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1 Chapter 3: Process Concept Lecture 3, 9.30.2008 Hao-Hua Chu.

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Presentation on theme: "1 Chapter 3: Process Concept Lecture 3, 9.30.2008 Hao-Hua Chu."— Presentation transcript:

1 1 Chapter 3: Process Concept Lecture 3, 9.30.2008 Hao-Hua Chu

2 2 A fun project: Lucid Touch (MSR) What more can we extend from multi-touch UI? –Use both front and back surfaces for interaction

3 3 Menu (are you hungry?) What is a process? –Process lifecycle, process state, process control block, process vs. thread Process scheduling –Long-term vs. short-term schedulers, ready vs. I/O queues, context switching Interprocess communication –Shared memory vs. message passing Client-server communication –Socket, RPC vs. RMI

4 4 Process overview What is a program? –A file containing instructions to be executed What is a process (also called a “job”)? –A program in execution on the computer What resources does OS provide to run a process (a web browser process)? –CPU, Memory, file (I/O), network (I/O) What is a process made up of in memory? What is the lifecycle of a process?

5 5 Process in Memory Text: program code Stack: temporary data (local variable, function parameters, return addresses) Data: global variables Heap: dynamic allocated memory, malloc() Process info not kept in memory: –Program counters & registers

6 6 Process Lifecycle What is the process lifecycle (and the 5 process state)? –Think about a web browser from birth to death …

7 7 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 process –terminated: The process has finished execution Multiprogramming! –What is the benefit? –What is the cost?

8 8 Process Control Block (PCB) What info does the system need to track for each process? –Save & reload each time the process goes in/out of the “running” state.

9 9 Process Control Block (PCB) What info does the system need to track for each process? –Save & reload each time the process goes in/out of the “running” state. –CPU-related –Memory-related –I/O related –Accounting –See 3.1.3 for a complete list

10 10 Process vs. Thread Why thread? –Say you start several web browsers, each with its own process. –What is the “memory” overhead? –Is there a way to reduce this memory overhead? –What memory sections can be shared vs. cannot be shared?

11 11 Process Scheduling Go back to multiprogramming What is the benefit of multiprogramming? –At the system-level: Maximize CPU utilization –At the user-level: Users can interact with several programs almost at the same time. What is the cost of providing multiprogramming? –Consider that only one running process per processor –Scheduling (overhead): decide the next “ready” process to run and dispatch it. Let’s look at scheduling data structure (& algorithms) –Called “queues”

12 12 Ready and I/O Queues 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 13 Process Migration among Queues

14 14 Process Scheduler Short-term scheduler –Select the next ready process to execute on CPU Also called CPU scheduler –What makes a good short-term scheduling algorithm (more on Ch5)? Long-term scheduler (in a batch system) –Select process(es) into the memory (and the ready queue) –Keep short-term schedule busy! –How does it differ from short-term scheduler? –What makes a good long- term scheduling algorithm?

15 15 Long vs. Short-term Schedulers Short-term scheduler is invoked very frequently (milliseconds)  (must be fast) –Context switch processes overhead is fast but significant –Say 1 ms, out of time quantum 20 ms -> Overhead 5% Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow) –Swapping processes (between disk and memory) is slow The long-term scheduler controls the degree of multiprogramming (# of processes in memory) What makes a “good” degree of multiprogramming?

16 16 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 Want to have a good process mix of both –Keep both CPU and I/O busy! What happen when most processes are I/O bound? What happen when most processes are CPU bound?

17 17 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 Context-switch time is overhead; the system does no useful work while switching Time dependent on hardware support

18 18 Process Operations Review from “System programming class”. Process creation –Parent-child tree relationship –Process identifier called PID –Resource sharing Process termination

19 19 Process Creation Parent process create children processes, which, in turn create other processes, forming a tree of processes 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

20 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

21 Process Creation

22 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"); exit(0); }

23 A tree of processes on a typical Solaris

24 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

25 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 –Communicate or share data –Need for OS support on interprocess communication (IPC)

26 Two IPC Mechanisms How can two (or more) processes communicate data? –Shared memory and message passing

27 Shared memory vs. Message passing What is shared memory? –Create a “shared memory region” between two processes –Exchange data by reading and writing to this shared region. What is message passing? –Create mailboxes between two processes to exchange data No perfect thing … –What are their strengths and weaknesses? –Think performance & concurrency –Systems people call it “Tradeoff”

28 Shared-memory vs. Message- passing Shared memory –OS does less work –Only one-time setup overhead –Fast communication, same as memory access time –Concurrent write to shared memory by both processes? Message passing –OS does more work –Data double-copy –No concurrent read & write problem

29 Producer-Consumer Problem As an example to illustrate the “concurrency problem” for shared memory 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

30 Bounded-Buffer: Shared-Memory Solution Shared data (in the shared memory) #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

31 Bounded-Buffer: Concurrent access to shared memory buffer (2 producers) while (true) { /* Produce an item */ while (((in + 1) % BUFFER_SIZE) == out) ; /* do nothing -- no free buffers */ buffer[in] = item; in = (in + 1) % BUFFER SIZE; } Remove() while (true) { while (in == out) ; // do nothing -- nothing to consume // remove an item from the buffer item = buffer[out]; out = (out + 1) % BUFFER SIZE; return item; } Insert()

32 Message-Passing IPC facility provides 2 basic 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 details … –Direct vs. indirect communication –Synchronous vs. asynchronous communication –Buffering size

33 Message-Passing Implementation Details Direct vs. indirect communication –Direct: send (P, message) & receive (Q, message) –Disadvantage: when either P/Q dies, re-establish communication –How to solve this issue? –Indirection using a “mailbox” or “port” – send (A, message) & receive (A, message), where A is a mailbox Synchronous vs. asynchronous communication –Blocking/noblocking send/receive Buffering size –Zero, bounded, and unbounded capacity –How to implement zero or bounded capacity? –What are their advantages & disadvantages?

34 Client-Server Communication Process communication across two different computers –Not IPC (on the same machine) Sockets Remove procedure call (RPC) Remote method call (RMI) in Java

35 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

36 Remote Procedure Calls (RPC) Call the function of a process running on another machine –Calling a remote function as easy as calling a “local function” –Used extensively in distributed file system (calling file servers) 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.

37 Execution of RPC

38 Remote Method Invocation (RMI) Java objected-oriented version of RPC –RMI allows a Java program on one machine to invoke a method on a remote object. –What does “OO” buy RMI over RPC?

39 Marshalling Parameters

40 RPC (procedure-based) vs. RMI (object- based) What are the differences between RPC and RMI? –RMI can pass of (remote) objects as parameters to remote methods –RMI can invoke remote methods on these (remote) objects –RPC cannot do both

41 41 Summary Process overview –Process lifecycle, process state, process control block, process vs. thread Process scheduling –Long-term vs. short-term schedulers, ready vs. I/O queues, context switching Interprocess communication –Shared memory vs. message passing Client-server communication –Socket, RPC vs. RMI

42 End of Chapter 3


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