chapter 3:Process concept Flashcards
(4)Process Concept, how execute
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 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
(4)Process Concept, what is process and its parts ?
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
(4)Process Concept, what is a program ?
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
(5)Process in Memory
(6)Process State, what are the states ?
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
(7)Diagram of Process State
(8)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
(9)CPU Switch From Process to Process
(10)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
(11)Process Representation in Linux
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 */
(12)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
(13)Ready Queue And Various
I/O Device Queues
(14)Representation of Process Scheduling-Queuing diagram
Queuing diagram represents queues, resources, flows
(15)Schedulers TYPES ?
- *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
- Sometimes the only scheduler in a system
(15)Schedulers- Short-term scheduler is invoked…?
Short-term scheduler is invoked very frequently (milliseconds) ⇒ (must be fast)
(15)Schedulers- Long-term scheduler is invoked…….?
Long-term scheduler is invoked very infrequently (seconds, minutes) ⇒ (may be slow)
The long-term scheduler controls the degree of multiprogramming
(15) Schedualers-process
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
(16)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
(17)Multitasking in Mobile Systems
Some systems / early systems 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
(17)Multitasking in Mobile Systems-Android ?
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
(18)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
(19)Operations on Processes
System must provide mechanisms for process creation, termination, and so on as detailed next
(20)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
(21)A Tree of Processes in Linux
(22)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
(23)C Program Forking Separate Process- example
(24)Creating a Separate Process via Windows API
(25)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
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
(26)Multiprocess Architecture – Chrome Browser
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 categories
Browser process manages user interface, disk and network I/O
Renderer process renders web pages, deals with HTML, Javascript, new one 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
(27)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
(28)Communications Models
(29)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
(30)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
(31)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
(32)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;
}
(33)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 \*/ }
(34)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)
(35)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?
(36)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
(37)Indirect Communication, massages and communcation link
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
(38)Indirect Communication, operations ans primitives ?
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
(39)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.
(40)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
}
(41)Synchronization (Cont.)
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 */
}
(42)Buffering
Queue of messages attached to the link; implemented in one of three ways
- Zero capacity – 0 messages
Sender must wait for receiver (rendezvous)
- Bounded capacity – finite length of n messages
Sender must wait if link full
- Unbounded capacity – infinite length
Sender never waits
(43)Examples of IPC Systems - POSIX
POSIX Shared Memory
Process first creates shared memory segment
shm_fd = shm_open(name, O CREAT | O RDRW, 0666);
Also used to open an existing segment to share it
Set the size of the object
ftruncate(shm fd, 4096); **
Now the process could write to the shared memory
** sprintf(shared memory, “Writing to shared memory”);
(44)IPC POSIX Producer
(45)IPC POSIX Consumer
(46)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()**
Send and receive are flexible, for example four options if mailbox full:
- Wait indefinitely
- Wait at most n milliseconds
- Return immediately
- Temporarily cache a message
(47)Examples of IPC Systems – Windows
Message-passing centric via advanced local procedure call (LPC) facility
Only works between processes on the same system
Uses ports (like mailboxes) 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.
(48)Local Procedure Calls in Windows XP
(49)Communications in Client-Server Systems
Sockets
Remote Procedure Calls
Pipes
Remote Method Invocation (Java)
(50)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
Special IP address 127.0.0.1 (loopback) to refer to system on which process is running
(51)Socket Communication
(52)Sockets in Java
Three types of sockets
- Connection-oriented (TCP)
- Connectionless (UDP)
- MulticastSocket class– data can be sent to multiple recipients
Consider this “Date” server:
(53)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
On Windows, stub code compile from specification written in Microsoft Interface Definition Language (MIDL)
Data representation handled via External Data Representation (XDL) format to account for different architectures
- Big-endian and little-endian
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
(54)Execution of RPC
(55)Pipes
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?
(56)Ordinary 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
Windows calls these anonymous pipes
See Unix and Windows code samples in textbook
(57)Named 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
