Lecture 2 1

Learning Objectives

• Understanding the difference between processes and threads.

• Understanding process migration and load distribution.

• Understanding Process Inter-process Communication in Distributed Systems

Lecture 2 2

The Kernel

• The kernel is a (often memory resident) part of the operating system that has access rights to ALL system resources.

• The kernel of a distributed system usually has access only to local resources.

• ? What o.s. components should be part of the kernel?

Lecture 2 3

Kernel Types

• Monolithic Kernel– Unix, VMS.

• Microkernel– Chorus (base technology for JavaOS)– Mach– more suitable for distributed computing

Lecture 2 4

Processes

• Process is a fundamental concept of computer system operation : it is a program in execution.

• ? What are properties of a process?

• ? What constitutes a process?

• ? What does a process need in order to accomplish its task?

Lecture 2 5

Process Management

• Process management in distributed systems involves:– process creation – process type identification– process migration – process scheduling– process status control– process termination and clean up

Lecture 2 6

Types of processes in distributed and parallel systems.

• Indivisible processes (entire process must be assign to a single processor).

• Divisible processes (a process may be subdivided into smaller sub-processes, tasks, or threads.)– TIG (Task Interaction Graph) represents

relationships between tasks of a divisible process. (see box 2.4 p.42)

Lecture 2 7

Load Distribution and Process Migration

• Process migration involves relocation of a process to remote processor (computer).

• Goal of load distribution:– balance the job load among computers in the

system– provide better response time of computations

Lecture 2 8

Two Components of Load Balancing Algorithms

• Information Gathering– gather info about loads of

other processors and select a suitable migration partner

– e.g. identifies idle processors, estimate cost of migration to various sites

• Process Selection– select a process to

migrate

– e.g. what is the communication delay for migrating a process, how to accommodate differences in heterogeneous systems

Lecture 2 9

External Data Representation

• Heterogeneous systems imply different CPUs, different software configurations and different data representation

• External Data Representation is a common representation of data used in heterogeneous systems. External Data Representation greatly reduces the amount of time required to perform cross-platform process migration.

Lecture 2 10

Benefits of External Data Representation

• Study figures 2.6 and 2.7 and solve problem 2.9 on page 51.

Lecture 2 11

Load balancing decision examples

• Consider the formula for Total_Weight on page 50. Use this formula with Weighting 1 parameters (0.25, 0.25, 0.25, 0.25) in order to determine which is the best location for process migration from exercise 2.6.

Lecture 2 12

Threads

• Thread is a basic unit of process execution.

• “Mini-process” (“lightweight process)

• Operating system allowing for multiple threads of execution are referred as multi-threaded (Windows NT, 2000, Unix)

• Languages that support multithreading include C, C++, Java.

Lecture 2 13

Thread Properties

• A process may have multiple threads of control.

• Each thread has its own program counter and stack, register set, child threads, and state.

• Threads share address space and global variables.

Lecture 2 14

Threads versus Processes

• Per Thread Items– PC

– Stack

– Register set

– Child threads

– State

• Per Process Items– Address space

– Global variables

– Open files

– Child processes

– Signals/IPC

Lecture 2 15

Multithreaded Support• Posix.1c specifies a standard for threads and

their implementation.– Posix Threads

• Java support for multithreaded programming– Threads are treated like other objects.– Must be instantiated with a parameter that is a

runable object (runable objects define a run() method)

– See box 2.2 p. 37 for Thread constructor prototypes and sample methods

Lecture 2 16

Multithreaded Code• Process versus threads example

– Posix Threads Example

• Multithreaded programming paradigms– specialist paradigm– client/server paradigm– assembly line paradigm

• ? Read paragraph 2.2.2 p.34 and answer question 2.2. on page 49.

Lecture 2 17

Processes and Threads in Windows’98

• Windows 95/98 NT and 2000 utilize multithreading.

• Check the following file to processes and threads in Windows

• C:\Program Files\DevStudio\Vc\bin\Win95\Pview95

Lecture 2 18

Inter-process Communication

• Inter-process Communication is an essential component of distributed computing.

• WHY?

Lecture 2 19

Common IPC Primitives

• Messages

• Pipes

• Sockets

• RPCs (Remote Procedure Calls)

Lecture 2 20

Message Passing

• Message passing involves copying of data from one process address space to another process address space.

• Primitives used:– msgSent (dest, text)– msgReceive (source, text)

Lecture 2 21

Blocking and Non-blocking Primitives

• Blocking msgSend waits (blocks awaiting) for acknowledgment

• Blocking msgReceive waits (block awaiting) for the messages

• Sometimes called synchronous

• Non-blocking msgSend sends the message and does not wait for ack.

• Non-blocking msgReceive does not wait for a messages.

• Sometimes called asynchronous

Lecture 2 22

Message Addressing

• One-to-many (multiple receivers, single sender)

• Many-to-one (multiple senders, one receiver)

• Many-to-many (multiple receivers, multiple senders)

Lecture 2 23

Pipes

• Communication takes via a memory buffer.

• One-to-one type

• Unnamed pipes– allow for communication between related

processes (e.g. parent and child)

• Named pipes– allow for communication between unrelated

processes (see Box 3.3 p.64 for details)

Lecture 2 24

Sockets

• Used for communication across the network.

• Low level primitives.

• Requires 6-8 steps (create socket, bind, connect, listen, send receive, shutdown.)

• Supported by Unix and Java (see box 3.4 and 3.5 pp. 70 and 71)

Lecture 2 25

Remote Procedure Calls

• RPC is a high level, blocking primitive.

• Allows communication between remote computers using techniques similar to traditional procedure calls.

• Allows for complex data structures to be passed as parameters.

Lecture 2 26

RPC features

• Parameter Type

• Data Type Support

• Parameter Marshalling

• RPC Binding

• RPC Authentication

• RPC Semantics

Lecture 2 27

RPC Semantics

• At-Most-Once

• At-Least-Once

• Last-of-Many-Call

• Idempotent

Lecture 2 28

Parameter Type

• Input only• per value

• Output only• receive from the server

• Input/output• call by value/results

Lecture 2 29

Data Type Support

• Allow/disallow some data types (e.g. pointers or complex structures.)

• Limit the number of parameters passed

Lecture 2 30

Parameter Marshalling

• Packing of parameters to minimize the amount of information sent across the network.

• Use of stubs on client and server sites.

Lecture 2 31

RPC Binding

• Binding involves – port mapping for the server– assigning a port handler for the client

• see fig. 3.10 p.74

• Binding can be accomplished at– compile time– link time– run time

Lecture 2 32

RPC Authentication

• Client/server authentication might be needed to assure security of data transmisison.