Task-Arena

Task-Arena (hereinafter referred to as Arena) is a set of header files that allow
for managing tasks executed in parallel. Arena works as a pool of threads, pre-allocated when
Arena is created and existing as long as Arena exists. In these posix threads, tasks
added by the user can be executed in parallel. Each of the posix threads of Arena has its
own task queue. Arena supports task stealing, i.e. when the queue of one posix thread is empty,
it takes a task from the queue of another posix thread. Arena implements a cancelation mechanism,
but for it to work, the user must include it in their tasks.

By default, the thread pool (Arena) uses two fewer POSIX threads than the hardware allows.
The user can also specify the number of posix threads pre-allocated by Arena.

When a task is passed to Arena, the user receives std::future as feedback through which he
can get the result of the task. Due to the possibility of interruption (cancellation), it
makes sense to declare the result returned by the task as optional (e.g. std::optional<int>).
(See examples)

Example of use:

#include <optional>
#include "task-arena/pool.h"

using namespace std::chrono_literals;

task_arena::ThreadPool pool;

/// Function used as task.
/// As return value we use std::optional<int> instead int.
/// This gives us the ability to return 'nothing' when a cancel occurs.
std::optional<int> task_as_function() {
    auto internal_task_future = pool.add_task([]() {
        std::this_thread::sleep_for(300ms);
        return 5;
    });

    std::this_thread::sleep_for(600ms);
    if (pool.canceled()) {
        task_arena::printer::print("-- Canceled task_as_function");
        return {};
    }
    task_arena::printer::print("-- Finished task_as_function");
    return 6 + internal_task_future.get();
}

/// Lambda used as task.
auto task_as_lambda = []() -> std::optional<int> {
    std::this_thread::sleep_for(900ms);

    if (pool.canceled()) {
        task_arena::printer::print("-- Canceled task_as_lambda");
        return {};
    }
    task_arena::printer::print("-- Finished task_as_lambda");
    return 100 + 200;
};

int main() {
    auto fut1 = pool.add_task(task_as_function);
    auto fut2 = pool.add_task(task_as_lambda);

    if (auto result = fut1.get(); result)
        task_arena::printer::print("** Result of task_as_function: ", result.value());

    pool.cancel();

    if (auto const result = fut2.get(); result)
        task_arena::printer::print("task_as_lambda result: ", *result);

    return 0;
}

Result of example execution:

Created thread: 0x16bb1f000, idx: 0
Created thread: 0x16bcc3000, idx: 3
Created thread: 0x16bd4f000, idx: 4
Created thread: 0x16bc37000, idx: 2
Created thread: 0x16bddb000, idx: 5
Created thread: 0x16bbab000, idx: 1
task added to pool queue
task added to pool queue
started task from pool queue: 2
task added to local queue
started task from pool queue: 0
started task stealed from other queue: 1
-- Finished task_as_function
** Result of task_as_function: 11
-- Canceled task_as_lambda
joiner