Building safety-critical real-time systems out of inexpensive, non-real-time, COTS components is challenging. Although COTS components generally offer high performance, they can occasionally incur significant timing delays. To prevent this, we propose controlling the operating point of each shared resource (like the cache, memory, and interconnection buses) to maintain it below its saturation limit. This is necessary because the low-level arbiters of these shared resources are not typically designed to provide real-time guarantees. In this work, we introduce a novel system execution model, the Predictable Execution Model (PREM), which, in contrast to the standard COTS execution model, coschedules at a high level all active components in the system, such as CPU cores and I/O peripherals. In order to permit predictable, system-wide execution, we argue that real-time embedded applications should be compiled according to a new set of rules dictated by PREM. To experimentally validate our theory, we developed a COTS-based PREM testbed and modified the LLVM Compiler Infrastructure to produce PREM-compatible executables.
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