Harel Paz

An on-the-fly mark and sweep garbage collector based on sliding views

With concurrent and garbage collected languages like Java and C# becoming popular, the need for a suitable non-intrusive, efficient, and concurrent multiprocessor garbage collector has become acute. We propose a novel mark and sweep on-the-fly algorithm based on the sliding views mechanism of Levanoni and Petrank. We have implemented our collector on the Jikes Java Virtual Machine running on a Netfinity multiprocessor and compared it to the concurrent algorithm and to the stop-the-world collector supplied with Jikes JVM. The maximum pause time that we measured with our benchmarks over all runs was 2ms. In all runs, the pause times were smaller than those of the stop-the-world collector by two orders of magnitude and they were also always shorter than the pauses of the Jikes concurrent collector. Throughput measurements of the new garbage collector show that it outperforms the Jikes concurrent collector by up to 60%. As expected, the stop-the-world does better than the on-the-fly collectors with results showing about 10% difference.On top of being an effective mark and sweep on-the-fly collector standing on its own, our collector may also be used as a backup collector (collecting cyclic data structures) for the Levanoni-Petrank reference counting collector. These two algorithms perfectly fit sharing the same allocator, a similar data structure, and a similar JVM interface.

An efficient on-the-fly cycle collection

A reference-counting garbage collector cannot reclaim unreachable cyclic structures of objects. Therefore, reference-counting collectors either use a backup tracing collector infrequently, or employ a cycle collector to reclaim cyclic structures. We propose a new concurrent cycle collector, i.e., one that runs concurrently with the program threads, imposing negligible pauses (of around 1ms) on a multiprocessor. Our new collector combines the state-of-the-art cycle collector [5] with the sliding-views collectors [20, 2]. The use of sliding views for cycle collection yields two advantages. First, it drastically reduces the number of cycle candidates, which in turn, drastically reduces the work required to record and trace these candidates. Therefore, a large improvement in cycle collection efficiency is obtained. Second, it eliminates the theoretical termination problem that appeared in the previous concurrent cycle collector. There, a rare race may delay the reclamation of an unreachable cyclic structure forever. The sliding-views cycle collector guarantees reclamation of all unreachable cyclic structures. The proposed collector was implemented on the Jikes RVM and we provide measurements including a comparison between the use of backup tracing and the use of cycle collection with reference counting. To the best of our knowledge, such a comparison has not been reported before.

Delay analysis of real-time data dissemination

The growing popularity of distributed real-time applications increases the demand for QoS-aware messaging systems. In the absence of transmission rate control, congestion may prevent a messaging system from meeting its timeliness requirements. In this paper, we develop an analytic model for congestion in data dissemination protocols and investigate the effect of transmission rate on message delivery latency. Unlike previous works, we take into account the processing overhead of receiver buffer overflow, which has a significant impact on the results. A simulation is used to obtain more insight into the problem and to study a number of additional effects ignored by the analytic model. The presented analysis can be incorporated into a transmission rate control logic, to enable it to rapidly converge to an optimal transmission rate.

Pulsar: a resource-control architecture for time-critical service-oriented applications

The complexity of real-time systems is growing extremely rapidly, as they move from isolated devices to multilevel networked systems. Traditional methodologies for developing and managing these systems are not scaling to meet the requirements of a new generation of distributed applications. While developers of complex real-time applications are looking to service-oriented architecture to address their needs for ease of development and flexibility of integration, current software infrastructures for service-oriented applications do not address the issue of predictable latency for the applications they host. In this paper, we present Pulsar, a resource-control architecture for managing the end-to-end latency of a set of distributed, time-critical applications. The primary entity of Pulsar is called a controller, which regulates an aspect of resource allocation or scheduling policy. Controllers utilize policy configurations, which may include latency targets to be achieved or resource allocations to be honored, and interact with resource allocators and schedulers (e.g., thread schedulers, memory allocators, or bandwidth reservation mechanisms) to effect local policy. Controllers also provide feedback on how well they are executing a policy. Pulsar includes an application model which captures resource-sensitive behavior and requirements and is independent of high-level programming models and application programming interfaces.

Using prefetching to improve reference-counting garbage collectors

Reference counting is a classical garbage collection method. Recently, a series of papers have extended the basic method to drastically reduce its notorious overhead and extend the basic method to run concurrently and efficiently on a modern computing platform. In this paper we investigate the use of prefetching to further improve the efficiency of the reference-counting collector. We propose potential prefetching opportunities for the advanced reference-counting collector and report an implementation of a collector that employs such prefetching. The proposed prefetch instructions were inserted into the Jikes reference-counting collector obtaining an average reduction of 8.7% of the memory management overheads.