Chapter 3: Processes
Process Concept
Process – a program in execution; process execution must progress in sequential fashion
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 One program can be several processes
Consider multiple users executing the same program
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
I/O status information – I/O devices allocated to process, list of open files
CPU Switch From Process to Process
Process Scheduling
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
Ready Queue And Various I/O Device Queues
Representation of Process Scheduling
Queueing diagram represents queues, resources, flows
Schedulers
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue – The long-term scheduler controls the degree of multiprogramming – Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow)
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 very frequently (milliseconds) (must be fast)
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
Long-term scheduler strives for good process mix
Addition of Medium Term Scheduling
Medium-term scheduler can be added if degree of multiple programming needs to decrease
Remove process from memory, store on disk, bring back in from disk to continue execution: swapping
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 -> longer the context switch
Time dependent on hardware support
Some hardware provides multiple sets of registers per CPU -> multiple contexts loaded at once
Context Switch
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 options
Parent and children share all resources
Children share subset of parent’s resources Parent and child share no resources
Execution options
Parent and children execute concurrently Parent waits until children terminate
A Tree of Processes in Linux
login pid = 8415
init pid = 1
kthreadd pid = 2
sshd pid = 3028
sshd pid = 3610
tcsch pid = 4005
bash pid = 8416
khelper pid = 6
pdflush pid = 200
ps pid = 9298
emacs pid = 9204
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
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 systems do not allow child to continue if
its parent terminates
All children terminated – cascading termination
Process Termination
Wait for termination, returning the pid:
pid t_pid; int status;
pid = wait(&status);
If no parent waiting, then terminated process is a zombie
If parent terminated, processes are orphans
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
Message Passing Vs. Shared Memory
Message passing:
Useful for exchanging smaller amounts of data
Easier to implement
Implemented using system calls
Every message requires kernel intervention – slower
Shared memory:
Useful for exchanging larger amounts of data
Requires synchronization
System calls are only required to set up shared-memory region
Once set up, all accesses are routine memory accesses, no further kernel assistance required – faster
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
… } item;
item buffer[BUFFER_SIZE]; int in = 0;
int out = 0;
Solution is correct, but can only use BUFFER_SIZE-1 elements
#define BUFFER_SIZE 10
typedef struct {
Bounded-Buffer – Producer
item next_produced; while (true) {
/* produce an item in next produced */
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = next_produced;
in = (in + 1) % BUFFER_SIZE;
}
Bounded Buffer – Consumer
item next_consumed;
while (true) {
while (in == out)
; /* do nothing */ next_consumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
/* consume the item in next consumed */
}
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., direct or indirect, synchronous or asynchronous, automatic or explicit buffering)
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
Solution to Producer-Consumer Problem using Blocking Send( ) and Receive( )
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 */ }
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
Communications in Client-Server Systems
Sockets
Remote Procedure Calls
Sockets
A socket is defined as an endpoint for communication
Concatenation of IP address and port – a number included at start of message packet to differentiate network services on a host
The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8
Communication consists between a pair of sockets
All ports below 1024 are well known, used for standard services
Socket Communication
Remote Procedure Calls
Remote procedure call (RPC) abstracts procedure calls between processes on networked systems
Again uses ports for service differentiation
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 performs the procedure on the server
Data representation handled via External Data Representation (XDL) format to account for different architectures
Remote communication has more failure scenarios than local
Messages can be delivered exactly once rather than at most once
OS typically provides a rendezvous (or matchmaker) service to connect client and server
Execution of RPC
End of Chapter 3