Process migration is the act of transferring a process between two machines. It enables dynamic load distribution, fault resilience, eased system administration, and data access locality. Despite these goals and ongoing research efforts, migration has not achieved widespread use. With the increasing deployment of distributed systems in general, and distributed operating systems in particular, process migration is again receiving more attention in both research and product development. As high-performance facilities shift from supercomputers to networks of workstations, and with the ever-increasing role of the World Wide Web, we expect migration to play a more important role and eventually to be widely adopted. This survey reviews the field of process migration by summarizing the key concepts and giving an overview of the most important implementations. Design and implementation issues of process migration are analyzed in general, and then revisited for each of the case studies described: MOSIX, Sprite, Mach, and Load Sharing Facility. The benefits and drawbacks of process migration depend on the details of implementation and, therefore, this paper focuses on practical matters. This survey will help in understanding the potentials of process migration and why it has not caught on.

The Sprite operating system allows executing processes to be moved between hosts at any time. We use this process migration mechanism to offload work onto idle machines, and also to evict migrated processes when idle workstations are reclaimed by their owners. Sprite's migration mechanism provides a high degree of transparency both for migrated processes and for users. Idle machines are identified, and eviction is invoked, automatically by daemon processes. On Sprite it takes up to a few hundred milliseconds on SPARCstation 1 workstations to perform a remote exec, whereas evictions typically occur in a few seconds. The pmake program uses remote invocation to invoke tasks concurrently. Compilations commonly obtain speed‐up factors in the range of three to six; they are limited primarily by contention for centralized resources such as file servers. CPU‐bound tasks such as simulations can make more effective use of idle hosts, obtaining as much as eight‐fold speed‐up over a period of hours. Process migration has been in regular service for over two years.

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A B S T R A C TMobile c o m p u t e r s such as notebooks, subnotebooks, a n d p a l m t o p s require low weight, low power consumption, a n d good interactive performance. These requirements impose m a n y challenges on architectures a n d operating systems. This chapter investigates t h r e e alternative storage devices for mobile c o m p u t e r s : magnetic h a r d disks, flash m e m o r y disk emulators, a n d flash m e m o r y cards.We have used h a r d w a r e m e a s u r e m e n t s a n d trace-driven simulation t o evaluate each of t h e alternative storage devices a n d their related design strategies. H a r d w a r e measurem e n t s on an H P O m n i B o o k 300 highlight differences in t h e performance of t h e t h r e e devices as used on t h e O m n i b o o k , especially t h e poor performance of version 2.00 of t h e Microsoft Flash File System [12] when accessing large flies. T h e traces used Permission has been granted by the USENIX Association to reprint the above paper. An earlier version of this appeared in the Proceedings of the First Symposium on Operating Systems Design and Implementation, USENIX Association, November, 1994, Copyright ©USENIX Association, 1994. Unlike that version, this chapter considers an SRAM buffer cache in addition to flash memory. It also uses updated parameters for one of the devices, and measures response time differently. A detailed description of the changes appears at the end of the chapter on page 503. This work was performed in part at the Matsushita Information Technology Laboratory of Panasonic Technologies, Inc. 474CHAPTER 18 in our study came from different environments, including mobile computers (Macintosh PowerBooks) and desktop computers (running Windows or HP-UX), as well as synthetic workloads. Our simulation study shows that flash memory can reduce energy consumption and mean read response time by up to two orders of magnitude, compared to magnetic disk, while providing acceptable write performance. These energy savings can translate into a 25-115% extension of battery life. We also find that the amount of unused memory in a flash memory card has a substantial impact on energy consumption, performance, and endurance: compared to low storage utilizations (40% full), running flash memory near its capacity (95% full) can increase energy consumption by 50-165%, degrade write response time by 24%, and decrease the lifetime of the memory card by up to 70%. For flash disks, asynchronous erasure can improve write response time by a factor of 2.5.

Abstract-Much work on the performance of Web proxy caching has focused on high-level metrics such as hit rates, but has ignored low-level details such as "cookies," aborted connections, and persistent connections between clients and proxies as well as between proxies and servers. These details have a strong impact on performance, particularly in heterogeneous bandwidth environments where network speeds between clients and proxies are significantly different than speeds between proxies and servers.We evaluate through detailed simulations the latency and bandwidth effects of Web proxy caching in such environments. We drive our simulations with packet traces from two scenarios: clients connected through slow dialup modems to a commercial ISP, and clients on a fast LAN in an industrial research lab. We present three main results. First, caching persistent connections at the proxy can improve latency much more than simply caching Web data. Second, aborted connections can waste more bandwidth than that saved by caching data. Third, cookies can dramatically reduce hit rates by making many documents effectively uncachable.