Guest blog by Achim Nohl
Developing embedded software often requires a physical target to run software for the purpose of validation and debug. As is often the case, the exact hardware may not exist yet. The software developer is faced with a few choices: explore using models, use a previous generation board or consider another solution where the exact hardware is available. Most software developers generally try to find a solution that is “good enough”, yet pragmatic, which serves their time and cost requirements. For example, in order to work on the Linux scheduler for big.LITTLE processing, software developers used “old” hardware such as those presented at the recent Linaro Connect event. Software developers gave an example of this exact scenario and how they leveraged “old” hardware to complete their Linux scheduler development for big.LITTLE processing. Here, the performance asymmetry of an ARM Cortex-A15/A7 MPCore big.LITTLE processing system is emulated using an off-the-shelf Cortex-A9 MPCore board In the case of big.LITTLE processing, two CPUs with different performance characteristics are combined together, while the Cortex-A9MPCore has common CPUs with identical performance. To mimic big.LITTLE processing on the Cortex-A9MPCore, asymmetry is emulated by running a so called “cycle stealer” software process on one of the Cortex-A9 CPUs, resulting in reduced processing bandwidth on the second CPU. This solution creates a set up that mirrors the expected big.LITTLE processing capabilities and the software under test takes longer to run on one CPU, than it takes for the same software to run on the second CPU. Is it cycle accurate? For sure not, but this is certainly a pragmatic, “good enough” solution to start optimizing the Linux kernel scheduler.
In the last month, I had the opportunity to get some hands-on experience with hardware virtualization and hypervisors. My knowledge so far on this has been mainly limited to what I could read about it and what other people are saying about it. However, the PowerPoint slides I’ve seen leave a lot of white fog between the bullet items. This didn’t make me feel very comfortable talking about this topic myself; but, there was no escape. Hypervisors play an increasingly important role for system designers in context of supporting multiple guest operating systems on the same device, or taking advantage of ARM®’s new big.LITTLETM processing. The fog is not all gone, but let me provide you some insight on what I found out. As a disclaimer, I’m not going to (and I cannot) write an expert almanac about all the aspects of virtualization covering Xen, VMWare, etc. Instead, I’m going to focus on my personal experience that I believe will be relevant to you as well. This post is the starting point for a series on this topic in this blog.