Abstract—Managed languages such as Java and C # are increasingly being considered for hard real-time applications because of their productivity and software engineering advantages. Automatic memory management, or garbage collection, is a key enabler for robust, reusable libraries, yet remains a challenge for analysis and implementation of real-time execution environments. This paper comprehensively compares the two leading approaches to hard real-time garbage collection. While there are many design decisions involved in selecting a real-time garbage collection algorithm, for timebased garbage collectors researchers and practitioners remain undecided as to whether to choose periodic scheduling or slackbased scheduling. A significant impediment to valid experimental comparison is that the commercial implementations use completely different proprietary infrastructures. Here, we present Minuteman, a framework for experimenting with realtime collection algorithms in the context of a high-performance execution environment for real-time Java. We provide the first comparison of the two approaches, both experimentally using realistic workloads, and analytically in terms of schedulability. I.

The disadvantages of unconstrained shared-memory multi-threading in Java, especially with regard to latency and determinism in realtime systems, have given rise to a variety of language extensions that place restrictions on how threads allocate, share, and communicate memory, leading to order-of-magnitude reductions in latency and jitter. However, each model makes different trade-offs with respect to expressiveness, efficiency, enforcement, and latency, and no one model is best for all applications. In this paper we present Flexible Task Graphs (Flexotasks), a single system that allows different isolation policies and mechanisms to be combined in an orthogonal manner, subsuming four previously proposed models as well as making it possible to use new combinations best suited to the needs of particular applications. We evaluate our implementation on top of the IBM Web-Sphere Real Time Java virtual machine using both a microbenchmark and a 30 KLOC avionics collision detector. We show that Flexotasks are capable of executing periodic threads at 10 KHz with a standard deviation of 1.2µs and that it achieves significantly better performance than RTSJ’s scoped memory constructs while remaining impervious to interference from global garbage collection.

The Real-time Specification for Java extends the Java platform

While managed languages such as C # and Java have become quite popular in enterprise computing, they are still considered unsuitable for hard real-time systems. In particular, the presence of garbage collection has been a sore point for their acceptance for low-level system programming tasks. Realtime extensions to these languages have the dubious distinction of, at the same time, eschewing the benefits of highlevel programming and failing to offer competitive performance. The goal of our research is to explore the limitations of high-level managed languages for real-time systems programming. To this end we target a real-world embedded platform, the LEON3 architecture running the RTEMS real-time operating system, and demonstrate the feasibility of writing garbage collected code in critical parts of embedded systems. We show that Java with a concurrent, real-time garbage collector, can have throughput close to that of C programs and comes within 10 % in the worst observed case on realistic benchmark. We provide a detailed breakdown of the costs of Java features and their execution times and compare to real-time and throughput-optimized commercial Java virtual machines.

Real-time systems, and in particular safety-critical systems, are a rich source of challenges for the program verification community as software errors can have catastrophic consequences. Unfortunately, it is nearly impossible to find representative safety-critical programs in the public domain. This has been significant impediment to research in the field, as it is very difficult to validate new ideas or techniques experimentally. This paper presents open challenges for verification of real-time systems in the context of the Real-time Specification for Java. But, our main contribution is a family of programs, called CDx, which we present as an open source benchmark for the verification community.

In recent years, various approaches to real-time execution of Java have proven their worth in numerous commercial and defense applications. The Real-time Specification for Java has extended the Java platform with a range of features needed for real-time computing. As the use of real-time Java has become more widespread, the demand for Java in real-time applications with safety requirements has led to an effort to define a new standard—JSR-302 Safety-Critical Java (SCJ). The goal of this standard is to facilitate the creation of safety-critical Java applications capable of certification under standards such as DO 178B level A or IEC61508 for SIL 4. JSR-302 is nearing completion and will soon be released for public review. This paper introduces some of the primary goals, challenges, and proposed solutions for safety-critical Java and its relationship with the Real-time Specification for Java. Keywords:

Managed languages such as Java and C # are increasingly being considered for hard real-time applications because of their productivity and software engineering advantages. Automatic memory management, or garbage collection, is a key enabler for robust, reusable libraries, yet remains a challenge for analysis and implementation of real-time execution environments. This paper comprehensively compares leading approaches to hard real-time garbage collection. There are many design decisions involved in selecting a real-time garbage collection algorithm. For time-based garbage collectors on uni-processors one must choose whether to use periodic, slack-based or hybrid scheduling. A significant impediment to valid experimental comparison of such choices is that commercial implementations use completely different proprietary infrastructures. We present Minuteman, a framework for experimenting with real-time collection algorithms in the context of a high-performance execution environment for real-time Java. We provide the first comparison of the approaches, both experimentally using realistic workloads, and analytically in terms of schedulability.