A real-time garbage collector has to fulfill two conflicting properties: avoid heap fragmentation and provide short blocking time. The heap needs to be compacted to avoid probably unbounded fragmentation. During compaction all objects are copied; copying is usually performed atomically to avoid interference with mutator threads. Copying of large objects and especially large arrays introduces long blocking times that are unacceptable for real-time systems. In this paper an interruptible copy unit is presented that implements non-blocking object copy. The unit intercepts object and array field access and redirects the access either to the source or destination part of the moving object. The unit can be interrupted after a single word move. The resulting maximum blocking time is the time for a memory word read and write. We have implemented the proposed non-blocking copy unit in the Java processor JOP and are able to run high priority real-time tasks at 10 kHz parallel to the garbage collection task on a 100 MHz system. 1.

There is increasing convergence between the fields of parallel and embedded computing. The demand for more functionality in embedded devices means that complex multicore architectures will be used. In order to promote scalability and obtain predictability, on-chip processor-local private memory subsystems will be used. Whilst at the hardware level this is technical feasible, the more pressing problem is how such memory is presented to the programmer and how its local access is policed. In this paper we illustrate how Java augmented by the Real-time Specification for Java can be used to present the abstraction of a thread-local scoped memory area. We show how to enforce access to the memory area to a single realtime thread. We implement the model on the JOP multiprocessor system and report on our experiences.

Automatic memory management or garbage collection greatly simplifies development of large systems. However, garbage collection is usually not used in real-time systems due to the unpredictable temporal behavior of current implementations of a garbage collector. In this paper we propose a real-time garbage collector that can be scheduled like a normal real-time thread with a deadline monotonic assigned priority. We provide an upper bound for the collector period so that the application threads will never run out of memory. Furthermore, we show that the restricted execution model of the Safety Critical Java standard simplifies root scanning and reduces copying of static data. Our proposal has been implemented and evaluated in the context of the Java processor JOP.

A real-time garbage collector has to fulfill two basic properties: ensure that programs with bounded allocation rates do not run out of memory and provide short blocking times. Even for incremental garbage collectors, two major sources of blocking exist, namely root scanning and heap compaction. Finding root nodes of an object graph is an integral part of tracing garbage collectors and cannot be circumvented. Heap compaction is necessary to avoid probably unbounded heap fragmentation, which in turn would lead to unacceptably high memory consumption. In this paper, we propose solutions to both issues. Thread stacks are local to a thread, and root scanning therefore only needs to be atomic with respect to the thread whose stack is scanned. This fact can be utilized by either blocking only the thread whose stack is scanned, or by delegating the responsibility for root scanning to the application threads. The latter solution eliminates blocking due to root scanning completely. The impact of this solution on the execution time of a garbage collector is shown for two different variants of such a root scanning algorithm. During heap compaction, objects are copied. Copying is usually performed atomically to avoid interference with application threads, which could render the state of an object inconsistent. Copying of large objects and especially large arrays introduces long blocking times that are unacceptable for real-time systems. In this paper an interruptible copy unit is presented that implements non-blocking object copy. The unit can be interrupted after a single word move. We evaluate a real-time garbage collector that uses the proposed techniques on a Java processor. With this garbage collector, it is possible to run high priority hard real-time tasks at 10 kHz parallel to the garbage collection task on a 100 MHz system. Categories and Subject Descriptors: C.3 [Special-Purpose and Application-Based Systems]: Real-time and embedded systems; D.3.4 [Programming Languages]: Processors—Memory management (garbage collection)

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.

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