Meet Linux's Little Brother Zephyr, a Tiny Open Source IoT RTOS (linuxgizmos.com)
DeviceGuru writes: The Linux Foundation has launched the Zephyr Project, to foster an open source, small footprint, modular, scalable, connected, real-time OS for IoT devices. The Zephyr Project's RTOS implements both a small footpoint microkernel and an even tinier nanokernel, and is the result of Wind River contributing its Rocket RTOS kernel to the Zephyr Project. (Wind's Rocket RTOS will now become a downstream commercial distribution based on Zephyr sources.) To get a sense of Zephyr's benefit, its nanokernel is said to be able to run in as little as 10KB of RAM on 32-bit microcontrollers, whereas a minimalistic Linux implementation like uClinux needs upwards of 200KB. The Linux Foundation hopes to see cross-project collaboration between the Zephyr and Linux communities. Technical details are at the Zephyr site.
IoT devices will typically only have a few kB of RAM, so yes, it does matter. As a data point, I'm working with an ARM Cortex-M0 processor at the moment and it has a grand total of 128kB program memory (flash, so you can't use it for read/write data storage) and 16kB RAM. This is an entirely typical processor for this sort of IoT application.