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Part two: Process managementPart two: Process management
A process can be thought of as a program in execution. A process will need certain resources-such as CPU time, memory, files, and I/O devices to accomplish its task. These resources are allocated to the process either when it is created or while it is executing.
All these processes may execute concurrently.
The operating system is responsible for the following activities in connection with process and thread management: the creation and deletion of both user and system processes; the scheduling of processes; and the provision of mechanisms for synchronization, communication, and deadlock handling for processes.
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Chapter 3: ProcessesChapter 3: Processes
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Chapter 3: ProcessesChapter 3: Processes
Process Concept
Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
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ObjectivesObjectives
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-sewer systems.
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Process ConceptProcess Concept
A question that arises in discussing operating systems involves what to call all the CPU activities.
在多道程序环境中，程序的执行是断断续续的，具备动态特征，程序和程序的执行不再一一对应，有必要引入一个新的概念
进程的定义：可并发执行的程序在一个数据集合上的运行过程。
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Process ConceptProcess Concept
a process is a program in execution. A process is more than the program code, which is
sometimes known as the text section. It also includes the current activity, as represented
by the value of the program counter and the contents of the processor's registers.
A process generally also includes the process stack, which contains temporary data (such as function parameters, return addresses, and local variables), and a data section, which contains global variables. A process may also include a heap, which is memory that is dynamically allocated during process run time.
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Process in MemoryProcess in Memory
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Program vs ProcessProgram vs Process
a program by itself is not a process; a program is a passive entity, such as a file containing a list of instructions stored on disk (often called an executable file), whereas a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources.
A program becomes a process when an executable file is loaded into memory.
Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences.
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Process StateProcess State
As a process executes, it changes state. The state of a process is defined in part by the current activity of that process. Each process may be in one of the following states:
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
only one process can be running on any processor at any instant. Many processes may be ready and waiting.
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Diagram of Process StateDiagram of Process State
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Process Control Block (PCB)Process Control Block (PCB)
Each process is represented in the operating system by a process control block
It contains many pieces of information associated with a specific process, including these:
Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information
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Process Control Block (PCB)Process Control Block (PCB)
PCB的作用 : 反映进程的动态特征，描述进程的执行情况，使操作系统能控制进程的运行。
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CPU Switch From Process to ProcessCPU Switch From Process to Process
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Characters of processCharacters of process
动态性：进程有一定的生命周期，它由创建而产生，由调度而执行，因得不到资源而暂停执行，由撤消而消亡。
并发性：多个进程实体，同存于内存中，能在一段时间内同时运行。
独立性：进程是独立运行的基本单位，是系统中独立获得资源和独立调度的基本单位。
异步性：进程按各自独立的、不可预知的速度向前推进。
结构特征：进程 = 程序 + 数据 + 进程控制块
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Process SchedulingProcess Scheduling
The objective of multiprogramming is to have some process running at all times, to maximize CPU utilization.
The objective of time sharing is to switch the CPU among processes so frequently that users can interact with each program while it is running.
To meet these objectives, the process scheduler selects an available process (possibly from a set of several available processes) for program execution on the CPU.
For a single-processor system, there will never be more than one running process. If there are more processes, the rest will have to wait until the CPU is free and can be rescheduled.
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Process Scheduling QueuesProcess Scheduling Queues
Job queue – set of all processes in the system
Only one
Ready queue – set of all processes residing in main memory, ready and waiting to execute
Only one
Device queues – set of processes waiting for an I/O device
Each device has its own device queue
Processes migrate among the various queues
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Ready Queue And Various I/O Device QueuesReady Queue And Various I/O Device Queues
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Representation of Process SchedulingRepresentation of Process Scheduling
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SchedulersSchedulers
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
Medium-term scheduler -- to remove processes from memory (and from active contention for the CPU) and thus the degree of multiprogramming. Later, the process can be reintroduced into memory, and its execution can be continued where it left off.
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Addition of Medium Term SchedulingAddition of Medium Term Scheduling
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Schedulers Schedulers
Short-term scheduler is invoked very frequently (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
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
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Schedulers Schedulers
It is important that the long-term scheduler select a good process mix of I/O-bound and CPU-bound processes.
If all processes are I/O bound, the ready queue will almost always be empty, and the short-term scheduler will have little to do. If all processes are CPU bound, the I/O waiting queue will almost always be empty, devices will go unused, and again the system will be unbalanced.
The system with the best performance will thus have a combination of CPU-bound and I/O-bound processes.
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long-term schedulerlong-term scheduler
the long-term scheduler may be absent or minimal.
time-sharing systems such as UNIX and Microsoft Windows systems often have no long-term scheduler but simply put every new process in memory for the short-term scheduler.
The stability of these systems depends either on a physical limitation (such as the number of available terminals) or on the self-adjusting nature of human users. If the performance declines to unacceptable levels on a multiuser system, some users will simply quit.
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Context SwitchContext Switch
interrupts cause the operating system to change a CPU from its current task and to run a kernel routine.
When an interrupt occurs, the system needs to save the current context of the process currently running on the CPU so that it can restore that context when its processing is done, essentially suspending the process and then resuming it.
The context is represented in the PCB of the process; it includes the value of the CPU registers, the process state, and memory-management information.
Generically, we perform a state save of the current state of the CPU, be it in kernel or user mode, and then a state restore to resume operations.
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Context SwitchContext 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
Context-switch time is pure overhead; the system does no useful work while switching
Time dependent on hardware support
the memory speed, the number of registers that must be copied, and the existence of special instructions (such as a single instruction to load or store all registers).
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Process CreationProcess Creation
Parent process create children processes, which, in turn create other processes, forming a tree of processes
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Process CreationProcess Creation
Resource sharing
Sub-process may be able to obtain its resources directly from the operating system,
or it may be constrained to a subset of the resources of the parent process.
The parent may have to partition its resources among its children, or it may be able to share some resources (such as memory or files) among several of its children.
Restricting a child process to a subset of the parent's resources prevents any process from overloading the system by creating too many sub-processes.
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Process CreationProcess Creation
execution
The parent continues to execute concurrently with its children.
The parent waits until some or all of its children have terminated.
address space
The child process is a duplicate of the parent process (it has the same program and data as the parent).
The child process has a new program loaded into it.
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Process Creation Process Creation
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
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C Program Forking Separate ProcessC Program Forking Separate Processint 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");exit(0);
}}
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Process TerminationProcess 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
Users could arbitrarily kill some jobs (kill).P95
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Cooperating ProcessesCooperating 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
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models of inter-process communicationmodels of inter-process communication
shared memory
a region of memory that is shared by cooperating processes is established. Processes can then exchange information by reading and writing data to the shared region.
message passing
communication takes place by means of messages exchanged between the cooperating processes.
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models of inter-process communicationmodels of inter-process communication
Both of the models just discussed are common in operating systems, and many systems implement both.
Message passing is useful for exchanging smaller amounts of data, because no conflicts need be avoided.
Message passing is also easier to implement than is shared memory for inter-computer communication.
Shared memory allows maximum speed and convenience of communication, as it can be done at memory speeds when within a computer.
Shared memory is faster than message passing.
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Communications Models Communications Models
Figure 3.13 Communications models. (a) Message passing. (b) Shared memory.
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Inter-process Communication (IPC)Inter-process 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 of communication link physical (e.g., shared memory, hardware bus) logical (e.g., logical properties)P99
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Implementation QuestionsImplementation 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?
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Direct CommunicationDirect 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
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Indirect CommunicationIndirect 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-directionalP101
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Indirect CommunicationIndirect 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
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Indirect CommunicationIndirect 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.
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SynchronizationSynchronization
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
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BufferingBuffering
Queue of messages attached to the link; implemented in one of three ways
1.Zero capacity – 0 messagesSender must wait for receiver (rendezvous)
2.Bounded capacity – finite length of n messagesSender must wait if link full
3.Unbounded capacity – infinite length Sender never waits
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Homework: 1Homework: 1 、、 22
End of Chapter 3End of Chapter 3
