【正文】 rou pG r o u ps e n dR e c e i v eR e c e i v e R e c e i v eR e c e i v eP1P2P3P4?Motivation of group multicasting Locating an object Faulttolerance Multiple update ., maintaining cache coherence under writeupdate mechanism ., Time synchronization, RAID Software Structure F i g u r e 2 . 3 La y e r s i n c e n t r a l i z e d c o m p u t e r s y s t e m sA p p l i c a t i o n sP r o g r a m m i n g l a n g u a g e s u p p o r tO p e r a t i n g s y s t e mH a r d w a r eF i g u r e 2 . 4 C a t e g o r i e s o f s o f t w a r e i n a d i s t r i b u t e d s y s t e m a n d t h e i r d e p e n d e n c i e sA p p l i c a t i o n sD i s t r i b u t e d p r o g r a m m i n gs u p p o r tO p e ns e r v i c e sO p e r a t i n g s y s t e m k e r n e l s e r v i c e sC o m p u t e r a n d n e t w o r k h a r d w a r e? ponents of DOS operating system kernel services extending conventional Unix kernel, like BSD Unix microkernels, like Mach, Amoeba and Chorus open services DFS DSM other services, like electronic mail delivery service Support for distributed programming RPC MPI or PVM Workload allocation ? two main workload allocation model processor pool model, the use of idle workstations The processor pool model W o r k s t a t i o n s X t e r m i n a l sL o c a l a r e a n e t w o r kW i d e a r e an e t w o r kg a t e w a y F i l e s e r v e r sL o g i n , p r i n t s e r v e r sR u ns e r v e rP r o c e s s o r p o o lFigure the processor pool model ? examples: Amoeba, Plan 9, Cambridge Distributed Computing System Dawning 2022 super server Use of idle workstation ? use of idle or underutilized workstations as a fluctuating pool of extra puters ? example: Sprite, LSF Sharedmemory multiprocessors also called Symmetric sharedmemory MultiProcessor (or SMP) F i g u r e 2 . 6 S t r u c t u r e o f S M PP r o c e s s o r P r o c e s s o r P r o c e s s o rC a c h e C a c h e C a c h eP r o g r a m 1 P r o g r a m 2 P r o g r a m 3 S h a r e d m e m o r yI / O B U S Consistency maintenance ? Update consistency there are likely to be many users accessing shared data。 the operation of the system itself depends on the consistency of certain databases ? Replication consistency ? Cache coherency hypothesis of locality ? Failure consistency ? Clock consistency ? User interface consistency User requirements ? Functionality what the system should do for users ? Reconfigurability the need for a system to acmodate changes without causing disruption to existing service provision ? Quality of service embracing issues of performance, reliability and security Functionality ? Key benefits of a distributed puter system: economy amp。 convenience from resource sharing。 potential improvement in performance amp。 reliability from distributed resource. ? Enhancements to the services provided by centralized puters: sharing across a work can bring access to a richer variety of resources than could be provided by any single puter。 utilization of the advantages of distribution enables explicit sharing, faulttolerant or parallel applications can be programmed. ? Three options when considering a migration from centralized puting to distributed puting: adapt existing operating systems for working example: BSD Unix + NFS move to an entirely new operating system designed specifically for distributed systems emulation: move to a new DOS, but can emulate one or more existing OS. examples: Mach amp。 Chorus Reconfigurability ? Requirements of a reconfigurable distributed system: the changes due to the scalability of a distributed system design and its ability to acmodate heterogeneity a failed process, puter or work ponent is replaced by another working counterpart。 putational load is shifted from overloaded to lessloaded machines, so as to increase the total throughput of the distributed system。 Quality of service ? Performance: in terms of the response times experienced by its users Optimizing the performance of all of the software ponents that involved OS’s munication services distributed programming support ( ., RPC) and the software that implements the service. ? Security es from two main threats against the privacy and integrity of users’ data as it travels over the work their openness to interference with system software: not all machines on a work can in general be made physically secure a faulttolerant system is one which can detect a fault either fail gracefully(that is, predictably) or mask the fault so that no failure is perceived by users of the system. ? Reliability and availability: Chapter 3 Networking amp。 Interworking ? Network technologies ? Protocols ? Technology case studies: Ether, Token Ring and ATM ? Protocol case studies: Inter protocols and FLIP In next class we’ll discuss: Thanks for your attention!