Embedded Linux utilizes Linux kernel for an embedded device, but it is quite different from the standard Linux OS. Its application to embedded systems is motivated by the availability of device support, file-systems, network connectivity, and UI support. It is a customized version of Linux for embedded systems, consequently having a much smaller size and minimal features and requires less processing power. Based on embedded system requirements, the Linux kernel is modified and optimized. Such embedded Linux can only run device-specific purpose-built applications.
The Real-Time Operating System (RTOS) with minimal code is used for such applications where least and fix processing time is required. RTOS is a time-sharing system based on clock interrupts that implement priority sequences to execute a process. In the event of a high priority, interrupt is generated by the system, the running low priority processes are stopped and the interrupt is served. The real-time operating system requires less operational memory and synchronizes the processes in such a way they can communicate with each other hence resources can be used efficiently without wastage of time.
The major difference between Embedded Linux and RTOS is in their sizes. RTOS running on an AVR requires approximately 4.4 kilobytes of ROM. Embedded Linux, on the other hand, is relatively larger. The kernel can be stripped of which are not required and even with that, the footprint is generally measured in megabytes.
Embedded Linux RAM requirement is in order of few megabytes. In practical applications, it requires more than that because some other tasks run under these Linux kernels. RTOS has much smaller memory requirements than Linux. A very simple setup, running two tasks, a scheduler, a queue for communication and a semaphore on an 8-bit architecture would use in the vicinity of 200 bytes.
The scheduler in an RT-system is important to ensure that tasks complete in a fixed time. Compared to a regular scheduler for a general-purpose system, it is not the main task of the scheduler to ensure ’fair’ distribution of CPU-time. A common technique is simply to let the task with the highest priority run before all tasks with lower priority. It works fine for a soft real-time system but for hard real-time, the system must provide a better guarantee.
RTOS uses the highest priority first scheduler. It means that the task having the highest priority is always running. This is achieved by having a preemptive scheduler that at a tick-interrupt decides if the currently running task is allowed to continue executing or it needs to be switched for another task based on priority. The scheduler uses the priority to schedule the task with the highest priority. Tasks having the same priority are given a “fair” process time. This schedular allows us to achieve soft real-time but it is difficult to achieve hard real-time by not having any kind of deadline-based scheduling.
For this purpose, there are choices of having a preemptive or a cooperative scheduler. In preemptive mode, a task can be preempted unlike in cooperative mode where it’s up to all tasks to give away the CPU “often” enough so higher priority tasks get to run. Typical RTOS real-time kernel achieves scheduler latencies from zero to a few microseconds.
Embedded Linux scheduler
In Embedded Linux, there are more choices to choose the scheduler. The modular of Embedded Linux allows to change different parts of the system. A simple insmod gives the possibility to change the scheduler. There are a couple of schedulers designed for different things.
First of all, it has a basic highest priority first scheduler that uses the priority of a task and schedules it first. Embedded Linux also implements the Earliest deadline first which uses the periodic feature of Embedded Linux. Assuming that the deadline for every task is when it is next to be run again one can implement a fast EDF. In theory, it is optimal since it can schedule tasks to 100% CPU-usages. In practice, it is not the same due to some overheads. As in idle process Embedded Linux runs a usual Linux kernel and when there are no rt-tasks that can run, Linux gets to run. which can lead to starvation of Linux and thus effectively disabling Linux. But the importance of a real-time system is to run the real-time tasks this is not a big problem for the system. Typical latencies in real-time Linux schedular are in the order of tens to hundreds of microseconds.
Embedded Linux requires a significant amount of CPU resources, perhaps >200MIPS, 32bit processor, ideally with an MMU, 4Mb of ROM and 16MB of RAM and boot may take several seconds.
An RTOS, on the other hand, runs in less than 10Kb, on microcontrollers from 8-bit up and boot in milliseconds.
IoT Implementation of OS
Embedded Linux is often preferred for extremely low-power applications, such as sensors, run for months on batteries. The low-power nature often precludes direct IP connectivity which serves as a gateway for Internet connectivity. The gateway communicates the low-power protocol to the sensors and would translate them to IP. Linux may have an existing protocol to fulfill the requirements.
The basic requirement of an IoT device is network connectivity, typically in the form of IP via a web server. An RTOS can offer IP connectivity but have a risk to be buggy unless you examine it. For example, usually, RTOSs do not isolate the IP stack user from the IP stack itself. Network connectivity requires potentially dealing with low speed or congested links which can lead to obscure and hard-to-debug buffer handling issues when the stack is intermingled with other code. On the other hand, an embedded Linux leverages hardware separation and a widely utilized IP stack that probably has been exposed to corner cases.
Security is essential in IoT devices, which are often exposed to open Internet. A system compromise on the Internet interface is prone to intruders and information or control of the device can be hijacked. Developers can leverage native, embedded Linux features—multiuser, SELinux, and containers—to contain and limit the damage.
Linux certainly is a robust and secure OS and the system has matured in an embedded operating system. Yet one of the drawbacks is its Memory footprint when compared to a real-time operating system even though it can be trimmed down by removing tools and system services that are not required in embedded systems, it still is a large software. It simply cannot run on 8 or 16-bit MCUs and requires more onboard RAM for the Linux kernel. For example, ARM Cortex-M architecture based MCUs, which typically have only a few hundred kilobytes of RAM, and Linux cannot run on these chips.
A common engineering solution for networked systems is to use two processors in the device. In this arrangement, an 8 or 16-bit MCU is used for the sensor or actuator, while a 32-bit processor is used for the network interface which runs an RTOS. Sales of 32-bit MCUs have exploded in the last several years, and have become the largest segment of the MCU market.