Ch4: Processes (OS)

Silberschatz, Galvin and Gagne ©20024.1
Chapter 4: Processes
Process Concept
Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
Communication in Client-Server Systems

Process Concept
An operating system executes a variety of programs:
Batch system – jobs
Time-shared systems – user programs or tasks
Textbook uses the terms job and process almost
interchangeably.
Process – a program in execution; process execution
must progress in sequential fashion.
A process includes:
program counter
stack
data section

Process State
As a process executes, it changes state
new: The process is being created.
running: Instructions are being executed.
waiting: The process is waiting for some event to occur.
ready: The process is waiting to be assigned to a process.
terminated: The process has finished execution.

Diagram of Process State

Process Control Block (PCB)
Information associated with each process.
Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information

Process Control Block (PCB)

CPU Switch From Process to
Process

Process Scheduling Queues
Job queue – set of all processes in the system.
Ready queue – set of all processes residing in main
memory, ready and waiting to execute.
Device queues – set of processes waiting for an I/O
device.
Process migration between the various queues.

Ready Queue And Various I/O Device
Queues

Representation of Process
Scheduling

Schedulers
Long-term scheduler (or job scheduler) – selects which
processes should be brought into the ready queue.
Short-term scheduler (or CPU scheduler) – selects which
process should be executed next and allocates CPU.

Addition of Medium Term
Scheduling

Schedulers (Cont.)
Short-term scheduler is invoked very frequently
(milliseconds) ⇒ (must be fast).
Long-term scheduler is invoked very infrequently
(seconds, minutes) ⇒ (may be slow).
The long-term scheduler controls the degree of
multiprogramming.
Processes can be described as either:
I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts.
CPU-bound process – spends more time doing
computations; few very long CPU bursts.

Context Switch
When CPU switches to another process, the system must
save the state of the old process and load the saved state
for the new process.
Context-switch time is overhead; the system does no
useful work while switching.
Time dependent on hardware support.

Process Creation
Parent process create children processes, which, in turn
create other processes, forming a tree of processes.
Resource sharing
Parent and children share all resources.
Children share subset of parent’s resources.
Parent and child share no resources.
Execution
Parent and children execute concurrently.
Parent waits until children terminate.

Process Creation (Cont.)
Address space
Child duplicate of parent.
Child has a program loaded into it.
UNIX examples
fork system call creates new process
exec system call used after a fork to replace the process’
memory space with a new program.

Processes Tree on a UNIX
System

Process Termination
Process executes last statement and asks the operating
system to decide it (exit).
Output data from child to parent (via wait).
Process’ resources are deallocated by operating system.
Parent may terminate execution of children processes
(abort).
Child has exceeded allocated resources.
Task assigned to child is no longer required.
Parent is exiting.
Operating system does not allow child to continue if its
parent terminates.
Cascading termination.

Cooperating Processes
Independent process cannot affect or be affected by the
execution of another process.
Cooperating process can affect or be affected by the
execution of another process
Advantages of process cooperation
Information sharing
Computation speed-up
Modularity
Convenience

Producer-Consumer Problem
Paradigm for cooperating processes, producer process
produces information that is consumed by a consumer
process.
unbounded-buffer places no practical limit on the size of the
buffer.
bounded-buffer assumes that there is a fixed buffer size.

Bounded-Buffer – Shared-Memory Solution
Shared data
#define BUFFER_SIZE 10
Typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
Solution is correct, but can only use BUFFER_SIZE-1
elements

Bounded-Buffer – Producer
Process
item nextProduced;
while (1) {
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
}

Bounded-Buffer – Consumer
Process
item nextConsumed;
while (1) {
while (in == out)
; /* do nothing */
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
}

Interprocess Communication (IPC)
Mechanism for processes to communicate and to
synchronize their actions.
Message system – processes communicate with each
other without resorting to shared variables.
IPC facility provides two operations:
send(message) – message size fixed or variable
receive(message)
If P and Q wish to communicate, they need to:
establish a communication link between them
exchange messages via send/receive
Implementation of communication link
physical (e.g., shared memory, hardware bus)
logical (e.g., logical properties)

Implementation Questions
How are links established?
Can a link be associated with more than two processes?
How many links can there be between every pair of
communicating processes?
What is the capacity of a link?
Is the size of a message that the link can accommodate
fixed or variable?
Is a link unidirectional or bi-directional?

Direct Communication
Processes must name each other explicitly:
send (P, message) – send a message to process P
receive(Q, message) – receive a message from process Q
Properties of communication link
Links are established automatically.
A link is associated with exactly one pair of communicating
processes.
Between each pair there exists exactly one link.
The link may be unidirectional, but is usually bi-directional.

Indirect Communication
Messages are directed and received from mailboxes
(also referred to as ports).
Each mailbox has a unique id.
Processes can communicate only if they share a mailbox.
Properties of communication link
Link established only if processes share a common mailbox
A link may be associated with many processes.
Each pair of processes may share several communication
links.
Link may be unidirectional or bi-directional.

Operations
create a new mailbox
send and receive messages through mailbox
destroy a mailbox
Primitives are defined as:
send(A, message) – send a message to mailbox A
receive(A, message) – receive a message from mailbox A

Mailbox sharing
P1, P2, and P3 share mailbox A.
P1, sends; P2 and P3 receive.
Who gets the message?
Solutions
Allow a link to be associated with at most two processes.
Allow only one process at a time to execute a receive
operation.
Allow the system to select arbitrarily the receiver. Sender is
notified who the receiver was.

Synchronization
Message passing may be either blocking or non-blocking.
Blocking is considered synchronous
Non-blocking is considered asynchronous
send and receive primitives may be either blocking or
non-blocking.

Buffering
Queue of messages attached to the link; implemented in
one of three ways.
1. Zero capacity – 0 messages
Sender must wait for receiver (rendezvous).
2. Bounded capacity – finite length of n messages
Sender must wait if link full.
3. Unbounded capacity – infinite length
Sender never waits.

Client-Server Communication
Sockets
Remote Procedure Calls
Remote Method Invocation (Java)

Sockets
A socket is defined as an endpoint for communication.
Concatenation of IP address and port
The socket 161.25.19.8:1625 refers to port 1625 on
host 161.25.19.8
Communication consists between a pair of sockets.

Socket Communication

Remote Procedure Calls
Remote procedure call (RPC) abstracts procedure calls
between processes on networked systems.
Stubs – client-side proxy for the actual procedure on the
server.
The client-side stub locates the server and marshalls the
parameters.
The server-side stub receives this message, unpacks the
marshalled parameters, and peforms the procedure on
the server.

Execution of RPC

Remote Method Invocation
Remote Method Invocation (RMI) is a Java mechanism
similar to RPCs.
RMI allows a Java program on one machine to invoke a
method on a remote object.

Marshalling Parameters

Ch4: Processes (OS)

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