How to Measure RTOS Performance
Why Make Measurements? Desktop or laptop computers are extremely powerful and available at amazingly low cost. This means that developers of software for desktop systems assume that there is infinite CPU power, so they worry very little about the speed of their code. They also assume that indefinite amounts of memory are available, so they do not worry about code size either.
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Embedded systems are different. Typically, there is enough CPU power to do the job, but only just enough – there is no excess. Memory size is limited. It is not normally unreasonably small, but there is unlikely to be any possibility of adding more. Power consumption is usually an issue and the software – its size and efficiency – can have a significant bearing on the number of Watts burned by the embedded device.
It is clear that, with an embedded system, it is vital that the RTOS has the smallest possible impact on memory footprint and makes very efficient use of the CPU.
There are three areas of interest if you are looking at the performance and usage characteristics of an RTOS:
- 1. Memory – how much ROM and RAM does the kernel need and how is this affected by options and configuration.
- 2. Latency, which is broadly the delay between something happening and the response to that occurrence. This is a particular minefield of terminology and misinformation, but there are two essential latencies to consider: interrupt response and task scheduling.
- 3. Performance of kernel services. How long does it take to perform specific actions?
Each of these measurements will be addressed in turn.
A key problem is that there is no real standardization. One possibility would be the Embedded Microprocessor Benchmark Consortium, but that is not widely adopted and, anyway, it is more oriented towards CPU benchmarking
RTOS Metrics – Memory Footprint
As all embedded systems have some limitations on available memory, the requirements of an RTOS, on a given CPU, need to be understood. An OS will use both ROM and RAM.
ROM, which is normally flash memory, is used to store the kernel code, along with the code for the runtime library and any middleware components. This code – or parts of it – may be copied to RAM on boot up, as this can offer improved performance. There is also likely to be some read only data. If the kernel is statically configured, this data will include extensive information about kernel objects. However, nowadays, most kernels are dynamically configured.
RAM space will be used for kernel data structures, including some or all of the kernel object information, again depending upon whether the kernel is statically or dynamically configured. There will also be some global variables.
If code is copied from flash to RAM, that space must also be accounted for.