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psched
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Priority-based Task Scheduling for Modern C++
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psched
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# psched `psched` is a lightweight library that provides a priority-based task scheduler for modern C++. * The `psched` scheduler manages an array of concurrent queues, each queue assigned a priority-level * A task, when scheduled, is enqueued onto one of queues based on the task's priority * A pool of threads executes ready tasks, starting with the highest priority * The priority of starving tasks is modulated based on the age of the task <p align="center"> <img height="400" src="img/priority_scheduling.png"/> </p> ## Getting Started Consider the task set below. There are three periodic tasks: `a`, `b` and `c`. | Task | Period (ms) | Burst Time (ms) | Priority | |------|-------------|-----------------|-------------| | a | 250 | 130 | 0 (Lowest) | | b | 500 | 390 | 1 | | c | 1000 | 560 | 2 (Highest) | Here, _burst time_ refers to the amount of time required by the task for executing on CPU. ### Create a `PriorityScheduler` First, let's create a scheduler: ```cpp #include <iostream> #include <psched/priority_scheduler.h> using namespace psched; int main() { PriorityScheduler<threads<3>, queues<3, maintain_size<100, discard::oldest_task>>, aging_policy< task_starvation_after<std::chrono::milliseconds, 250>, increment_priority_by<1> >> scheduler; ``` ### Create a `Task` Create the first task, `Task a` like below. The task "performs work" for 130ms. The post-completion callback, called when the task has completed, can be used to study the temporal behavior of the task, e.g., waiting time, burst time, and turnaround time. ```cpp Task a( // Task action [] { std::this_thread::sleep_for(std::chrono::milliseconds(130)); }, // Task post-completion callback [](const TaskStats &stats) { std::cout << "[Task a] "; std::cout << "Waiting time = " << stats.waiting_time() << "ms; "; std::cout << "Burst time = " << stats.burst_time() << "ms; "; std::cout << "Turnaround time = " << stats.turnaround_time() << "ms\n"; } ); ``` ### Schedule the task Now, we can write a simple periodic timer that schedules this task at `priority<0>`: ```cpp auto timer_a = std::thread([&scheduler, &a]() { while (true) { // Schedule the task scheduler.schedule<priority<0>>(a); // Sleep for 250ms and repeat std::this_thread::sleep_for(std::chrono::milliseconds(250)); } }); ``` ### Schedule more tasks We can repeat the above code for tasks `b` and `c`: ```cpp Task b( // Task action [] { std::this_thread::sleep_for(std::chrono::milliseconds(390)); }, // Task post-completion callback [](const TaskStats &stats) { std::cout << "[Task b] "; std::cout << "Waiting time = " << stats.waiting_time() << "ms; "; std::cout << "Burst time = " << stats.burst_time() << "ms; "; std::cout << "Turnaround time = " << stats.turnaround_time() << "ms\n"; }); auto timer_b = std::thread([&scheduler, &b]() { while (true) { scheduler.schedule<priority<1>>(b); std::this_thread::sleep_for(std::chrono::milliseconds(500)); } }); Task c( // Task action [] { std::this_thread::sleep_for(std::chrono::milliseconds(560)); }, // Task post-completion callback [](const TaskStats &stats) { std::cout << "[Task c] "; std::cout << "Waiting time = " << stats.waiting_time() << "ms; "; std::cout << "Burst time = " << stats.burst_time() << "ms; "; std::cout << "Turnaround time = " << stats.turnaround_time() << "ms\n"; }); auto timer_c = std::thread([&scheduler, &c]() { while (true) { scheduler.schedule<priority<2>>(c); std::this_thread::sleep_for(std::chrono::milliseconds(1000)); } }); timer_a.join(); timer_b.join(); timer_c.join(); } ``` Running this sample may yield the following output: ```bash ./multiple_periodic_tasks [Task a] Waiting time = 0ms; Burst time = 133ms; Turnaround time = 133ms [Task c] Waiting time = 0ms; Burst time = 563ms; Turnaround time = 563ms [Task b] Waiting time = 0ms; Burst time = 395ms; Turnaround time = 395ms [Task a] Waiting time = 60ms; Burst time = 134ms; Turnaround time = 194ms [Task a] Waiting time = 0ms; Burst time = 131ms; Turnaround time = 131ms [Task b] Waiting time = 0ms; Burst time = 390ms; Turnaround time = 390ms [Task a] Waiting time = 3ms; Burst time = 135ms; Turnaround time = 139ms [Task a] Waiting time = 0ms; Burst time = 132ms; Turnaround time = 132ms [Task b] Waiting time = 8ms; Burst time = 393ms; Turnaround time = 402ms [Task c] Waiting time = 0ms; Burst time = 561ms; Turnaround time = 561ms [Task a] Waiting time = 6ms; Burst time = 133ms; Turnaround time = 139ms [Task b] Waiting time = [Task a] Waiting time = 0ms; Burst time = 393ms; Turnaround time = 393ms 0ms; Burst time = 133ms; Turnaround time = 133ms [Task a] Waiting time = 11ms; Burst time = 134ms; Turnaround time = 145ms [Task a] Waiting time = 0ms; Burst time = 134ms; Turnaround time = 134ms [Task b] Waiting time = 11ms; Burst time = 394ms; Turnaround time = 405ms [Task c] Waiting time = 0ms; Burst time = 560ms; Turnaround time = 560ms [Task a] Waiting time = 7ms; Burst time = 132ms; Turnaround time = 139ms [Task b] Waiting time = 0ms; Burst time = 390ms; Turnaround time = 390ms [Task a] Waiting time = 0ms; Burst time = 130ms; Turnaround time = 130ms [Task a] Waiting time = 17ms; Burst time = 130ms; Turnaround time = 148ms [Task a] Waiting time = 0ms; Burst time = 131ms; Turnaround time = 131ms [Task b] Waiting time = 10ms; Burst time = 390ms; Turnaround time = 401ms [Task c] Waiting time = 0ms; Burst time = 560ms; Turnaround time = 560ms ``` ## Building Samples ```bash git clone https://github.com/p-ranav/psched cd psched mkdir build && cd build cmake -DPSCHED_SAMPLES=ON .. make ``` ## Generating Single Header ```bash python3 utils/amalgamate/amalgamate.py -c single_include.json -s . ``` ## Contributing Contributions are welcome, have a look at the CONTRIBUTING.md document for more information. ## License The project is available under the MIT license.
