03 - 05 > Process [프로세스의 이해, 생성, 통신]

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[  ] Process Concept.

- A process is a program in execution.

- The memory layout of a process is divided in to multiple sections :

 Text Section ( The executable code ), Data Section ( Global variables ),

 Heap Section ( Memory that is dynamically allocated during program run time )

 Stack Section ( temporary data storage when invoking functions + such as function parameters, return addresses, and local variables. )

Figure 3.1 Layout of a process in memory.

 

- As a process executes, it changes its state.

 New : the process is being created.

 Running : Instructions are begin executed.

 Waiting : the process is waiting for some event to occur. ( +such as an I/O completion or reception of a signal )

 Ready : the process is waiting to be assigned to a processor.

 Terminated : the process has finished execution.

Process는 위와 같이 5개의 Life cycle을 가지고 있고, 이것을 따라 OS는 Process를 관리한다.

Waiting은 모종의 이유로 CPU의 점유를 놓은 상태, Ready는 Waiting이 끝난 후 다시 CPU 자원을 할당 받기 위해 대기하는 상태.

 

Figure 3.2 Diagram of process state.

 

 

[  ] How to manage the process in OS?

 - PCB ( Process Control Block ) or TCB ( Task Control Block) 을 만들어 관리한다.

   struct 안에 Process가 가져야 할 모든 정보를 저장해서 관리하는 것이 PCB.

   Linux는 작업을 Task라 사용해서 TCB라고도 불린다.

 - A PCB contains many pieces of information associated with a specific process:

   Process state, ( Program Counter, CPU registers ) => 이 둘을 묶어 Context라 부른다.

   CPU-scheduling information, Memory-management information, Accounting information, I/O status information.

 

Figure 3.3 Process control block(PCB).

 

[  ] The objective

The objective of multiprogramming is

- to have some process running at all times.

- so as to maximize CPU utilization.

The objective of time sharing is

- to switch a CPU core among processes so frequently

- that users can interact with each program while it is running.

시분할은 시간을 쪼개 여러 개의 Process을 조금씩 처리해 나가서 동시적으로 처리하는 것처럼 보이게 하는 것.

 

[  ] Scheduling Queues

- As processes enter the system, they are put into a ready queue, where they are ready and waiting to execute on a CPU's core.

- Processes that are waiting for a certain event to occur are placed in a wait queue.

Figure 3.4 The ready queue and wait queues.

 

[  ] Queueing Diagram

Figure 3.5 Queueing-diagram representation of process scheduling.

 

[  ] Context Switch

- The context of a process is represented in the PCB.

- The context switch is a task that

   switches the CPU core to another process.

   performs a state save of the current process and a state restore of a different process.

Figure 3.6 Diagram showing context switch from process to process.

Multiprocessing은 OS에 의해서 Context switch로 이루어 진다.

 

[  ] Zombie and Orphan.

- Zombie process : a process that has terminated, but whose parent has not yet called wait().

- Orphan process : a process that has a parent process who did not invoke wait() and instead terminated.

 

[  ] In UNIX-like O/S,

- A new process is created by the fork() system call.

 

[  ] After a fork() system call,

- the parent can continue its execution; or if it has nothing else to do while the child runs, it can issue a wait() system call to move itself off the ready queue until the termination of the child.

 

Child PID 는  0 을   가지므로  wait() 을   실행하지   않는다 .  조건   아래의   문장은  Child 가   먼저   실행   후  Parent Process  가   실행된다 .

fork는 context가 복제가 되므로 Child Process는 복제된 이후의 Code를 계속해서 실행하게 된다.

 

Process 는  2^4 = 16 개가   생성된다 .

1번째 Process가 복제, 복제된 것과 부모 process는 다시 fork()을 Call하고 뒤의 코드가 복제되면서 반복되어 생성된다.

 

 

CHILD : 0,1,4,9,16 / Parent : 0,1,2,3,4

wait가 걸려있으므로, Child가 먼저 실행된다. Context가 전체 복제됨으로 전역변수 또한 복사가 되어 Parent 출력은 nums elements가 값의 변화가 없이 순서대로 출력된다.

 

[  ] Processes executing concurrently.

- Independent processes = A process is independent if it does not share data with any other processes.

- Cooperating processes = A process is cooperating if it can affect or be affected by the other processes.

   Cleary, any processes that shares data with other processes is a cooperating process.

Cooperating processes 간의 메시지를 주고받을 때 발생하는 문제를 어떻게 해결할 것인가? = IPC

 

[★] IPC : Inter-Process Communication.

 - Cooperating processes require an IPC mechanism that will allow them to exchange data that is, send data to and receive data from each other.

 - Two fundamental models of IPC : shared memory, message passing.

Figure 3.11 Communication models. (a) Shared memory, and (b) Message passing.

 

[  ] Consider the Producer-Consumer Problem.

- to illustrate the concept of cooperating processes.

- a common paradigm for cooperating processes.

 

- A Producer produces info that is consumed by a consumer.

  ex) a complier produces assembly code, and an assembler consumes it.

       a web server produces an HTML file, and a browser consumes it.

 

- A solution using shared-memory :

  To allow producer and consumer to run concurrently.

  Let a buffer of items be available.

  (Producer) can fill the buffer.   (Consumer) can empty the buffer.

 

[★] The scheme of using shared-memory

- requires that these processes share a region of memory and that the code for accessing and manipulation the shared memory - be written explicitly by the application programmer.

P와 C 가 1 by 1 매칭이 되면 괜찮겠지만, 만약 P가 N이고 C가 M일 경우 어떻게 Shared buffer를 구성할 것인가?

이는 전적으로 프로그래머의 몫이다.

 

[★] Message-Passing

- O/S provides the means for cooperating processes to communicate with each other via a message-passing facility.

Message만 던지고 나머지 처리는 OS에게 넘긴다.

 

- The properties of communication links in this scheme( Direct )

  반드시 누구에게 줄 것인지, 누구에게 받을 것인지 명시해 준다. > Links are established automatically.

  There exists exactly one link between each pair of processes.

 

- The properties of communication links in this scheme( Indirect )

  the massages are sent to and received from mailboxes, or ports.

  두 명의 member가 공유할 수 있는 port가 있을 때, 더 많은 process 도 가능하다.

  A number of different links may exist, between each pair of processes with each link corresponding to one port.

 

- Different design options for implementation:

  blocking or non-blocking : synchronous or asynchronous.

  Blocking send : the sender is blocked until the message is received. (Receiver 도 마찬가지)

  Non-blocking send : the sender is sends the message and continue. (Receiver 도 마찬가지)

 

 

[  ] Examples of IPC Systems

- Shared Memory: POSIX Shared Memory. ( POSIX : Portable Operating System Interface (for UNIX)

  OS의 표준화를 꿰차기 위해 만듦.

- Message passing : Pipes ( One of the earliest IPC mechanisms on UNIX systems.)

 

[  ] POSIX shared memory.

- is organized using memory-mapped files, which associate the region of shared memory with a file.

UNIX에서 구현한다면 일일이 Open하고 Write하고 Read하고 Close해주고 등 할 것이 많다.

 

[  ] Pipes

- Pipes were one of the first IPC mechanism in early UNIX systems.

- A pipe acts as a conduit allowing two processes to communicate.

*conduit n. 전달자[기관, 정부], 도관(액체나 기체를 통하게 하는)

 

[  ] Four issue of pipe implementation.

- Does the pipe allow unidirectional or bidirectional communication?

uni(한쪽으로만 전송) VS bi (양쪽 모두 전송)

- In the case of two-way comm., is it half-duplex or full-duplex?

둘이 왔다 갔다 할 수 있는지,  uni pipe를 2개 만들면 간단한 문제.

- Must a relationship exist between the communicating process?

  + such as parent-child. / 사실 주고 받고라서 그럴 필요는 없지만 pipe를 사용하는 편의상 가진다.

- Can the pipes communicate over a network?

  이때는 pipe라 안 부르고, Socket이라 부른다.

 

[  ] Ordinary Pipes

- allow two processes to communicate in producer-consumer fashion.

  the producer writes to one end of the pipe. (write end)

  the consumer reads from the other end (read end)

- unidirectional : only one-way communication is possible.

- two-way communication? use two pipes!

 

Figure 3.20 File descriptors for an ordinary pipe.

 

 

[  ] Two other strategies in client-server system.

- Sockets are defined endpoints for communication.

데이터를 주고 받을 때, 한쪽은 32bit, 한쪽은 64bit 일 경우 둘의 통신에 문제가 발생한다. 일일이 용량별로 보내는 데이터를 결정했다. 그래서 이것을 해결하기 위해 RPCs가 등장했다.

- RPCs ( Remote Procedure Calls )

  abstracts procedure calls between processes on networked systems.

 

[  ] Socket

- A socket is identified by an IP address concatenated with a port number.

 

[  ] Java provides

- Socket class : connection -oriented(TCP)

- DatagramSocket : connectionless(UDP)

- MulticastSocket : multiple recipients.

Java 제공하는

 

[  ] RPC + java에서는 RMI (Remote Method Invocattion) 이라 부른다.

- one of the most common forms of remote service.

- designed as a way to abstract the procedure-call mechanism for use between systems with network connections.

 

[  ] The RPC System

- hides the details that allow communication to take place by providing a stub on the client side.

IPC 의 확장 개념 > RPC는 Network 간에 있는 두 PC끼리 데이터를 주고받는다.