Operating Systems

Practical Session 3 Threads

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Threads

• Executed within a process.• Allow multiple independent executions under the

same process (container).• Possible states: running, ready, blocked, terminated. • In most of today’s operating systems, a process is

created with at least one thread but may have more than one thread (multithreading).

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Threads - Advantages

• Share open files, data structures, global variables, child processes, etc.

• Peer threads can communicate without using System calls.

• Threads are faster to create/terminate/switch than processes (have no resources attached).

• Parallelism which improves overall performance:– A single core CPU and a substantial amount of computing

and I/O– Multiple cores

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Threads - Disadvantages

• Share open files, data structures, global variables, child processes, etc.

• No protection between threads – one can read/write/wipe out/corrupt the other’s data.

• Sending some signals (such as SIGSTOP) to a process affects all threads running within it.

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Threads vs. Processes(“classic” approach – Linux’s clone results in some ambiguity)

Threads Processes

shared data unique data

shared code unique code

shared open I/O unique open I/O

shared signal table unique signal table

unique stack unique stack

unique PC unique PC

unique registers unique registers

unique state unique state

light context switch heavy context switch

Signal handlers must be shared among all threads of a multithreaded application; however, each thread must have its own mask of pending and blocked signals (POSIX 1003.1).

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Threads - motivation

Web page cache

Workers

Dispatcher

Webpage request

Dispatcher thread:while (TRUE) { get_next_request(&buf); handoff_work(&buf);}

Worker thread:while (TRUE) { wait_for_work(&buf); look_for_page_in_cache(&buf, &page); if (page_not_in_cache(&page)) read_page_from_disk(&buf, &page); return page(&page); }

Example from “Modern Operating Systems”, 2nd Edition, pg. 88

Why are threads better in this case?

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Threads – some known Issues

• Does the fork() command duplicate just the calling thread or all threads of the process? – POSIX defines that only the calling thread is replicated in the

child process.– Solaris 10 defines fork1() and forkall() which attempt to

better define the relation between fork and threads, however, this is not POSIX compliant.

– Many issues arise from using fork in a multithreaded code. Unless calling exec immediately after fork, try to avoid it!

• Does the exec() command replace the entire process?– The entire process is replaced including all its threads.

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User-level and Kernel-level Threads

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ThreadsLibrary

P

User space

Kernelspace

P

User space

Kernelspace

(a) Pure user-level (b) Pure kernel-level

ThreadsLibrary

User-level and Kernel-level Threads

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P

User space

Kernelspace

(c) Combined

P

Source: Stallings, Operating Systems: Internals and design principles 7th ed.

User-level threads

• The kernel sees just the main thread of the process (all other threads that run within the process’ context are “invisible” to the OS).

• The user application – not the kernel – is responsible for scheduling CPU time for its internal threads within the running time scheduled for it by the kernel.

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User-level threads (cont’d)

• The kernel’s inability to distinguish between user level threads makes it difficult to design preemptive scheduling.– When context switching is made directly towards the

entire process, clock interrupts are usually used for this purpose.

– User level thread will usually have to voluntarily give up the CPU.

• If a thread makes a blocking system call, the entire process is blocked.

• Will only utilize a single CPU.11

Kernel-level threads

• All threads are visible to the kernel.• The kernel manages the threads.

• The kernel schedules each thread within the time-slice of each process.

• The user cannot define the scheduling policy.• Context switching is slower for kernel threads than

for user-level threads.• Because the kernel is aware of the threads, in

multiple CPU machines, each CPU can run a different thread of the same process, at the same time.

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Kernel-level threads User-level threads

Visible to the kernel Invisible to the kernel Threads

Kernel defined User defined Scheduling policy

Preemptive Non-preemptive* Thread switching

Slower, done by the kernel Faster, done by the runtime Context switch

Block the single thread Block the whole process Blocking calls

Held by the kernel Held by the process Thread table

User-level vs. kernel-level threads

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A tech. note on POSIX threads

• When the first Unix and POSIX functions were designed, it was assumed that there will be a single thread of execution.

• Consider a naïve implementation of errno in a multi threaded environment for example. – Hence, the need for reentrant functions.– While this is supported by many standard functions, the

compiler must be aware of the need for re-entrant functions:

gcc –D_REENTRANT –lpthread …

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Threads in POSIX (pthreads)int pthread_create( pthread_t* thread, pthread_attr_t* attr, void* (*start_func)(void*) , void* arg)Creates a new thread of control that executes concurrently with the calling thread.

On success, the identifier of the newly created thread is stored in the location pointed by the thread argument, and a 0 is returned. On error, a non-zero error code is returned.attr specifies thread attributes that will be applied to the new thread (e.g. detached, scheduling-policy). Can be NULL (default attributes).start_func is pointer to the function the thread will start executing; start_func receives one argument of type void* and returns a void*. arg is the parameter to be given to func.

pthread_t pthread_self()

return this thread’s identifier.

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Threads in POSIX (pthreads) – cont’dint pthread_join( pthread_t th, void** thread_return )

Suspends the execution of the calling thread until the thread identified by th terminates.On success, the return value of th is stored in the location pointed by thread_return, and a 0 is returned. On error, a non-zero error code is returned.At most one thread can wait for the termination of a given thread. Calling pthread_join on a thread th on which another thread is already waiting for termination returns an error. th is the identifier of the thread that needs to be waited for.

thread_return is pointer to the returned value of the th thread (can be NULL).

void pthread_exit( void* ret_val )

Terminates the execution of the calling thread. Doesn’t terminate the whole process if called from the main function.If ret_val is not null, then ret_val is saved, and its value is given to the thread who performed join on this thread; that is, it will be written to the thread_return parameter in the pthread_join call.

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Hello World!#include <pthread.h>

#include <stdio.h>

 

void *printme() {

printf("Hello World!\n");

return NULL;

}

 

void main() {

pthread_t tcb;

void *status;

if (pthread_create(&tcb, NULL, printme, NULL) != 0) {

perror("pthread_create");

exit(1);

}

if (pthread_join(tcb, &status) != 0) {

perror("pthread_join");

exit(1);

}

}

When compiling a multi-threaded app:gcc –D_REENTRANT –o myprog myprog.c –lpthread

What can happen if we remove the join part?

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Example A – Version 1void *printme(void *id) { int *i; i = (int *)id; printf("Hi. I'm thread %d\n", *i); return NULL;}

void main() { int i, vals[4]; pthread_t tids[4]; void *retval; for (i = 0; i < 4; i++) { vals[i] = i; pthread_create(tids+i, NULL, printme, vals+i); } for (i = 0; i < 4; i++) { printf("Trying to join with tid%d\n", i); pthread_join(tids[i], &retval); printf("Joined with tid%d\n", i); }}

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Example A – Version 1possible outputTrying to join with tid0 Hi. I'm thread 0 Hi. I'm thread 1 Hi. I'm thread 2 Hi. I'm thread 3 Joined with tid0 Trying to join with tid1 Joined with tid1 Trying to join with tid2 Joined with tid2 Trying to join with tid3 Joined with tid3 19

Example A – Version 2void *printme(void *id) { int *i; i = (int *)id; printf("Hi. I'm thread %d\n", *i); pthread_exit(NULL);}

void main() { int i, vals[4]; pthread_t tids[4]; void *retval; for (i = 0; i < 4; i++) { vals[i] = i; pthread_create(tids+i, NULL, printme, vals+i); } for (i = 0; i < 4; i++) { printf("Trying to join with tid%d\n", i); pthread_join(tids[i], &retval); printf("Joined with tid%d\n", i); }

pthread_exit(NULL);} 20

Example A – Version 2possible outputTrying to join with tid0 Hi. I'm thread 0 Hi. I'm thread 1 Hi. I'm thread 2 Hi. I'm thread 3 Joined with tid0 Trying to join with tid1 Joined with tid1 Trying to join with tid2 Joined with tid2 Trying to join with tid3 Joined with tid3 21

Example A – Version 3void *printme(void *id) { int *i; i = (int *)id; printf("Hi. I'm thread %d\n", *i); pthread_exit(NULL);}

void main() { int i, vals[4]; pthread_t tids[4]; void *retval; for (i = 0; i < 4; i++) { vals[i] = i; pthread_create(tids+i, NULL, printme, vals+i); }

pthread_exit(NULL); for (i = 0; i < 4; i++) { printf("Trying to join with tid%d\n", i); pthread_join(tids[i], &retval); printf("Joined with tid%d\n", i); }} 22

Example A – Version 3output

Hi. I'm thread 0 Hi. I'm thread 1 Hi. I'm thread 2 Hi. I'm thread 3

If the main thread calls pthread_exit(), the process will continue executing until the last thread terminates or the whole process is terminated

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Example A – Version 4void *printme(void *id) { int *i = (int *)id; sleep(5); printf("Hi. I'm thread %d\n", *i); pthread_exit(NULL);}

int main() { int i, vals[4]; pthread_t tids[4]; void *retval; for (i = 0; i < 4; i++) { vals[i] = i; pthread_create(tids+i, NULL, printme, vals+i); }

return 0;}

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Example A – Version 4possible output

No Output!

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Example A – Version 5void *printme(void *id) { int *i; i = (int *)id; printf("Hi. I'm thread %d\n", *i); exit(0);}

main() { int i, vals[4]; pthread_t tids[4]; void *retval; for (i = 0; i < 4; i++) { vals[i] = i; pthread_create(tids+i, NULL, printme, vals+i); }

for (i = 0; i < 4; i++) { printf("Trying to join with tid%d\n", i); pthread_join(tids[i], &retval); printf("Joined with tid%d\n", i); }

pthread_exit(NULL);} 26

Example A – Version 5possible output

Trying to join with tid0 Hi. I'm thread 0

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Midterm – 2006

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מצביע על g. קודקוד בעץ תהליכים כל קודקוד מייצג תהליך.q יצר את g, כלומר אם q הוא אבא של g אם"ם qקודקוד

g

q

שרטטו את עץ התהליכים הנוצר ע"י )א( . (תנו שמות Cהרצת הקוד הבא בשפת

;int x .1שרירותיים לתהליכים הנוצרים.)2. fork();3. x = fork();4. if(x != 0)6. fork();7. printf(“pid= %d”,getpid());

Midterm – 2006 (cont’d)

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2

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1

:פתרון )א(

Midterm – 2006 (cont’d)

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ב. מהו הפלט של הרצת התוכנית מסעיף א'? האם זהו הפלט היחיד

שורות). 3 האפשרי? הסבירו. (עד  

. הפלט אינו יחיד, כל 0שישה מספרים גדולים מ :פתרון )ב( מספרים נכונים.שישה

  

;kill(x, SIGINT). 5 נוסיף את השורה: 6 ו 4ג. אם בין שורות מה ישתנה בעץ התהליכים ובפלט?

  ימותו. הפלט עשוי להישאר זהה או 4 ו 3: התהליכים פתרון )ג( 

מספרים.4 מספרים או רק 5 שיודפסו רק

Midterm – 2006 (cont’d)

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ד. האם ייתכן תסריט שבו לאחר השינוי נקבל פלט זהה לפלט אותו קיבלנו לפני השינוי? אם כן, מהו תסריט זה? אם לא, נמקו

מדוע לא יתכן כי נקבל פלט זהה. 

: כן, יתכן כזה תסריט. נניח שהמתזמן נותן לכל בן שנוצר פתרון )ד(ב-

fork לרוץ עד אשר הוא מסיים, הרי שכל אחד יספיק להגיע לשורת ההדפסה.   

Midterm – 2006 (cont’d)

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, שתרוץ על threadsה. נניח כי תידרשו לכתוב תוכנית מרובת kernel וגם ב-user threadsמערכת הפעלה התומכת גם ב-

threads-באיזו אפשרות תבחרו אם ה .threads מבצעים פעולות I/O שורות). הסבירו באילו נסיבות (כלומר, 3 רבות? הסבירו (עד

עבור איזה סוג תוכנית) הייתם בוחרים באפשרות השנייה. 

: פתרון )ה( שכן blocking כולם לעבור ל user threads גורמת ל I/Oפעולת

מערכת ההפעלה לא מודעת לקיומם ולכן לא סביר לבחור . I/Oבאופציה זו במקרה של ריבוי פעולות

במקרים בהם רוצים user threadsלעומת זאת, כדאי לבחור ב למשל שליטה מלאה על התזמון. בנוסף, אם מדובר במערכת עם

שכן החלפה ביניהם user threadsיחסית מעט מעבדים נעדיף היא מהירה יותר.

 

Thread-specific data

• Programs often need global or static variables that have different values in different threads: Thread-specific data (TSD).

• Each thread possesses a private memory block, the TSD area.

• This area is indexed by TSD keys (Map).• TSD keys are common to all threads, but the

value associated with a given TSD key can be different in each thread.

• Defined in POSIX.33

Thread-specific data (cont’d)

• Question: Why can’t we achieve this by using regular variables?

• Because threads share one memory space.• Usage examples:

– Separate log for each thread.– errno variable.

• C99 standard defines __thread storage specifier, which specifies that a variable should be distinct per thread. Supported in GCC since version 3.3.

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Thread-specific data (cont’d)int pthread_key_create(pthread_key_t* key, void (*destr_func)(void*))

Allocates a new TSD key. Return 0 on success and a non-zero error code on failure.

key the key is stored in the location pointed to by key.

destr_func if not NULL, specifies a destructor function associated with the key. When a thread terminates via pthread_exit, destr_func is called with arguments – the value associated with the key in that thread. The order in which destructor functions are called at thread termination time is unspecified.

int pthread_key_delete(pthread_key_t key)

Deallocates a new TSD key. Return 0 on success and a non-zero error code on failure.

It does not check whether non-NULL values are associated with that key in the currently executing threads, nor call the destructor function associated with the key.key the key of the value to delete.

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Thread-specific data (cont’d)

int pthread_setspecific(pthread_key_t key, const void* pointer)

Changes the value associated with key in the calling thread, storing the given pointer instead.

void* pthread_getspecific(pthread_key_t key)

Returns the value currently associated with key in the calling thread, or NULL on error.

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TSD Usage example

Suppose, for instance, that your application divides a task among multiple threads. For audit purposes, each thread is to have a separate log file, in which progress messages for that thread's tasks are recorded.

The thread-specific data area is a convenient place to store the file pointer for the log file for each individual thread.

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#include <malloc.h> #include <pthread.h> #include <stdio.h> // The key used to associate a log file pointer with each thread. static pthread_key_t thread_log_key; // Write MESSAGE to the log file for the current thread.void write_to_thread_log(const char* message) { FILE* thread_log = (FILE*)pthread_getspecific (thread_log_key); fprintf(thread_log, "%s\n", message); } // Close the log file pointer THREAD_LOG. void close_thread_log (void* thread_log) { fclose((FILE*) thread_log); }

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void* thread_function (void* args) { char thread_log_filename[20]; FILE* thread_log;

sprintf(thread_log_filename,"thread%d.log",(int) pthread_self()); thread_log = fopen (thread_log_filename, "w");

pthread_setspecific (thread_log_key, thread_log); write_to_thread_log ("Thread starting.");

/* Do work here... */ return NULL; }

int main () { int i; pthread_t threads[5];

pthread_key_create (&thread_log_key, close_thread_log); for (i = 0; i < 5; ++i)

pthread_create (&(threads[i]), NULL, thread_function, NULL); for (i = 0; i < 5; ++i)

pthread_join (threads[i], NULL); return 0; }

Threads in XV6

• XV6 doesn’t support threads.• Only processes can be created using the fork()

system call.• Thread support can be added to XV6:

– Implementing the clone(…) system call.– Configuring clone(…) to create a process with the same

memory space as its parent process.– Adding some basic threads functionality (create,

terminate, join…)

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