We propose a lightweight process migration mechanism and an adaptive memory prefetching scheme called AMPoM (adaptive memory prefetching in openMosix), whose goal is to reduce the migration freeze time in openMosix while ensuring the execution efficiency of migrants. To minimize the freeze time, our system transfers only a few pages to the destination node during process migration. After the migration, AMPoM analyzes the spatial locality of memory access and iteratively prefetches memory pages from remote to hide the latency of inter-node page faults. AMPoM adopts a unique algorithm to decide which and how many pages to prefetch. It tends to prefetch more aggressively when a sequential access pattern is developed, when the paging rate of the process is high or when the network is busy. This advanced strategy makes AMPoM highly adaptive to different application behaviors and system dynamics. The HPC Challenge benchmark results show that AMPoM can avoid 98% of migration freeze time while preventing 85-99% of page fault requests after the migration. Compared to openMosix which does not have remote page fault, AMPoM induces a modest overhead of 0-5% additional runtime. When the working set of a migrant is small, AMPoM outperforms openMosix considerably due to the reduced amount of data transfer. These results indicate that by exploiting memory access locality and prefetching, process migration can be a lightweight operation with little software overhead in remote paging.

Achieving IP handoff with a short latency and minimal packet loss is essential for mobile devices that roam across IP subnets. Many existing solutions require changes to be made to the network or transport layer, and they tend to suffer from long handoff latency in either soft or hard hand-off scenario, or both; and some are difficult to deploy in practice. We propose a new protocol, called the adaptive multipath protocol, to achieve efficient IP handoff. Based on link-layer signal strength measurements, two different schemes are used to handle soft and hard handoff respectively. Seamless IP handoff is achieved by using multiple transport layer connections on top of persistent link-layer connectivity during soft handoff. To achieve low hand-off latency during hard handoff, a set of distributed sessions repositories (SRs), which are independent of the end hosts, are employed. Simulation results clearly support our claims. In particular, the latency for hard handoff is found to be as low as 50% of that of Fast handoff.

This paper presents a new distributed disk-array architecture for achieving high I/O performance in scalable cluster computing. In a serverless cluster of computers, all distributed local disks can be integrated as a distributed-software redundant array of independent disks (ds-RAID) with a single I/O space. We report the new RAID-x design and its benchmark performance results. The advantage of RAID-x comes mainly from its orthogonal striping and mirroring (OSM) architecture. The bandwidth is enhanced with distributed striping across local and remote disks, while the reliability comes from orthogonal mirroring on local disks at the background. Our RAID-x design is experimentally compared with the RAID-5, RAID-10, and chained-declustering RAID through benchmarking on a research Linux cluster at USC. Andrew and Bonnie benchmark results are reported on all four disk-array architectures. Cooperative disk drivers and Linux extensions are developed to enable not only the single I/O space, but also the shared virtual memory and global file hierarchy. We reveal the effects of traffic rate and stripe unit size on I/O performance. Through scalability and overhead analysis, we find the strength of RAID-x in three areas: 1) improved aggregate I/O bandwidth especially for parallel writes, 2) orthogonal mirroring with low software overhead, and 3) enhanced scalability in cluster I/O processing. Architectural strengths and weakness of all four ds-RAID architectures are evaluated comparatively. The optimal choice among them depends on parallel read/write performance desired, the level of fault tolerance required, and the cost-effectiveness in specific I/O processing applications.

A new RAID-x (redundant array of inexpensive disks at level x) architecture is presented for distributed I/O processing on a serverless cluster of computers. The RAID-x architecture is based on a new concept of orthogonal striping and mirroring (OSM) across all distributed disks in the cluster. The primary advantages of this OSM approach lie in: (1) a significant improvement in parallel I/O bandwidth; (2) hiding disk mirroring overhead in the background; and (3) greatly enhanced scalability and reliability in cluster computing applications. All claimed advantages are substantiated with benchmark performance results on the Trojans cluster built at USC in 1999. The authors discuss the issues of scalable I/O performance, enhanced system reliability, and striped checkpointing on distributed RAID-x in a serverless cluster environment.

Support of single system image (SSI) services is the main approach that enables better utilization of PC/workstation clusters. Some SSI services can be easily built with the support of other low-level, elementary, SSI services. In this paper, we describe a single I/O space architecture for achieving a SSI at the I/O subsystem level. Furthermore, we demonstrate how the single I/O space can facilitate the development of other key SSI services. Typical SSI services which can benefit from the single I/O space include single file hierarchy, single memory space, checkpointing systems and single process space with process migration facilities. Benchmark performance results show that our design achieves both performance and storage size scalabilities that are essential to building I/O-intensive clusters.