Filip Pizlo

Real-time Java scoped memory: design patterns and semantics

One of the most powerful features of the real-time specification for Java (RTSJ) is the new memory management model based on scoped memory areas. This model allows programmers to ensure timely reclamation of memory and predictable performance, at the cost of an unfamiliar programming model. We report on experience using and implementing scoped memory areas. Our contribution is twofold: (i) we give an informal introduction to the semantics of the scope management rules of the RTSJ, (ii) we present a number of design patterns for effectively using the scoped memory area API

CD x : a family of real-time Java benchmarks

Java is becoming a viable platform for hard real-time computing. There are production and research real-time Java VMs, as well as applications in both military and civil sector. Technological advances and increased adoption of Real-time Java contrast significantly with the lack of real-time benchmarks. The few benchmarks that exist are either low-level synthetic micro-benchmarks, or benchmarks used internally by companies, making it difficult to independently verify and repeat reported results. This paper presents the x (Collision Detector) benchmark suite, an open source application benchmark suite that targets different hard and soft real-time virtual machines. x is, at its core, a real-time benchmark with a single periodic task, which implements aircraft collision detection based on simulated radar frames. The benchmark can be configured to use different sets of real-time features and comes with a number of workloads. We describe the architecture of the benchmark and characterize the workload based on input parameters.

Stopless: a real-time garbage collector for multiprocessors

We present Stopless: a concurrent real-time garbage collector suitable for modern multiprocessors running parallel multithreaded applications. Creating a garbage-collected environment that supports real-time on modern platforms is notoriously hard,especially if real-time implies lock-freedom. Known real-time collectors either restrict the real-time guarantees to uniprocessors only, rely on special hardware, or just give up supporting atomic operations (which are crucial for lock-free software). Stopless is the first collector that provides real-time responsiveness while preserving lock-freedom, supporting atomic operations, controlling fragmentation by compaction, and supporting modern parallel platforms. Stopless is adequate for modern languages such as C# or Java. It was implemented on top of the Bartok compiler and runtime for C# and measurements demonstrate high responsiveness (a factor of a 100 better than previously published systems), virtually no pause times, good mutator utilization, and acceptable overheads.

Schism: fragmentation-tolerant real-time garbage collection

Managed languages such as Java and C# are being considered for use in hard real-time systems. A hurdle to their widespread adoption is the lack of garbage collection algorithms that offer predictable space-and-time performance in the face of fragmentation. We introduce SCHISM/CMR, a new concurrent and real-time garbage collector that is fragmentation tolerant and guarantees time-and-space worst-case bounds while providing good throughput. SCHISM/CMR combines mark-region collection of fragmented objects and arrays (arraylets) with separate replication-copying collection of immutable arraylet spines, so as to cope with external fragmentation when running in small heaps. We present an implementation of SCHISM/CMR in the Fiji VM, a high-performance Java virtual machine for mission-critical systems, along with a thorough experimental evaluation on a wide variety of architectures, including server-class and embedded systems. The results show that SCHISM/CMR tolerates fragmentation better than previous schemes, with a much more acceptable throughput penalty.

A study of concurrent real-time garbage collectors

Concurrent garbage collection is highly attractive for real-time systems, because offloading the collection effort from the executing threads allows faster response, allowing for extremely short deadlines at the microseconds level. Concurrent collectors also offer much better scalability over incremental collectors. The main problem with concurrent real-time collectors is their complexity. The first concurrent real-time garbage collector that can support fine synchronization, STOPLESS, has recently been presented by Pizlo et al. In this paper, we propose two additional (and different) algorithms for concurrent real-time garbage collection: CLOVER and CHICKEN. Both collectors obtain reduced complexity over the first collector STOPLESS, but need to trade a benefit for it. We study the algorithmic strengths and weaknesses of CLOVER and CHICKEN and compare them to STOPLESS. Finally, we have implemented all three collectors on the Bartok compiler and runtime for C# and we present measurements to compare their efficiency and responsiveness.

High-level programming of embedded hard real-time devices

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. Real-time extensions to these languages have the dubious distinction of, at the same time, eschewing the benefits of high-level 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 Java on resource-constrained platforms with Fiji VM

Real-time Java is quickly emerging as a platform for building safety-critical embedded systems. The real-time variants of Java, including [8, 15], are attractive alternatives to Ada and C since they provide a cleaner, simpler, and safer programming model. Unfortunately, current real-time Java implementations have trouble scaling down to very hard real-time embedded settings, where memory is scarce and processing power is limited. In this paper, we describe the architecture of the Fiji VM, which enables vanilla Java applications to run in very hard environments, including booting on bare hardware with only very rudimentary operating system support. We also show that our minimalistic approach delivers comparable performance to that of server-class production Java Virtual Machine implementations.

Loci: Simple Thread-Locality for Java

This paper presents a simple type system for thread-local data in Java. Classes and types are annotated to express thread-locality and unintended leaks are detected at compile-time. The system, called Loci, is minimal, modular and compatible with legacy code. The only change to the language is the addition of two new metadata annotations. We implemented Loci as an Eclipse plug-in and used it to evaluate our design on a number of benchmarks. We found that Loci is compatible with how Java programs are written and that the annotation overhead is light thanks to a judicious choice of defaults.

An Emprical Evaluation of Memory Management Alternatives for Real-Time Java

Memory management is a critical issue for correctness and performance of hard-real time systems. Java environments usually incorporate high-throughput garbage collection algorithms, but these algorithms can induce pause times in excess of 100 milliseconds. This is not acceptable for a real-time system. Two approaches for remedying this problem are being investigated. The pause times can be bounded with a real-time garbage collector; or a means to program around the collector entirely can be provided, as is done in the real-time specification for Java with its scoped memory interface. This paper presents the first side-by-side empirical evaluation of the impact of memory management regimes on realistic realtime applications. We present usability arguments as well as a detailed performance comparison of scoped memory and real-time garbage collection. Experience with medium sized systems suggests that while programming with scoped memory is error prone, it provides substantially better throughput. We have observed a throughput reduction of up to 37% and, in the worst-case, an 80% latency penalty for real-time garbage collection

A Real-time Java Virtual Machine for Avionics - An Experience Report

We report on our experience with the implementation of the Real-time Specification for Java (RTSJ) in the DARPA Program Composition for Embedded System (PCES) program. Within the scope of PCES, Purdue University and the Boeing Company collaborated on the development of Ovm, an open source implementation of the RTSJ virtual machine. Ovm was deployed on a ScanEagle Unmanned Aerial Vehicle and successfully flight tested during the PCES Capstone Demonstration.