffee9223970 Taskflow: F2 cluster_p0x7ffee92238d0 Taskflow: F1 p0x7f816f402b60 f2A p0x7f816f402d80 f2C p0x7f816f402b60->p0x7f816f402d80 p0x7f816f402fa0 module [Taskflow: F1] p0x7f816f402d80->p0x7f816f402fa0 p0x7f816f402c70 f2B p0x7f816f402c70->p0x7f816f402d80 p0x7f816f402fa1 module [Taskflow: F1] p0x7f816f402fa0->p0x7f816f402fa1 p0x7f816f402e90 f2D p0x7f816f402fa1->p0x7f816f402e90 p0x7f816f402830 f1A p0x7f816f402a50 f1C p0x7f816f402830->p0x7f816f402a50 p0x7f816f402940 f1B p0x7f816f402940->p0x7f816f402a50

However, the following composition is invalid. Both module tasks refer to the same taskflow. They can not run at the same time because they are associated with the same graph.

Taskflow cluster_p0x7ffee9223970 Taskflow: F2 cluster_p0x7ffee92238d0 Taskflow: F1 p0x7f816f402b60 f2A p0x7f816f402d80 f2C p0x7f816f402b60->p0x7f816f402d80 p0x7f816f402fa0 module [Taskflow: F1] p0x7f816f402d80->p0x7f816f402fa0 p0x7f816f402fa1 module [Taskflow: F1] p0x7f816f402d80->p0x7f816f402fa1 p0x7f816f402c70 f2B p0x7f816f402c70->p0x7f816f402d80 p0x7f816f402e90 f2D p0x7f816f402fa0->p0x7f816f402e90 p0x7f816f402fa1->p0x7f816f402e90 p0x7f816f402830 f1A p0x7f816f402a50 f1C p0x7f816f402830->p0x7f816f402a50 p0x7f816f402940 f1B p0x7f816f402940->p0x7f816f402a50