distributed operating system - pritee

Notes by Pritee Parwekar,ANITS 27-Nov-16 1

Credits:4

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Notes by Pritee Parwekar,ANITS

Understand the Distributed Systems and the challenges involved in Design of the Distributed Systems.

Understand how communication is created and synchronized in Distributed systems

Design and Implement Distributed applications using Technologies like RPC, threads.

Learn how to store data in Distributed File System.

Understand How Distributed Shared Memory is managed.

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Introduction to Distributed Systems, What is a Distributed System?, Hard ware concepts, Software concepts, Design issues.

Communication in Distributed Systems, Lay red Protocols, ATM networks, The Client – sever model, Remote Procedure call, Group communication.

Synchronization in Distributed System, Clock Synchronization, Mutual Exclusion, Election algorithms, Atomic transactions, Deadlocks in Distributed Systems.

Process and processors in Distributed System threads, System Models, Processors allocation, Scheduling in Distributed System, Fault tolerance, Real time Distributed System.

Distributed File Systems, Distributed File System Design, Distributed File System implementation, Trends in Distributed File System.

Distributed Shared Memory, Introduction, What is Shared memory?, Consistency models, Page based Distributed Shared memory, Shared – variable Distributed Shared memory, Object based Distributed Shared Memory.

27-Nov-16Notes by Pritee Parwekar,ANITS 3

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TEXT BOOK:

Distributed Operating Systems, Andrew S. Tanenbanm

REFERENCE BOOK:

Advanced Concepts in Operating Systems, Makes Singhal and Niranjan G.Shivaratna.

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To understand the function of Operating Systems (OS) and what are the types of operating systems.

To make the students learn about the concept of Distributed Operating Systems (DOS).


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By end of this lecture, the students would know the following:-

What are operating systems?

What are the functions of operating systems?

Types and operating systems

Functions of operating systems

Introduction to distributed operating systems

Difference between distributed and network operating systems

Examples of operating systems


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Batch Operating Systems

Timesharing Operating Systems

Real Time Systems

Network Operating Systems (NOS)

Distributed Operating Systems (DOS)

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The users of a batch operating system do not interact with the computer directly.

Each user prepares his job on an off-line device like punch cards and submits it to the computer operator.

To speed up processing, jobs with similar needs are batched together and run as a group.

The programmers leave their programs with the operator and the operator then sorts the programs with similar requirements into batches.


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Time-sharing is a technique which enables many people, located at various terminals, to use a particular computer system at the same time.

Time-sharing or multitasking is a logical extension of multiprogramming.

Processor's time which is shared among multiple users simultaneously is termed as time-sharing.


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A real-time system is defined as a data processing system in which the time interval required to process and respond to inputs is so small that it controls the environment.

The time taken by the system to respond to an input and display of required updated information is termed as the response time. So in this method, the response time is very less


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Hard real-time systems guarantee that critical tasks complete on time.

In hard real-time systems, secondary storage is limited or missing and the data is stored in ROM. In these systems, virtual memory is almost never found.


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Extremely Fast Reaction Time

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Soft real-time systems are less restrictive. A critical real-time task gets priority over other tasks and retains the priority until it completes.

Soft real-time systems have limited utility than hard real-time systems.

For example, multimedia, virtual reality, Advanced Scientific Projects like undersea exploration and planetary rovers, etc.


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Multimedia / Sending & Receiving Packets

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A Network Operating System runs on a server and provides the server the capability to manage data, users, groups, security, applications, and other networking functions.

The primary purpose of the network operating system is to allow shared file and printer access among multiple computers in a network, typically a local area network (LAN), a private network or to other networks.


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Centralized servers are highly stable.

Security is server managed.

Upgrades to new technologies and hardware can be easily integrated into the system.

Remote access to servers is possible from different locations and types of systems.


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High cost of buying and running a server.

Dependency on a central location for most operations.

Regular maintenance and updates are required.


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Distributed Operating System is a model where distributed applications are running on multiple computers linked by communications.

Distributed system is a collection of independent computers that appear to the user of the system as a single computer.

A distributed operating system is an extension of the network operating system that supports higher levels of communication and integration of the machines on the network.


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Network Operating Systems :- Contains N copies of Operating Systems, communication between machines is via shared files.

Distributed OS : - Contains N copies of Operating systems, communication between nodes is via messages over a network. These messages pass the necessary parameters for the task and on completion messages return the results. Its as if computers sends emails to other computers with request and answer.


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Give more performance than single system

If one pc in distributed system malfunction or corrupts then other node or pc will take care of More resources can be added easily.

Resources like printers can be shared on multiple pc’s


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Item Description

Economics Microprocessors offer a better price/performance than mainframes

Speed A distributed system may have more total computing power than a mainframe

Inherent Distribution Some applications involve spatially separated machines

Reliability If one machine crashes, the system as a whole can still survive

Incrementatl Growth Computing power can be added in small increment


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Item Description

Data Sharing Allow many users access to a common data base

Device Sharing Allow many users to share expensive peripherals like color printers

Communication Make human-to-human communication easier, for example, by electronic mail

Flexibility Spread the workload over the available machines in the mosteffective way


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Security problem due to sharing, easy access also applies to secret data

Some messages can be lost in the network system.

Bandwidth is another problem if there is large data then all network wires to be replaced which tends to become expensive

Software : Little software exists at present for distributed systems

Networking : Network can saturate or cause other problems


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In this lecture we have learnt

What are operating systems?

What are the functions of operating systems?

Types and operating systems

Functions of operating systems

Introduction to distributed operating systems

Difference between distributed and network operating systems

Examples of operating systems


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To understand the hardware and software concepts of distributed operating concepts

To understand the design issues comes with hardware and software concepts


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By end of this lecture, the students would know the following:-

What is the term multiprocessor?

What is the term multicomputer?

Flynn’s Classification

How multiple processors are connected ?

How multiple processors communicate ?

What are the hardware and software concepts ?

Design issues involved


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All Distributed Systems consist of multiple CPUs and there are different ways of interconnecting them and how they communicate


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SISD

MISD

MISD

MIMD


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MIMD (Multiple-Instruction Multiple-Data)

MIMD can be split into two classifications

Multiprocessors - CPUs share a common memory

Multicomputers - CPUs have separate memories


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Tightly-coupled - short delay in communication between computers, high data rate (e.g., Parallel computers working on related computations)

Loosely-coupled - Large delay in communications, Low data rate (Distributed Systems working on unrelated computations)


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Bus - All machines connected by single medium (e.g., LAN, bus, backplane, cable)

Switched - Single wire from machine to machine, with possibly different wiring patterns (e.g, Internet)


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Up to 64 Tightly-coupled CPUs on a bus, backplane or motherboard with a shared memory module

Plus point is - coherent

Disadvantage is Bus traffic

So add high speed memory cache to each CPU

Snoopy cache


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To build a multiprocessor with more than 64 processors, a different method is needed to connect the CPUs with the memory.

Two switching techniques are employed for it.

a. Crossbar Switch

b. Omega Switch


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Memory is divided into the modules and are connected to the CPUs with the crossbar switch.

At every intersection is a tiny electronic crosspointswitch that can be opened and closed in hardware.

When a CPU wants to access a particular memory, the crosspoint switch connecting them is closed, to allow the access to take place.

If two CPUs try to access the same memory simultaneously, one of them will have to wait.

The downside of the crossbar switch is that with n CPUs and n memories, n2 crosspoint switches are needed. For large n this number can be prohibitive.


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It contains 2x2 switches, each having two inputs and two outputs.

Each switch can route either input to either output. A careful look at the figure will show that with proper setting of the switches, every CPU can access every memory.

In general case, with n CPUs and n memories, the omega network requires log2n switching stages, each containing n/2 switches, for a total of (nlog2n)/2 switches.


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Bus-Based Multicomputers

easy to build

communication volume much smaller

relatively slow speed LAN (10-100 MIPS, compared to 300 MIPS and up for a backplane bus)

Switched Multicomputers

interconnection networks: E.g., grid, Hypercube

Hypercube: n-dimensional cube bus)


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Software more important for users

Three types:

1. Network Operating Systems

2. (True) Distributed Systems

3. Multiprocessor Time Sharing


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loosely-coupled software on loosely-coupled hardware

A network of workstations connected by LAN each machine has a high degree of autonomy

rlogin machinercp machine1:file1 machine2:file2

Files servers: client and server model Clients mount directories on file servers Best known network OS: Sun’s NFS (network file servers) for shared file

systems a few system-wide requirements: format and meaning of all the messages exchanged


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NFS Architecture– Server exports directories– Clients mount exported directories

NSF Protocols– For handling mounting– For read/write: no open/close, stateless


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tightly-coupled software on loosely-coupledhardware

provide a single-system image or a virtualuniprocessora single, global interprocess communicationmechanism, process management, file system; thesame system call interface everywhere

Ideal definition:

“ A distributed system runs on a collection of computersthat do not have shared memory, yet looks like a singlecomputer to its users.”


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Tightly-coupled software on tightly-coupledhardware

Examples: high-performance serversshared memory

single run queue

traditional file system as on a single-processorsystem: central block cache


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Transparency

Flexibility

Reliability

Performance

Scalability


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How to achieve the single-system image, i.e.,how to make a collection of computers appearas a single computer.

• Hiding all the distribution from the users aswell as the application programs can beachieved at two levels:

1) hide the distribution from users2) at a lower level, make the system look transparentto programs.1) and 2) requires uniform interfaces such as accessto files, communication.


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In flexibility there are two schools of thoughts

One school maintains that each machine should run a traditional kernel that provides most services itself.

The other maintains that the kernel should provide as little as possible with the bulk of the operating system services available from user level servers.

This two models known as the monolithic kernel and microkernel respectively.


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Make it easier to change

• Monolithic Kernel: systems calls are trapped andexecuted by the kernel. All system calls are served bythe kernel, e.g., UNIX.

• Microkernel: provides minimal services. 1) IPC2) some memory management3) some low-level process management and scheduling4) low-level i/o

E.g., Mach can support multiple file systems, multiplesystem interfaces.


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Distributed system should be more reliablethan single system.

Example: 3 machines with 0.95 probability of being up. The probability of all being down will be 0.05 3

– Availability: fraction of time the system is usable.Redundancy improves it.– Need to maintain consistency– Need to be secure– Fault tolerance: need to mask failures, recoverfrom errors.


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Without gain on this, why bother withdistributed systems.

• Performance loss due to communicationdelays:

– fine-grained parallelism: high degree of interaction

– coarse-grained parallelism: data is communicated infrequently, after larger amounts of computation.

• Fault Tolerance also exacts its price.


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Systems grow with time or become obsolete. Techniques that require resources linearly in terms of the size of the system are not scalable.

e.g., broadcast based query won't work forlarge distributed systems.

• Examples of bottlenecksCentralized components: a single mail serverCentralized tables: a single URL address bookCentralized algorithms: routing based on complete information


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Omega network: 2x2 switches for n CPUs and n memories,log2 n switching stages, each with n/2 switches, total (n log2 n)/2 switches


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If n=1024, How many switched stages will be there ? How many will be the total switches stages

(CPU to Memory & Memory –CPU) ? If a processor takes 10 nsec instruction

execution time then now many a RISC processor executes instructions in MIPS ?

What would be the switching time for one instructions ?


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NUMA (Non-Uniform Memory Access): placement of program and databuilding a large, tightly-coupled, shared memory multiprocessor is possible, but is difficult and expensive


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A multi-computer with 256-CPUs is organized as a 16X16 grid. What is the worst-case delay (in hops) that a message might have to take?

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Consider a 256 CPU-hypercube what is the worst case delay here again in hops?


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A multiprocessor has 4096 CPUs connected to memory by an omega network.

Find out how many switches stages required ?

How many total switches stages required ?

How many total switches required ?


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A multiprocessor has 4096 50MIPS CPUs connected to memory by an omega network. How fast do the switches have to be to allow a request to go to memory and back in one instruction time ?


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1.Explain the following in brief :a) Scalability can be achieved by applying

different techniques. What are these techniques? -2M

b) Advantages of distributed systems over independent PCs -2M

2.Discuss software concepts of distributed system. -6M3.Explain design issues of distributed systems-6M(CO-1)


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distributed operating system - pritee

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