release-2-2-0

# Release 2.2.0 (2019/06/15)

Download
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New Features
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Breaks and Deprecated Features
release-2-2-0_1release-2-2-0_breaks_and_deprecated_features

Cpp-Taskflow 2.2.0 is the 3rd release in the 2.x line! This release includes several new changes such as tf::ExecutorObserverInterface, tf::Executor, isolation of taskflow graph and executor, benchmarks, and so forth. In particular, this release improve the performance of the work stealing scheduler.

# Download

Cpp-Taskflow 2.2.0 can be downloaded from here.

# New Features

- A new executor class to isolate the execution module from a taskflow

- A new observer interface to inspect the activities of an executor

- A decomposable taskflow construction interface

- A new work-stealing algorithm to improve the performance

# Breaks and Deprecated Features

In this release, we isolated the executor interface from tf::Taskflow, and merge tf::Framework with tf::Taskflow. This change largely improved the modularity and composability of Cpp-Taskflow in creating clean task dependency graphs and execution flows. Performance is also better. While this introduced some breaks in tf::Taskflow, we have managed to make it as less painful as possible for users to adapt to the new change.

Previously, tf::Taskflow is a hero class that manages both a task dependency graph and the execution of all graphs including frameworks. For example:

    //beforev2.2.0,tf::Taskflowmanagesbothgraphandexecution
    tf::Taskflowtaskflow(4);//createataskflowobjectwith4threads
    taskflow.emplace([](){std::cout<<"taskA\n";});
    taskflow.wait_for_all();//dispatchthepresentgraph

    tf::Frameworkframework;//createaframeworkobject
    framework.emplace([](){std::cout<<"taskB\n";});
    taskflow.run(framework);//runtheframeworkonce
    taskflow.wait_for_all();//waituntiltheframeworkfinishes

However, this design is awkward in many aspects. For instance, calling `wait_for_all` dispatches the present graph and the graph vanishes when the execution completes. To reuse a graph, users have to create another special graph called framework and mix its execution with the one in a taskflow object. Given the user feedback and lessons we have learned so far, we decided to isolate the executor interface out of tf::Taskflow and merge tf::Framework with tf::Taskflow. All execution methods such as `dispatch` and `wait_for_all` have been moved from tf::Taskflow to tf::Executor.

    //startingfromv2.2.0,tf::Executormanagestheexecutionofgraphs
    tf::Taskflowtaskflow;//createataskflowtobuilddependenttasks
    tf::TaskA=taskflow.emplace([](){std::cout<<"taskA\n";});
    tf::TaskB=taskflow.emplace([](){std::cout<<"taskB\n";});
    A.precede(B);

    tf::Executorexecutor(4);//createanexecutorof4threads
    executor.run(taskflow);//runthetaskflowonce
    executor.run(taskflow,2);//runthetaskflowtwice
    executor.wait_for_all();//waitforthethreerunstofinish

The new design has a clean separation between a task dependency graph builder tf::Taskflow and the execution of graphs tf::Executor. Users are fully responsible for the lifetime of a taskflow, and need to ensure a taskflow is alive during its execution. Besides, all task constructs remain unchanged in tf::Taskflow. In most situations, you will just need to add an executor to your program to run your taskflow graphs.

Again, we apologize this breaking change! I hope you can understand what we did is to make Cpp-Taskflow provide good performance scaling and user experience.