Languages and Tools for Real-Time Systems: Problems, Solutions and Opportunities

This report summarizes two talks I gave at the ACM SIGPLAN Workshop on Language, Compiler, and Tool Support for Real-Time Systems, which took place on June 21, 1994, in in Orlando, Florida. The workshop was held in concert with ACM SIGPLAN Conference on Programming Languages Design and Implementation. The first talk ("Statements about Real-Time: Truth or Bull?") was given in the early morning. At the behest of the workshop's organizers, its primary function was to seed the ongoing discourse and provoke some debate. Besides asking controversial questions, and positing opinions, the talk also identified some several fertile research areas that might interest PLDI attendees . The second talk ("Languages and Transformations: Some Solutions") was more technical, and it reviewed our research on program optimizations for real-time domains. However, I tried as much as possible to revisit the research problems raised in the morning talk, and present some possible approaches to them. The following paragraphs contain the text from my viewgraphs, laced with some commentary. Since so much work has been done in real-time systems - and even more in programming languages - my references are by necessity incomplete. (Also cross-referenced as UMIACS-TR-94-117

Compiling Real-Time Programs into Schedulable Code

We present a programming language with first-class timing constructs, whose semantics is based on timeconstrained relationships between observable events. Since a system specification postulates timing relationships between events, realizing the specification in a program becomes a more straightforward process. Using these constraints, as well as those imposed by data and control flow properties, our objective is to transform the code so that its worst-case execution time is consistent with its real-time requirements. To accomplish this goal we first translate an event-based source program into intermediate code, in which the timing constraints are imposed on the code itself, and then use a compilation technique which synthesizes feasible code from the original source program. 1 Overview The construction of a hard real-time system can, in practice, be a painful process. Many factors conspire to make this the case, among which are inflexible scheduling paradigms and the lack of high-le..

Compiler-Assisted Scheduling for Real-Time Applications: A Static Alternative to Low-Level Tuning

Developing a real-time system requires finding a balance between the timing constraints and the functional requirements. Achieving this balance often requires last-minute, low-level intervention in the code modules -- via intensive hardware-based instrumentation and manual program optimizations. In this dissertation we present an automated, static alternative to this kind of human-intensive work. Our approach is motivated by recent advances in compiler technologies, which we extend to two specific issues on real-time programming, that is, feasibility and schedulability. A task is infeasible if its execution time stretches over its deadline. To eliminate such faults, we have developed a synthesis method that (1) inspects all infeasible paths, and then (2) moves instructions out of those paths to shorten the execution time. On the other hand, schedulability of a task set denotes an ability to guarantee the deadlines of all tasks in the application. This property is affected by interactions between the tasks, as well as their individual execution times and deadlines. To address the schedulability problem, we have developed a task transformation method based on program slicing. The method decomposes a task into two subthreads: the IO-handler component that must meet the original deadline, and the state-update component that can be postponed past the deadline. This delayed-deadline approach contributes to the schedulability of the overall application. We also present a new fixed-priority preemptive scheduling strategy, which yields both a feasible priority ordering and a feasible task-slicing metric. (Also cross-referenced as UMIACS-TR-95-33

Safe code transfromations for speculative execution in real-time systems

Although compiler optimization techniques are standard and successful in non-real-time systems, if naively applied, they can destroy safety guarantees and deadlines in hard real-time systems. For this reason, real-time systems developers have tended to avoid automatic compiler optimization of their code. However, real-time applications in several areas have been growing substantially in size and complexity in recent years. This size and complexity makes it impossible for real-time programmers to write optimal code, and consequently indicates a need for compiler optimization. Recently researchers have developed or modified analyses and transformations to improve performance without degrading worst-case execution times. Moreover, these optimization techniques can sometimes transform programs which may not meet constraints/deadlines, or which result in timeouts, into deadline-satisfying programs. One such technique, speculative execution, also used for example in parallel computing and databases, can enhance performance by executing parts of the code whose execution may or may not be needed. In some cases, rollback is necessary if the computation turns out to be invalid. However, speculative execution must be applied carefully to real-time systems so that the worst-case execution path is not extended. Deterministic worst-case execution for satisfying hard real-time constraints, and speculative execution with rollback for improving average-case throughput, appear to lie on opposite ends of a spectrum of performance requirements and strategies. Deterministic worst-case execution for satisfying hard real-time constraints, and speculative execution with rollback for improving average-case throughput, appear to lie on opposite ends of a spectrum of performance requirements and strategies. Nonetheless, this thesis shows that there are situations in which speculative execution can improve the performance of a hard real-time system, either by enhancing average performance while not affecting the worst-case, or by actually decreasing the worst-case execution time. The thesis proposes a set of compiler transformation rules to identify opportunities for speculative execution and to transform the code. Proofs for semantic correctness and timeliness preservation are provided to verify safety of applying transformation rules to real-time systems. Moreover, an extensive experiment using simulation of randomly generated real-time programs have been conducted to evaluate applicability and profitability of speculative execution. The simulation results indicate that speculative execution improves average execution time and program timeliness. Finally, a prototype implementation is described in which these transformations can be evaluated for realistic applications