Processes COMPSCI 386

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1 Processes COMPSCI 386

2 Elements of a Process A process is a program in execution. Distinct processes may be created from the same program, but they are separate execution sequences. call stack heap STACK data section (globals) text section (code) HEAP DATA TEXT

3 States of a Process As a process executes, it changes state. new (being created) running (currently executing) waiting (for an event) ready (waiting for CPU) terminated scheduler dispatch running interrupt or trap terminated exit I/O pending new ready waiting I/O completion

4 Process Control Block process state process number register contents I/O status info devices allocated open files accounting info time used time limits CPU scheduling info process priority pointers to queues memory management info base and limit registers page tables In Linux, PCB implemented as task_struct in linux/sched.h

5 Process Scheduler Part of the OS that enables time sharing. New processes spooled to a job queue on disk. Long-term scheduler selects processes to be loaded into the ready queue. Controls degree of multiprogramming. Good mix of I/O-bound and CPU-bound processes. Short-term (CPU) scheduler selects process from ready queue to execute. Must be very fast invoked every few milliseconds. Typical time for context switch: a few milliseconds Hardware support with multiple register sets Each device has its own queue of waiting processes.

6 Migration Among Queues READY QUEUE CPU I/O I/O QUEUE I/O REQUEST TIME SLICE EXPIRED CHILD FORKED INTERRUPT

7 Process Creation A process identifier (pid) is assigned A process can fork children, so there is a natural tree structure of processes in the system. Child may share all, some, or none of parent s resources. Parent and child may execute concurrently, or parent may wait until child terminates. Consider: gedit filename vs gedit filename &

8 Process Termination What happens to child of process that terminates? In UNIX, adopted by init process (pid = 1). Reasons to terminate child: Child has exceeded allotted limits (e.g., time). Task to be performed by child no longer needed. Some systems to not allow child to continue if parent has terminated (cascading termination).

9 IPC: Shared Memory Producer creates region within its address space to be shared with consumers. Consumers attach the memory to own address space. Processes responsible for: formatting data synchronizing data Fast and relatively simple from OS point of view. More complicated from programmer s point of view.

10 Shared Memory in C const int N = 4096; // create shared memory object and configure size int shm_fd = shm_open("os", O_CREAT O_RDWR, 0666); ftruncate(shm_fd, N); // memory map the shared memory object void* ptr = mmap(0, N, PROT_WRITE, MAP_SHARED, shm_fd, 0); char* msg = "Hello"; sprintf(ptr, "%s", msg); ptr += strlen(msg); msg = "World!"; sprintf(ptr, "%s", msg); ptr += strlen(msg); Code for producer. Must be linked with rt library.

11 Shared Memory in C const int N = 4096; // create shared memory object int shm_fd = shm_open("os", O_RDONLY, 0666); // establish memory mapped file for shared memory object void* ptr = mmap(0, N, PROT_READ, MAP_SHARED, shm_fd, 0); printf("%s\n", (char*) ptr); shm_unlink("os"); Code for consumer. Must be linked with rt library.

12 IPC: Message Passing Processes must establish communication link. Implementation issues: capacity of link, one-way or bi-directional, fixed- or variable-length messages. Once the link established, processes simply invoke send and receive operations. Extra overhead: system call for each send/receive. Life easier for programmers: OS worries about data format and synchronization. Convenient for distributed systems where processes may be executing on different machines.

13 IPC in Mach Part of macos How does Mach avoid typical performance hit in message passing systems due to double copying of messages (sender mailbox, maibox receiver)? Virtual memory techniques. Maps sender s message directly into receiver s address space. Shared memory under the covers, with illusion of message passing through abstract mailboxes. Only works for IPC on a single machine.

14 Client-Server Communications Sockets (a communication endpoint) Specified by IP address and port: :1625 Servers listening for http, telnet, ftp requests or other standard services use ports numbered less than Web servers, for example, listen on port 80. Efficient, but only raw byte streams exchanged. Process must impose structure on the data. Remote procedure calls Remote method invocation (java.rmi)

15 Explore in Code Process management in C fork, exec, wait getpid, getppid redirecting standard output stream communicating with pipes Signals in UNIX Creating processes in Java Process and ProcessBuilder Socket programming in Java?

16 wait When a parent calls wait, it blocks until a child that it forked terminates. If process has no children, call to wait does nothing. int status; pid_t pid = wait(&status);

17 Orphans and Zombies Orphan Parent of a process terminates. Orphaned process adopted by init (pid = 1), which simply waits perpetually on its children. Zombie Child terminates before parent calls wait. Terminated state. Entry remains in process table. Allows parent to read child s exit status Reaped (entry removed from process table)

18 File Descriptors Every process starts with three open files. stdin 0 stdout 1 stderr 2 We can write to cerr in Java and C/C++. The open call returns a file desciptor, which is just an index into a file descriptor table in kernel space. The table allows read/write calls to be mapped to a file or file-like object. Abstraction: everything is a file.

19 File Descriptors

20 Pipes Connects output of one process to input of another. UNIX philosophy: chaining small tools together. The transfer takes place via a buffer in kernel space. But why bother with pipes? The producer could just open a file for writing and the consumer could open the same file for reading. No special mechanism is needed...?

21 Pipes Producer blocks on write if the pipe is full. Consumer blocks on read if the pipe is empty. If not empty, a call to read drains the buffer. A pipe is a pair of file descriptors, one for writing and one for reading. The kernel buffer is unnamed. It can only be accessed via these descriptors. If parent and child do not close off unused ends of a pipe, EOF will never be returned by read/write.

22 Redirection Basic idea (for output) int fd = open( output.txt, O_CREAT O_WRONLY, 0666) close(1) creates an open slot in kernel s FD table. dup(fd) puts file descriptor for output.txt in first open slot. Child process inherits copy of parent s fd table. But if pipes or redirection are being used for parent/child IPC, we really want to use dup instead of open. A call to dup creates a reference in child s fd table to existing entry in global table. A call to open would create a new entry in global table that refers to same file object. But think about file pointer!

23 Parent/Child IPC If pipes or redirection are being used for parent/child IPC, we really want to use dup instead of open file obj file obj dup open

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