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Considerations on Combining Vestal's Mixed-criticality Task Model and the Predictable Execution Model (PREM) for Real-time Systems
Ref: CISTER-TR-210906       Publication Date: 2, Sep, 2021

Abstract:
In the design of critical real-time embedded systems, predictability in timing behavior and in system resource usage is necessary. Vestal’s mixed-criticality task model and the Predictable Execution Model (PREM) help achieve these objectives in different ways. Under Vestal’s model, multiple worst-case execution time (WCET) estimates are considered for each task, with corresponding degree of confidence, and associated with a different criticality level. The schedulability analysis can derive the appropriate timing safety guarantees for each task without using more conservative estimates than needed, thereby avoiding overengineering. The adaptive variant of Vestal’s model also allows for system modes, with some tasks idled at mode change and more conservative WCET estimates thereafter assumed for remaining tasks. Meanwhile, the 2-phase PREM model, via compiler support, first fetches from memory (into the cache) all the locations that a task will access, and only subsequently proceeds with computation. This removes a lot of the uncertainty in WCET estimation stemming from the cache state and memory access delays, leading to better predictability and tighter WCET estimates. Vestal’s model and the PREM model, however, were independently conceived, and never combined. In this work, we explore different possibilities about how these two models could be combined. We focus on the semantics of multiple (static or probabilistic) per-task estimates of processor computation time and number of memory accesses, how these can be derived, the associated compiler and O/S support required, and the implications for timing analysis.

Ishfaq Hussain

,

Muhammad Ali Awan

,

Konstantinos Bletsas

,

Pedro Souto

,

Eduardo Tovar

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