Transcript Lecture 10

Operating Systems
Lecture 10
Processes II
Read Ch 4.4 - 4.6
Operating System Concepts
4.1
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
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
 Question: Why would you want to have cooperating
processes? (we will discuss this in class)
 Information sharing
 Computational speedup
 Modularity
 Convenience
Operating System Concepts
4.2
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Producer-Consumer Problem
 Paradigm for cooperating processes, producer
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process produces information that is consumed by a
consumer process.
 E.g. Compiler and assembler
Use a buffer that is filled by the producer and emptied
by the consumer.
These must be synchronized (so consumer does not
try to consume an item that has not yet been
produced).
An unbounded-buffer places no practical limit on the
size of the buffer.
A bounded-buffer assumes that there is a fixed buffer
size.
Operating System Concepts
4.3
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Bounded-Buffer – Shared-Memory Solution
 A buffer may be provided by the OS through an IPC
(interprocess communication) facility, or it may be
coded by the application through shared memory.
 Shared data
#define BUFFER_SIZE 10
typedef struct {
...
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
Operating System Concepts
4.4
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Buffer is a Circular Array
 The buffer is implemented as a circular array.
 Two logical pointers:
 in--Next free position in buffer
 out--First full position in buffer
 Buffer is empty when in == out
 Buffer is full when out == ((in + 1) % BUFFER_SIZE);
 Question: How many items can actually be held in
the buffer?
Operating System Concepts
4.5
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Bounded-Buffer – Producer Process
item nextProduced;
while (1) {
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
}
Operating System Concepts
4.6
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Bounded-Buffer – Consumer Process
item nextConsumed;
while (1) {
while (in == out)
; /* do nothing */
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
}
Operating System Concepts
4.7
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Interprocess Communication (IPC)
 OS provides a means of communication between
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processes.
OS provides a mechanism for processes to
communicate and to synchronize their actions.
Message system – processes communicate with each
other without resorting to shared variables.
This is particularly useful in distributed environments.
Best accomplished by a message passing system.
Operating System Concepts
4.8
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
The IPC concept
 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)
Operating System Concepts
4.9
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Implementation Questions
 How are links established?
 Can a link be associated with more than two
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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?
Operating System Concepts
4.10
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
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.
 Disadvantage of direct communication: (we will discuss
in class)
 Limited modularity. If change the name of one process, may
have to change multiple other processes that communicate
with it.
Operating System Concepts
4.11
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
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.
Operating System Concepts
4.12
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
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
Operating System Concepts
4.13
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Indirect Communication
 Mailbox sharing
 P1, P2, and P3 share mailbox A.
 P1, sends; P2 and P3 receive.
 Who gets the message?
 Solutions (we will discuss these in class).
 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.
Operating System Concepts
4.14
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
Synchronization
 Message passing may be either blocking (synchronous)
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or non-blocking (asynchronous).
Blocking send: Sending process is blocked until the
message is received by the receiving process or by the
mailbox.
Non-blocking send: Sending process sends message
and resumes execution without waiting.
Blocking receive: Receiver blocks until a message is
available.
Non-blocking receive: Receiver retrieves either a valid
message or a null.
When both send and receive are blocking, there is a
rendezvous of the sender and receiver.
Operating System Concepts
4.15
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005
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.
Operating System Concepts
4.16
Silberschatz, Galvin and Gagne 2002
Modified for CSCI 399, Royden, 2005