available (continues to provide service) and does not fail. A common way that developers categorize highly reliable systems is by quantifying their downtime per year, as shown in Table 4.1. The percentages under the 'Number of 9s' column indicate the percent of the total time that a system must be available.
While RTOSes must be reliable, note that the RTOS by itself is not what is measured to determine system reliability. It is the combination of all system elements-including the hardware, BSP, RTOS, and application-that determines the reliability of a system.
Table 4.1: Categorizing highly available systems by allowable downtime. [2]
| Number of 9s | Downtime per year | Typical application |
|---|---|---|
| 3 Nines (99.9%) | ~9 hours | Desktop |
| 4 Nines (99.99%) | ~1 hour | Enterprise Server |
| 5 Nines (99.999%) | ~5 minutes | Carrier-Class Server |
| 6 Nines (99.9999%) | ~31 seconds | Carrier Switch Equipment |
4.7.2 Predictability
Because many embedded systems are also real-time systems, meeting time requirements is key to ensuring proper operation. The RTOS used in this case needs to be predictable to a certain degree. The term deterministic describes RTOSes with predictable behavior, in which the completion of operating system calls occurs within known timeframes.
Developers can write simple benchmark programs to validate the determinism of an RTOS. The result is based on timed responses to specific RTOS calls. In a good deterministic RTOS, the variance of the response times for each type of system call is very small.
4.7.3 Performance
This requirement dictates that an embedded system must perform fast enough to fulfill its timing requirements. Typically, the more deadlines to be met-and the shorter the time between them-the faster the system's CPU must be. Although underlying hardware can dictate a system's processing power, its software can also contribute to system performance. Typically, the processor's performance is expressed in million instructions per second (MIPS).
Throughput also measures the overall performance of a system, with hardware and software combined. One definition of throughput is the rate at which a system can generate output based on the inputs coming in. Throughput also means the amount of data transferred divided by the time taken to transfer it. Data transfer throughput is typically measured in multiples of bits per second (bps).
Sometimes developers measure RTOS performance on a call-by-call basis. Benchmarks are written by producing timestamps when a system call starts and when it completes. Although this step can be helpful in the analysis stages of design, true performance testing is achieved only when the system performance is measured as a whole.
4.7.4 Compactness
Application design constraints and cost constraints help determine how compact an embedded system can be. For example, a cell phone clearly must be small, portable, and low cost. These design requirements limit system memory, which in turn limits the size of the application and operating system.
In such embedded systems, where hardware real estate is limited due to size and costs, the RTOS clearly must be small and efficient. In these cases, the RTOS memory footprint can be an important factor. To meet total system requirements, designers must understand both the static and dynamic memory consumption of the RTOS and the application that will run on it.
4.7.5 Scalability
Because RTOSes can be used in a wide variety of embedded systems, they must be able to scale up or down to meet application-specific requirements. Depending on how much functionality is required, an RTOS should be capable of adding or deleting modular components, including file systems and protocol stacks.
If an RTOS does not scale up well, development teams might have to buy or build the missing pieces. Suppose that a development team wants to use an RTOS for the design of a cellular phone project and a base station project. If an RTOS scales well, the same RTOS can be used in both projects, instead of two different RTOSes, which saves considerable time and money.
4.8 Points to Remember
Some points to remember include the following:
· RTOSes are best suited for real-time, application-specific embedded systems; GPOSes are typically used for general-purpose systems.
· RTOSes are programs that schedule execution in a timely manner, manage system resources, and
