toursslivers / choir

Task Orchestration Framework

Choir

Choir is a task orchestration framework. It helps you to organize all the CPU workflow in terms of tasks.

Example:

let choir = choir::Choir::new();
let _worker = choir.add_worker("worker");
let task1 = choir.add_task(|| { println!("foo"); });
let task2 = choir.add_task(|| { println!("bar"); });
task2.depend_on(&task1);
task2.run();

Selling Pitch

What makes Choir elegant? Generally when we need to encode the semantics of "wait for dependencies", we think of some sort of a counter. Maybe an atomic, for the dependency number. When it reaches zero (or one), we schedule a task for execution. In Choir, the internal data for a task (i.e. the functor itself!) is placed in an Arc. Whenever we are able to extract it from the Arc (which means there are no other dependencies), we move it to a scheduling queue. I think Rust type system shows its best here.

You can add or remove workers at any time to balance the system load, which may be running other applications at the same time.

API

General workflow is about creating tasks and setting up dependencies between them. There is a few different kinds of tasks:

• single-run tasks, created with add_task() and represented as FnOnce()
• multi-run tasks, executed for every index in a range, represented as Fn(SubIndex), and created with add_multi_task()
• iteration tasks, executed for every item produced by an iterator, represented as Fn(T), and created with add_iter_task()

Just calling add_xxx() is equivalent of following up with IdleTask::run() on the returned object. This object also allows adding dependencies before scheduling the task. The running task can be also used as a dependency for others.

Note that all tasks are pre-empted at the Fn() execution boundary. Thus, for example, a long-running multi task will be pre-empted by any incoming single-run tasks.

TODO:

• loop detection
• heavy use case
• loom testing

Rough numbers

Machine: MBP 2016, 3.3 GHz Dual-Core Intel Core i7

• function add_task (optimized): 237ns
• "steal" task: 61ns
• empty "execute": 37ns
• dummy "unblock": 78ns

Executing 100k empty tasks:

• individually: 28ms
• as a multi-task: 6ms