Transcript Module 4: Processes

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:
 Operating System’s code
 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
 CPU registers
 Stack, containing temporary variables
 data section, containing global variables
 A C++ source file on disk is not a process
 Several processes can be associated with the same program
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 processor.
 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)
new, ready,
running,
waiting,
terminated
General memory
management
information such
as base and limit
registers
Address of the
next
instruction to
be executed
CPU Switch From Process to Process
Process Scheduling
 Some systems allow execution of only a single process
at a time. Such are called uni-programming systems.
 Others allow more than one process, concurrent
execution of many processes. These are called multiprogramming (or multi-processing, but not multiprocessor) system.
 In a multiprogramming system, the CPU switches
automatically among processes running each for tens or
hundreds of milliseconds. Hence, users can interact with
each program.
Process Scheduling Queues
 Job queue – set of all processes in the system.
 Ready queue – set of all processes residing in main
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memory, ready and waiting to execute.
Device queues – set of processes waiting for an I/O
device. Each device has its own queue.
Processes that reside in main memory that are ready to
execute are put on the ready queue.
If a process issues an I/O request, it is placed in the
corresponding I/O queue.
When a process terminates, it is removed from all queues
and its PCB is deleted.
Ready Queue And Various I/O Device Queues
Representation of Process Scheduling
Process Schedulers
 The selection of processes from the queues is managed
by schedulers.
 Long-term scheduler (or job scheduler) – selects
processes from a mass-storage (i.e., hard disk) device
where they are spooled and loads them into memory for
execution.
 Short-term scheduler (or CPU scheduler) – selects
which ready process that is already loaded into memory
should be executed next and allocates CPU.
Schedulers (Cont.)
 Short-term scheduler is invoked very frequently
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(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. This corresponds to the number of
processes in memory.
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.
Schedulers (Cont.)
 Long-term scheduler should pick a relatively good mix of
I/O- and CPU-bound processes so that the system
resources are better utilized.
 If all processes are I/O-bound, the ready queue will
almost always be empty.
 If all processes are CPU-bound, I/O queue will be almost
always empty.
 A balanced combination should be selected for system
efficiency.
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.
 Switching involves copying registers, local and global
data, file buffers and other information to the PCB.
 Context-switch time is overhead; the system does no
useful work while switching.
 Time is 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 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 – terminating the children
processes by a terminated parent process.
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 – e.g. a shared database
 Computation speed-up
 Modularity – system can be constructed in modular form
 Convenience – different system tools may be used in
parallel printing, editing, compiling etc.
Producer-Consumer Problem
 Paradigm for cooperating processes, producer process
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produces information that is consumed by a consumer
process.
Buffer is defined to be a shared memory space
Producer places the produced item into a buffer and
consumer takes its consumption from the buffer.
When buffer is full, producer must wait until consumer
consumes at least an item; likewise, when buffer is
empty, consumer must wait until producer places at least
an item into the buffer.
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
buffer
Consumer
process
Producer
process
memory
Producer-Consumer Problem
Shared variables:
const int N=?;
int buffer[N];
int in=0, out=0;
// buffer size
// kind of items kept in the buffer
// in & out indexes to buffer.
out
N-1
Buffer is empty when:
in = out
0
x
y 1
z 2
Buffer is full when:
(in + 1) %N == out
3
in
Producer – Consumer Processes
Producer code:
Consumer code:
int nextp;
// local variable of item produced
do{
….
produce an item in nextp;
….
while ((in + 1)% N == out); //buffer full
buffer [in] = nextp;
in = (in + 1) % N;
} while (true);
int nextc; // local variable of item consumed
do{
while (in == out);
//buffer empty
nextc = buffer [out];
out = (out + 1) % N;
….
Consume the item in nextc;
….
} while (true);
REMARK: At most (N-1) items can be available in the buffer
Producer – Consumer Processes
(alternative solution)
Shared variables:
const int N = ?;
int full[N] = {0}; // the buffer is initially empty
int buffer[N], in=0, out=0;
Producer code:
Consumer code:
int nextp;
do
{
…
produce an item in nextp
…
while (full[in]) /* do nothing*/;
buffer[in] = nextp;
full[in] = 1;
in = (in + 1) % N;
} while(true);
int nextc;
do
{
while (!full[out]) /* do nothing*/;
nextc = buffer[out];
full[out] = 0;
out = (out + 1) % N;
…
consume the item in nextc
…
} while(true);
REMARK: At most N items can be available in the buffer
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 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.
Direct Communication
Producer-Consumer problem
Producer code:
Consumer code:
int nextp;
do
{
int nextc;
do
{
receive(producer,nextc);
…
produce an item in nextp
…
…
consume the item in nextc
send (consumer,nextp);
} while(true);
…
} while(true);
The main disadvantage is limited modularity in direct communication.
The names of producer and receiver should be correctly set in every time they
are used
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.
Indirect Communication
 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
Indirect Communication
 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.
Buffering
 Queue of messages attached to the link; implemented in
one of three ways.
1. Zero capacity – 0 messages
Sender must wait for receiver.
2. Bounded capacity – finite length of n messages
Sender must wait if link full.
3. Unbounded capacity – infinite length
Sender never waits.
Acknowledgement
 After receiving a message, in order to inform sender of the
receipt of the message, receiver sends an acknowledgement
message.
Process P
sends a message to
send (Q, message)
receive (Q, message)
Process Q
receive (P, message)
send (Q, “Acknowledgement”)
Exception Conditions
 Message system is useful in distributed environments since the
probability of error during communication is larger than that of a
single machine environment.
 In a single machine environment, shared memory system is
generally used.
 If a communication error occurs, error recovery or exception
condition handling must take place.
 Possible errors are:
 A process terminates
 Lost messages
 Scrambled messages
Exception Conditions – Terminated Process
 A sender or receiver process may terminate before a
message is processed. This situation will leave message
that will never be received or processes waiting for
messages that will never be sent.
P
Q
waiting
P
Q
sender
Receiver process (P) waits a
message from Q that has
terminated. Then P will be blocked
forever.
P sends a message to a terminated
process. If P requires an
acknowledgement from Q for further
processing, P will be blocked
forever.
 In these cases, the operating system should either
terminate P or notify P that Q has terminated.
Exception Conditions – Lost Messages
 P send a message to Q, which is lost
somewhere in the link due to link
failure.
 In such cases,
P
Q
The message is lost due to
hardware or communication
line failure.
 The OS is responsible for detecting
and responding by notifying the
sending process that the message is
lost, or
 The sending process is responsible for
detecting and should re-transmit the
message.
 The most common method for detecting lost messages is to use
timeouts. For instance, if the acknowledgement signal does not
arrive at the sending process in a specified time interval, it is
assumed that the signal is lost and resent.
Exception Conditions – Scrambled messages
 Because of the noise in communication channels, the delivered
message may be scrambled (modified) in on the way. Error
checking codes are commonly used to detect this type of error.