Composable Tasking
Contents
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::
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);
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::module1
and module2
refer to the same taskflow F1
, the dependency link prevents F1
from multiple executions at the same time.
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.