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Tuesday, June 13, 2006

... Our continuing mission: To seek out knowledge of C, to explore strange

UNIX commands, and to boldly code where no one has man page 4

- Brett L. Huber

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Announcement

Assignment 1 is up on the website.

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Output?int main(int argc, char** argv){ int session = 0; char str[] = "a = "; char ptr[] = "x = "; printf("Start.\n"); ++session; fork(); printf("%s %i . \n", str, session); fork(); printf("%s %i . \n", ptr, session);}

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Interprocess Communication

Pipes

Shared Memory

Message Queues

Sockets

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Pipescat file1 file2 | sortOnly processes with common ancestorOne-way communicationData write order

What is write end of pipe connected to?

What is read end of pipe connected to?

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Pipes

pipe() system call

Takes an array of 2 integers

int pipesfd[2];

int ret=pipe(pipesfd);

if (ret==-1){

perror(“pipe”);

exit(1);

}

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Pipes

pipesfd[0] contains the read file descriptor

pipesfd[1] contains the write file descriptor

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int main(int argc, char* argv[]){ int data_pipe[2]; int pid; int rc; rc = pipe(data_pipe); if (rc == -1) { perror("pipe"); exit(1); } pid = fork(); switch (pid) { case -1:/* fork failed. */ perror("fork"); exit(1); case 0:/* inside child process. */ do_child(data_pipe); default:/* inside parent process. */ do_parent(data_pipe); } return 0;}

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void do_parent(int data_pipe[]){ char ch; int rc;

close(data_pipe[0]);

while ((ch = getchar()) > 0) { /* write the character to the pipe. */ rc = write(data_pipe[1], &ch, 1); if (rc == -1) { perror("Parent: write"); close(data_pipe[1]); exit(1); } }

close(data_pipe[1]); exit(0);}

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void do_child(int data_pipe[]) {

char ch;/*data received from parent*/ int rc;/* return status of read(). */

close(data_pipe[1]);

while ((rc = read(data_pipe[0], &ch, 1)) > 0) {

putchar(ch); } close(data_pipe[0]);

exit(0);}

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Two way communication ...

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Named Pipes

Limitation of anonymous pipes: related

processes

IPC in Unrelated Processes?

Named Pipe or FIFO

If file opened for reading, it is blocked until

another process opens it for writing.

• Named pipes use between two processes on same system.

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Named Pipes

mknod sample p

prw-r--r--

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int main(int argc, char* argv[]){ FILE* pfile; while (1) { pfile = fopen("/export/home/sample", "w");

if (!pfile) { perror("fopen"); exit(1); } fprintf(pfile, "hello from write process"); fclose(pfile);

sleep(1); } return 0;}

writefifo.c

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int main(int argc, char* argv[]){ char buf[200]; FILE* pfile; while (1) { pfile = fopen("/export/home/sample", "r"); if (!pfile) { perror("fopen"); exit(1); } fgets(buf, 200, pfile); printf("%s\n", buf);

sleep(1); } return 0;}

readfifo.c

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Run the previous as separate processes …

Another way of making named pipes:

int mkfifo(const char *path, mode_t mode);

Upon successful completion, 0 is returned. Otherwise -1 is returned and errno is set to indicate the error.

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Message Queues

With pipes Parse bytes FIFO

Message queues can be public or private “Message based” Several processes may read / write to a

queue Messages may be read by type

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Sockets

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Threads

Traditionally a process has an address space and a single thread of execution

Process: thread + address space Keeps track of what to execute next Registers hold current working variables It has a stack (execution history)

Threads allow multiple executions to take place in the same process environment

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(a) Three processes each with one thread(b) One process with three threads

Environment (resource) execution

THREADS

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Threads

Multiple threads is analogous to multiple processes running in parallel

Light weight process vs Heavy weight process

Threads share an address space open files other resources

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Threads in the same process share resourcesEach thread executes separately

THREADS

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Threads can be in one of the following states: Running, Blocked Ready Terminated

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Thread idEach thread has its own stack. Why?

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Thread idEach thread has its own stack. Why?

Threads may resume out of order: • Different execution history

• Cannot use a single LIFO stack to save state

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THREADS

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Motivation

Spread sheetsLarge database accessSpell check, grammar checkServersetc.

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BenefitsResponsiveness

Substantial computing + substantial I/O leads to overlap

Resource Sharing

Economy Process creation is time consuming. Solaris

• Creating a process is 30 times slower (In some system 100 times faster)

• Context switch is 5 times slower

Utilization of MP Architectures

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Ability of threads to share a set of resources, so that they can work closely together

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Thread Usage: word processor

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Thread Usage: word processor

Closely collaborate with each other

Threads unavoidable.

Programming model simpler

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Single threaded web server

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Thread Usage: Web Server

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Multithreaded kernels Solaris has a set of threads for interrupt

handling

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Real OS permutations

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Threads

Not independent as processesSame address spaceShare same global variablesCan read, write another thread’s stackNo protection between threads:

What if a thread runs endlessly? Necessary?

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Threads in user space

Threads package entirely in user space Kernel thinks it is managing a traditional single

threaded process Private thread table + scheduling

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A user-level threads package

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Threads in user space

If a thread has to wait for another thread scheduler can choose another ready thread to run

Thread yield No trap needed No context switch etc Scheduler is just like a local procedure and

much faster than making a kernel call

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Threads in user space

Customized scheduling algorithms

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Threads in user space

However, there are disadvantages as well…

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Threads in user space

Blocking system calls?• Read from keyboard

– (e.g. in our word processor example)

• Disk read

No clock interrupts

Multithreaded webserver : threads often block

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Many-to-One

Many User-Level Threads Mapped to Single Kernel Thread.

Used on Systems That Do Not Support Kernel Threads.

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Many-to-one Model

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Threads in Kernel

When a thread wants to create a new thread or terminate an existing one – it makes a kernel call

Kernel thread table• Registers

• State

• Etc

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Threads in Kernel

When thread blocks the kernel can select from same process or different one

Overhead of creation etc is larger

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One-to-One

Each User-Level Thread Maps to Kernel Thread.

Examples

Windows 95/98/NT/XP Linux Solaris 9

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One-to-one Model

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Many-to-many Model

Example- Version older than Solaris 9

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Hybrid Implementations

Multiplexing user-level threads onto kernel- level threads

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Process creation is time consuming

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Library procedures to: Create threads Exit when finished Wait for another thread Yield – voluntarily give up CPU