RT-Thread

RT-Thread is an open-source real-time operating system (RTOS) for Embedded devices and Internet of things (IoT). It is developed by the RT-Thread Development Team based in China. RT-Thread is aimed to change the current situation in China that there is no well used open-source real-time operating system in the microcontroller area. 
As of August 2020, RT-Thread was reported to be #3 on the list of RTOSes with the largest number of contributors (behind Zephyr and mbed OS).
Overview.
RT-Thread was started in 2006 as an open-source real-time operating system (RTOS) that is mainly written in C language. There is two types of RT-Thread: Standard and Nano. For resource-constrained microcontrollers, RT-Thread Nano was released in 2017 and requires a minimum of 3KB Flash/ROM and 1.2KB RAM.