Operating Systems

1. Operating Systems Practical Session 4Threads

2. Threads • Executed within a process. • Allow multiple independentexecutions 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).

3. Threads - Advantages • Shareopen 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

4. 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.

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

6. Threads - motivation • Dispatcher thread: • while (TRUE) { • get_next_request(&buf); • handoff_work(&buf); • } Dispatcher • 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); • } Workers Web page cache Webpage request Why are threads better in this case? Example from “Modern Operating Systems”, 2nd Edition, pg. 88

7. 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.

8. User-level and Kernel-level Threads Threads Library User space User space Kernel space Kernel space P P (a) Pure user-level (b) Pure kernel-level

9. User-level and Kernel-level Threads • Threads • Library User space Kernel space P P (c) Combined Source: Stallings, Operating Systems: Internals and design principles 7th ed.

10. 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.

11. User-level threads (cont’d) • User-level threads implement in user-level libraries, rather than via systems calls, so thread switching does not need to call operating system and to cause interrupt to the kernel. • The kernel’s inability to distinguish between user level threads makes it difficult to design preemptive scheduling. • If a thread makes a blocking system call, the entire process is blocked. • Will only utilize a single CPU.

12. 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.

13. User-level vs. kernel-level threads

14. 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. • 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 …

15. Threads in POSIX (pthreads)

16. Threads in POSIX (pthreads) – cont’d

17. Hello World! When compiling a multi-threaded app: gcc–D_REENTRANT –o myprogmyprog.c –lpthread #include <pthread.h> #include <stdio.h> void *printme() { printf("Hello World!\n"); return NULL; } void main() { pthread_ttcb; 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); } } What can happen if we remove the join part?

18. Example A – Version 1 void *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); } }

19. Example A – Version 1possible output Trying 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

20. Example A – Version 2 void *printme(void *id) { int*i; i = (int*)id; printf("Hi. I'm thread %d\n", *i); pthread_exit(NULL); } void main() { inti, vals[4]; pthread_ttids[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); }

21. Example A – Version 2possible output Trying 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

22. Example A – Version 3 void *printme(void *id) { int*i; i = (int*)id; printf("Hi. I'm thread %d\n", *i); pthread_exit(NULL); } void main() { inti, vals[4]; pthread_ttids[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); } }

23. 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

24. Example A – Version 4 void *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; }

25. Example A – Version 4possible output No Output!

26. Example A – Version 5 void *printme(void *id) { int*i; i = (int*)id; printf("Hi. I'm thread %d\n", *i); exit(0); } main() { inti, vals[4]; pthread_ttids[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); }

27. Example A – Version 5possible output Trying to join with tid0 Hi. I'm thread 0

28. Midterm – 2006 בעץ תהליכים כל קודקוד מייצג תהליך.קודקודg מצביע על קודקודqאם"םg הוא אבא של q, כלומר אם g יצר את q. g q (א) שרטטו את עץ התהליכים הנוצר ע"י הרצת הקוד הבא בשפת C. (תנו שמות שרירותיים לתהליכים הנוצרים.) 1. int x; 2. fork(); 3. x = fork(); 4. if(x != 0) • fork(); • 7. printf(“pid= %d”,getpid());

29. Midterm – 2006 (cont’d) פתרון (א): 1 5 4 2 3 6

30. Midterm – 2006 (cont’d) ב. מהו הפלט של הרצת התוכנית מסעיף א'? האם זהו הפלט היחיד האפשרי? הסבירו. (עד 3 שורות). פתרון (ב):שישה מספרים גדולים מ 0. הפלט אינו יחיד, כל שישה מספרים נכונים. ג. אם בין שורות 4 ו 6 נוסיף את השורה: 5. kill(x, SIGINT); מה ישתנה בעץ התהליכים ובפלט? פתרון (ג): התהליכים 3 ו 4 ימותו. הפלט עשוי להישאר זהה או שיודפסו רק 5 מספרים או רק 4 מספרים.

31. Midterm – 2006 (cont’d) 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. פתרון (ד): כן, יתכן כזה תסריט. נניח שהמתזמן נותן לכל בן שנוצר ב- forkלרוץ עד אשר הוא מסיים, הרי שכל אחד יספיק להגיע לשורת ההדפסה.

32. Midterm – 2006 (cont’d) ה. נניח כי תידרשו לכתוב תוכנית מרובת threads, שתרוץ על מערכת הפעלה התומכת גם ב-user threads וגם ב-kernel threads. באיזו אפשרות תבחרו אם ה-threads מבצעים פעולות I/O רבות? הסבירו (עד 3 שורות). הסבירו באילו נסיבות (כלומר, עבור איזה סוג תוכנית) הייתם בוחרים באפשרות השנייה. פתרון (ה): פעולת I/O גורמת ל user threads כולם לעבור ל blocking שכן מערכת ההפעלה לא מודעת לקיומם ולכן לא סביר לבחור באופציה זו במקרה של ריבוי פעולות I/O. לעומת זאת, כדאי לבחור ב user threads במקרים בהם רוצים למשל שליטה מלאה על התזמון. בנוסף, אם מדובר במערכת עם יחסית מעט מעבדים נעדיף user threads שכן החלפה ביניהם היא מהירה יותר.

33. 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.

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

35. Thread-specific data (cont’d)

36. Thread-specific data (cont’d)

37. 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.

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

39. void* thread_function (void* args) { charthread_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 () { inti; 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; }