Low-latency time-portable real-time programming with Exotasks

Abstract

Exotasks are a novel Java programming construct that achieve three important goals. They achieve low latency while allowing the fullest use of Java language features, compared to previous attempts to restrict the Java language for use in the submillisecond domain. They support pluggable schedulers, allowing easy implementation of new scheduling paradigms in a real-time Java system. They can achieve deterministic timing, even in the presence of other Java threads, and across changes of hardware and software platform. To achieve these goals, the program is divided into tasks with private heaps. Tasks may be strongly isolated, communicating only with each other and guaranteeing determinism, or weakly isolated, allowing some communication with the rest of the Java application. Scheduling of the tasks' execution, garbage collection, and value passing is accomplished by the pluggable scheduler. Schedulers that we have written employ logical execution time (LET) in association with strong isolation to achieve time portability. We have also built a quad-rotor model helicopter, the JAviator, which we use to evaluate our implementation of Exotasks in an experimental embedded version of IBM's J9 real-time virtual machine. Our experiments show that we are able to maintain very low scheduling jitter and deterministic behavior in the face of variations in both software load and hardware platform. We also show that Exotasks perform nearly as well as Eventrons on a benchmark audio application.

Design of safety-critical Java level 1 applications using affine abstract clocks

“…Dataflow models of computation have been used in many real-time Java programming models such as Eventrons [39], Reflexes [40], Exotasks [4], StreamFlex [41], and Flexotasks [3]. The main goal of those restricted subsets of Java is to achieve lower latencies than what can be achieved by the real-time garbage collector.…”

Flexible task graphs

“…We will use this summary in explaining how the features of Flexotasks were chosen. For more details, the reader is referred to published descriptions of Eventrons [31], Reflexes [33], Exotasks [5], StreamFlex [32], and RTSJ [14]. For simplicity, we describe Exotasks as they were presented in [5], although a later version [20] includes a unification with Eventrons that is carried forward and extended to the other models by the present work.…”

“…As with Exotasks, a Flexotask program is instantiated from a template (called a "specification graph" in [5]). In contrast, other models used APIs to construct graphs (or isolated tasks) programmatically.…”

“…The RTSJ introduced the NoHeapRealtimeThread for this purpose. More recently, alternatives to NoHeapRealtimeThread have been proposed, such as Eventrons [31], Reflexes [33], Exotasks [5], and StreamFlex [32]. Typically, time critical tasks account for only a fraction of the code of the entire application.…”

The JAviator: A High-Payload Quadrotor UAV with High-Level Programming Capabilities

“…In particular, we tested EControl in the presence of different software workloads (and on different hardware) and verified closeto-identical real-time behavior. 2,3 In this context, Figure 20 (left) depicts the interarrival times for the fromJaviator Exotask, which is part of the JAviator control program and the first one to run in each 20ms controller period. In the present case, fromJaviator is executed on an XScale PXA255 CPU, whereas Figure 20 (right) illustrates the execution of this Exotask on the same CPU when a concurrently running thread is allocating 256KB of memory per second to exercise the runtime system's garbage collector.…”