Module 4: Processes
Chapter 3: Processes
Processes
Process Concept
Process Scheduling
Operations on Processes
Interprocess Communication
Examples of IPC Systems
Communication in Client-Server Systems
Objectives
To introduce the notion of a process — a program in execution, which forms the basis of all computation
To describe the various features of processes, including scheduling, creation and termination, and communication
To describe 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
Process Concept
Multiple parts
The program code, also called text section
Current activity including program counter, processor registers
Stack containing temporary data
Function parameters, return addresses, local variables
Data section containing global variables
Heap containing memory dynamically allocated during run time
Program is passive entity stored on disk (executable file), process is active
Program becomes process when executable file loaded into memory
Execution of program started via GUI mouse clicks, command line entry of its name, etc
One program can be several processes
Consider multiple users executing the same program
Process Concept (Cont.)
Process in Memory
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
(also called task control block)
Process state – running, waiting, etc
Program counter – location of instruction to next execute
CPU registers – contents of all process-centric registers
CPU scheduling information- priorities, scheduling queue pointers
Memory-management information – memory allocated to the process
Accounting information – CPU used, clock time elapsed since start, time limits
I/O status information – I/O devices allocated to process, list of open files
CPU Switch From Process to Process
Threads
So far, process has a single thread of execution
Consider having multiple program counters per process
Multiple locations can execute at once
Multiple threads of control -> threads
Must then have storage for thread details, multiple program counters in PCB
See next chapter
So far, process has a single thread of execution
Consider having multiple program counters per process
Multiple locations can execute at once
Multiple threads of control -> threads
Must then have storage for thread details, multiple program counters in PCB
See next chapter
Threads
Represented by the C structure task_struct
pid t_pid; /* process identifier */
long state; /* state of the process */
unsigned int time_slice /* scheduling information */
struct task_struct *parent; /* this process’s parent */
struct list_head children; /* this process’s children */
struct files_struct *files; /* list of open files */
struct mm_struct *mm; /* address space of this process */
Process Representation in Linux
Maximize CPU use, quickly switch processes onto CPU for time sharing
Process scheduler selects among available processes for next execution on CPU
Maintains scheduling queues of processes
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
Processes migrate among the various queues
Process Scheduling
Ready Queue And Various I/O Device Queues
Representation of Process Scheduling
Schedulers
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU
Sometimes the only scheduler in a system
Short-term scheduler is invoked frequently (milliseconds) (must be fast)
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue
Long-term scheduler is invoked infrequently (seconds, minutes) (may be slow)
The long-term scheduler controls the degree of multiprogramming
Addition of Medium Term Scheduling
Some mobile systems (e.g., early version of iOS) allow only one process to run, others suspended
Due to screen real estate, user interface limits iOS provides for a
Single foreground process- controlled via user interface
Multiple background processes– in memory, running, but not on the display, and with limits
Limits include single, short task, receiving notification of events, specific long-running tasks like audio playback
Multitasking in Mobile Systems
Android runs foreground and background, with fewer limits
Background process uses a service to perform tasks
Service can keep running even if background process is suspended
Service has no user interface, small memory use
Multitasking in Mobile Systems
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 via a context switch
Context of a process represented in the PCB
Context-switch time is overhead; the system does no useful work while switching
The more complex the OS and the PCB the longer the context switch
Time dependent on hardware support
Process Creation
Parent process create children processes, which, in turn create other processes, forming a tree of processes
Generally, process identified and managed via a process identifier (pid)
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
C Program Forking Separate Process
int main() {
pid_t pid;
/* fork another process */
pid = fork();
if (pid < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0) { /* child process */
execlp("/bin/ls", "ls", NULL);
}
else { /* parent process */
/* parent will wait for the child to complete */
wait (NULL);
printf ("Child Complete");
}
return 0;
}
Creating a Separate Process via Windows API
Process Termination
Process executes last statement and asks the operating system to delete 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
If parent is exiting
Some operating system do not allow child to continue if its parent terminates
All children terminated - cascading termination
Process Termination
Some operating systems do not allow child to exists if its parent has terminated. If a process terminates, then all its children must also be terminated.
cascading termination. All children, grandchildren, etc. are terminated.
The termination is initiated by the operating system.
The parent process may wait for termination of a child process by using the wait()system call. The call returns status information and the pid of the terminated process
pid = wait(&status);
If no parent waiting (did not invoke wait()) process is a zombie
If parent terminated without invoking wait , process is an orphan
Many web browsers ran as single process (some still do)
If one web site causes trouble, entire browser can hang or crash
Google Chrome Browser is multiprocess with 3 different types of processes:
Browser process manages user interface, disk and network I/O
Renderer process renders web pages, deals with HTML, Javascript. A new renderer created for each website opened
Runs in sandbox restricting disk and network I/O, minimizing effect of security exploits
Plug-in process for each type of plug-in
Multiprocess Architecture – Chrome Browser
Interprocess Communication
Processes within a system may be independent or cooperating
Cooperating process can affect or be affected by other processes, including sharing data
Reasons for cooperating processes:
Information sharing
Computation speedup
Modularity
Convenience
Cooperating processes need interprocess communication (IPC)
Two models of IPC
Shared memory
Message passing
Communications Models
(a) Message passing. (b) shared memory.
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;
Bounded-Buffer – Producer
while (true) {
/* Produce an item */
while ((((in + 1) % BUFFER SIZE)== out) /*NOP*/ ;
/* i.e., no free buffers */
buffer[in] = item;
in = (in + 1) % BUFFER SIZE;
}
Bounded Buffer – Consumer
while (true) {
while (in == out) /*NOP*/; // nothing to consume
// remove an item from the buffer
item = buffer[out];
out = (out + 1) % BUFFER SIZE;
return item;
}
Solution is correct, but can only use how many slots when full?
Ans: BUFFER_SIZE-1
Interprocess Communication – Shared Memory
An area of memory shared among the processes that wish to communicate
The communication is under the control of the users processes not the operating system.
Major issues is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory.
Synchronization is discussed in great details in Chapter 5.
Interprocess Communication – Message Passing
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
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.
Synchronization
Message passing may be either blocking or non-blocking
Blocking is considered synchronous
Blocking send has the sender block until the message is received
Blocking receive has the receiver block until a message is available
Non-blocking is considered asynchronous
Non-blocking send has the sender send the message and continue
Non-blocking receive has the receiver receive a valid message or null
Synchronization
Message passing may be either blocking or non-blocking
Blocking is considered synchronous
Blocking send -- the sender is blocked until the message is received
Blocking receive -- the receiver is blocked until a message is available
Non-blocking is considered asynchronous
Non-blocking send -- the sender sends the message and continue
Non-blocking receive -- the receiver receives:
A valid message, or
Null message
Different combinations possible
If both send and receive are blocking, we have a rendezvous
Producer-consumer becomes trivial
message next_produced;
while (true) {
/* produce an item in next produced */
send(next_produced);
}
message next_consumed;
while (true) {
receive(next_consumed);
/* consume the item in next consumed */
}
Synchronization (Cont.)
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
Examples of IPC Systems - POSIX
POSIX Shared Memory
Process first creates shared memory segment
segment id = shmget(IPC PRIVATE, size, S IRUSR | S IWUSR);
Process wanting access to that shared memory must attach to it
shared memory = (char *) shmat(id, NULL, 0);
Now the process could write to the shared memory
sprintf(shared memory, "Writing to shared memory");
When done a process can detach the shared memory from its address space
shmdt(shared memory);
Examples of IPC Systems - Mach
Mach communication is message-based
Even system calls are messages
Each task gets two mailboxes at creation- Kernel and Notify
Only three system calls needed for message transfer
msg_send(), msg_receive(), msg_rpc()
Mailboxes needed for commuication, created via
port_allocate()
Examples of IPC Systems – Windows XP
Message-passing centric via local procedure call (LPC) facility
Only works between processes on the same system
Uses ports (like mboxes) to establish and maintain communication channels
Communication works as follows:
The client opens a handle to the subsystem’s connection port object
The client sends a connection request
The server creates two private communication ports and returns the handle to one of them to the client
The client and server use the corresponding port handle to send messages or callbacks and to listen for replies
Local Procedure Calls in Windows XP
Communications in Client-Server Systems
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
Acts as a conduit allowing two processes to communicate
Issues:
Is communication unidirectional or bidirectional?
In the case of two-way communication, is it half or full-duplex?
Must there exist a relationship (i.e., parent-child) between the communicating processes?
Can the pipes be used over a network?
Ordinary pipes – cannot be accessed from outside the process that created it. Typically, a parent process creates a pipe and uses it to communicate with a child process that it created.
Named pipes – can be accessed without a parent-child relationship.
Pipes
Pipes
Ordinary Pipes allow communication in standard producer-consumer style
Producer writes to one end (the write-end of the pipe)
Consumer reads from the other end (the read-end of the pipe)
Ordinary pipes are therefore unidirectional
Require parent-child relationship between communicating processes
Windows calls these anonymous pipes
See Unix and Windows code samples in textbook
Pipes
Named Pipes are more powerful than ordinary pipes
Communication is bidirectional
No parent-child relationship is necessary between the communicating processes
Several processes can use the named pipe for communication
Provided on both UNIX and Windows systems
Print
process A
message queue
kernel
(a) (b)
process A
shared memory
kernel
process B
m0 m1 m2 ...m3 mn
process B