The document discusses several topics related to distributed operating systems including: - Distributed shared memory, which implements shared memory across distributed systems without physical shared memory. - Central server and migration algorithms for managing shared data in distributed shared memory systems. - Read replication and full replication algorithms that allow multiple nodes to read or write shared data. - Memory coherence and coherence protocols for maintaining consistency across processor caches. - Key components of distributed file systems such as naming, caching, writing policies, availability, scalability, and cache consistency.

1. Distributed Operating
System
MOEEZ AHMAD 1421-316267
DANISH DAUD 1421-316255
EJAZ ALI 1421-316269
QADEER AYUB 1421-316273
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2. Distributed Shared Memory:
 It implements the shared memory model in distributed
systems, which have no physical shared memory.
 The shared memory model provides a virtual address space
shared between all nodes.
 Data moves between main memory and secondary memory
(within a node) and between main memories of different
nodes
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3. Central Server Algo:
 Central server maintains all shared data.
 Read request: returns data item
 Write request: updates data and returns acknowledgement
message.
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Data
Access
Requests
Clients

4. Migration Algo:
 Ship (migrate) entire data object (page, block) containing
data item to requesting location
 Allow only one node to access a shared data at a time
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Node i Node j
Data Access Request
Data Migration

5. Algorithms of DSM
 Read Replication Algo:
 Replicates data objects to multiple nodes.
 Multiple nodes can have read access or one node write access
(multiple readers-one writer protocol).
 After a write, all copies are invalidated or updated
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Node i Node j
Data Access Request
Data Replication

6. Algorithms of DSM
 Full Replication Algo:
 Extension of read-replication algorithm
 Multiple nodes can read and multiple nodes can write
(multiple-readers, multiple-writers protocol)
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Write
Requests
Clients
Update
Multicast

7. Memory Coherence:
 Memory coherence is a desirable condition in which
corresponding memory locations for each processing
element in a multi-core processor always contain the same
cached data.
 Without memory coherence, programs can be adversely
affected.
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8. Coherence Protocols:
 Coherence protocols apply cache coherence in multiprocessor systems.
 The intention is that two clients must never see different values for the
same shared data.
 Protocols can also be classified as snoopy or directory-based.
 Early systems used directory-based protocols where a directory would
keep a track of the data being shared and the sharers.
 In snoopy protocols, the transaction requests (to read, write, or upgrade)
are sent out to all processors.
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9. Distributed File System:
 Naming & Naming Resolution:
 Name refers to an object such as file or a directory.
Example: Home, Public, Documents, Movies etc.
 Name space is a collection of names.
 Name Resolution refers to the process of mapping a name to
an object that is physical.
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10. Distributed File System:
 Caches on Disk or Main Memory:
 Main Memory:
 Faster than disks.
 Enables diskless workstations.
 Disks:
 Caches larger files
 Good when local usage dominates
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11. Distributed File System:
 Writing Policy:
 Write through -- reliable, but not cache for write.
 delayed write:
 delay the writes to the server
 write on close:
 not much difference from delay write for short opens.
 for long open: less write but more susceptible to losing data
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12. Distributed File System:
 Availability:
 Overcome the failure of servers or network links.
 Replication - Maintain copies of files at different servers.
 Scalability:
 Adaptation of distributed systems to accommodate more users.
 Usually done by adding more and/or faster processors.
 Design components to be scalable!
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13. Distributed File System:
 Cache Consistency:
 server-initiated: the server inform cache managers whenever the
data in the client caches become stale
 client-initiated: the client make sure the cache is clean before
returning the data to the application
 not allow caching when concurrent-write sharing occurs.
 sequential-write sharing may also cause problems: a client
opens a files that has recently modified and closed by another
client
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14. Distributed File System:
 Semantics:
 What a user wants? strict consistency.
 Users can usually tolerate a certain degree of errors in file
handling -- no need to enforce strict consistency.
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15. Ejaz Ali 1421-316267
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16. Architecture of Distributed system:
 Presented on different platforms and several components can
cooperate with one another over a communication network.
 There are several technology frameworks to support
distributed architectures, including .NET, J2EE, CORBA, .NET
Web services, AXIS Java.
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17. Monolithic Kernel
 Is an operating system is an operating
system architecture where the entire
operating system is working in kernel space.
 It holds all privileges to access input/output
devices, memory, hardware interrupts and
CPU stack.
 Tends to be larger than other kernel.
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18. Communication Primitives:
 High level construct help the program in using underlying
communication network
 Two types of communication primitive
 Message passing
 Remote procedure calls
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19. Message passing:
 SEND – Message and its destination
 RECEIVER – source of message and buffer for storing the
message
 Client server computing model:
 Client sent message to server and waits
 Server replies after computing
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20. Remote procedure call
 RPC is a protocol that one program can use to request a
service from a program located in other computer in a
network.
 RPC use client server model
 The main idea is to allow a local computer (client) to
remotely call procedure on a remote computer (server)
 RPC is a interaction between a client and server
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21. Message passing:
 SEND – Message and its destination
 RECEIVER – source of message and buffer for storing the
message
 Client server computing model:
 Client sent message to server and waits
 Server replies after computing
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22. Remote procedure call
 RPC is a protocol that one program can use to request a
service from a program located in other computer in a
network.
 RPC use client server model
 The main idea is to allow a local computer (client) to
remotely call procedure on a remote computer (server)
 RPC is a interaction between a client and server
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23. Synchronization mechanism in shared
memory
 Shared Memory Synchronization. In sharing memory, a
portion of memory is mapped into the address space of one
or more processes.
 No method of coordinating access is automatically provided,
so nothing prevents two processes from writing to
the shared memory at the same time in the same place.
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24. Communication sequential process:
 CSP, is a language for describing patterns of interaction. It is
supported by an elegant, mathematical theory, a set of proof
tools, and an extensive literature.
 It is a member of the family of mathematical theories of
concurrency known as process algebras,
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25. Axiomatic verification of parallel programs
 A language for parallel programming, with a primitive
construct for synchronization and mutual exclusion, is
presented. Hoare's deductive system for proving partial
correctness of sequential programs is extended to include the
parallelism described by the language.
 The proof method lends insight into how one should
understand and present parallel programs.
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26. The sprite file system
 Unix file system developed for diskless workstations with
large memories at UCB.
 Considers memory as a huge cache of disk blocks. Memory is
shared between file system and VM.
 Files are stored on servers. Servers have a large memory that
acts as a cache as well.
 If a file is being written by more than 1 machine, client
caching is turned off -- all requests go to the server.
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27. Apollo domain distributed file system
 The Apollo DOMAIN® system is a fully operational
distributed computing environment for a network of personal
workstations and network servers.
 Its distributed system focus was on a file system that
provided users of autonomous workstations with the same
ease of file sharing.
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28. X-kernel
 X-kernel is an experimental distributed operating system which
allows uniform access to resource on a nationwide internet.
 Provide a uniform interface with which to access heterogenous
physical file system.
 Two unique features:
 Each user define her or his own private file system out of the
existing physical file system.
 Logical directories with physical directories, identifies the type pf
the physical file system and identifies the server (host)
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29. Thank You!
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