How to handle sub task

[Alex-PLACET]

Hello,

In case that you create tasks inside a task:

auto main_task = tw::make_task(tw::wait, [exec]{
std::vector<int> vec = {1, 2, 3, 4, 5, 6, 7};
auto sub_task = tw::for_each(exec, vec.begin(), vec.end(), [](int& x){ x *= 2; });
sub_task-wait();
});

How can we handle this issue? If we imagine that I have a single worker in my pool, my main task will be paused until my sub task is finished but because the worker is already used by the main_task, my sub_task is blocked because there is no free worker.

Regards

[bloomen]

Creating tasks within tasks is not supported by transwarp (unless you're using a separate executor but it still feels somewhat hacky). What is encouraged instead is to construct a graph of tasks where your current "sub_task" would be one of the parents to "main_task". Makes it much easier to reason about your tasks and their inter-dependencies. Does that make sense?

[Alex-PLACET]

Do you have some ideas in mind about this hacky executor ?
In my case I already have a codebase, and it will be hard to put every tasks in a single function.
I will try to dedicate time next weeks to clone the repo and add features like the support of subgraph, subtask. As you know well the code do you think it's possible ?

[bloomen]

To make your code above work you need to use a separate executor, e.g.:

tw::parallel main_exec{2};
tw::parallel exec{2};
auto main_task = tw::make_task(tw::wait, [&exec]{
std::vector<int> vec = {1, 2, 3, 4, 5, 6, 7};
auto sub_task = tw::for_each(exec, vec.begin(), vec.end(), [](int& x){ x *= 2; });
sub_task->wait();
});
main_task->schedule(main_exec);

Adding some sort of subgraph support to transwarp doesn't really make sense. With transwarp you can build up graphs and that's it. Sure, you can create another graph within a task. I wouldn't call that a subgraph tho. A task is generic, it can do whatever.