Operating systems. Lecture 3 Interprocess communication. Narcis ILISEI, 2018

Transcription

1 Operating systems Lecture 3 Interprocess communication Narcis ILISEI, 2018

2 Goals for today Race Condition Critical Region Mutex / Monitor Semaphore

3 Inter-process communication (IPC) A mechanism that allows access to shared data Race condition: Two processes want to access shared memory at the same time. Source: Modern operating systems - A.Tannenbaum, 3 rd edition, Pearson Prentice Hall

4 Critical Regions A piece of code that it is only run by one thread at any given time It is a good instrument to be used against race conditions

5 Strict Alternation One type of mutual exclusion implementation Does not guarantee progress! No Progress == The critical region is free AND a process wants it, BUT it cannot enter it. Example: If Process 1 is inside the noncritical_region() for a long time, Then Process 2 may finish its turn and wish to run again, but it cannot. Strict alternation solution to critical region problem. (a) Process 0. (b) Process 1 Source: Modern operating systems - A.Tannenbaum, 3 rd edition, Pearson Prentice Hall

6 "Test and Set" Instruction (TSL) Hardware support for critical region implementation TSL = a CPU instruction used to write to a memory location AND return its old value as a single, non interruptible operation Entering and leaving a critical region using the TSL instruction.

7 Busy waiting, spinlock Busy-waiting or spinning is a technique in which a thread repeatedly checks to see if a condition ( lock ) is true EnterRegion(int *LOCK) { while (test_and_set(lock) == 1); } Spinlock = acquiring of a lock using busy waiting CPU intensive but efficient from the OS scheduler point of view

8 Critical region implementation with TSL instruction function EnterCriticalRegion(boolean *ptr_shared_mem) { } while (test_and_set(*ptr_shared_mem) == 1) {; //loop} function ExitCriticalRegion(boolean *ptr_shared_mem) { *ptr_shared_mem = 0; }

9 Mutex Special OS object used to implement exclusive access to a sequence of code. Mutex objects can be: Process wide: in PThreads System wide: in Win32, C# Mutex objects support: lock() / acquire() / down() unlock() / release() / up()

10 Mutex Mutexes have two basic operations, lock and unlock Implementation of mutex lock and mutex unlock. Mutexes can be shared by threads only, or can be system wide. (inter-process locking) Source: Modern operating systems - A.Tannenbaum, 3 rd edition, Pearson Prentice Hall

11 Race condition solution with mutex THREAD 1 pthread_mutex_lock (&mut); a = data; a++; data = a; pthread_mutex_unlock (&mut); THREAD 2 pthread_mutex_lock (&mut); b = data; b--; data = b; pthread_mutex_unlock (&mut);

12 Mutexes in Pthreads Process wide objects. Some of the Pthreads calls relating to mutexes.

13 Condition variables Special OS objects used to signal to a thread that a variable has changed. Benefit: No need of polling loops Usage in conjunction with a Mutex Operations: pthread_cond_wait (condition, mutex) pthread_cond_signal (condition)

14 Condition variables in POSIX Calls relating to condition variables.

15 Mutexes and conditional variables... Producer-consumer problem solved with mutex Source: Modern operating systems - A.Tannenbaum, 3 rd edition, Pearson Prentice Hall

16 Mutexes in Pthreads Some of the Pthreads calls relating to condition variables.

17 Win32 API Mutex, CriticalRegion handle = CreateMutex (,,Name) OpenMutex() if needed from a different process WaitForSingleObject(handle, timeout) ReleaseMutex(handle) Note: Win32 Mutex is SYSTEM WIDE CriticalRegion object can be used as lightweight Mutex

18 Monitors Special programming language keyword that hides acquisition/release of a mutex. Example: synchronized in Java lock in C# Does not exist in C/C++

19 Semaphores Sempahore = Mutex + max count Mutex = Semaphore with max count = 1. Operations V() / release() / up() / sem_post() > counter ++ P() / acquire() / down() / sem_wait() -> counter -- The semaphore can be acquired max count number of times

20 Semaphores Example... Picture(c): Modern operating systems - A.Tannenbaum, 3 rd edition, Pearson Prentice Hall The producer-consumer problem using semaphores.

21 Semaphores vs Mutexes Semaphores can replace mutexes Semaphores are system wide object, mutexes are local to processes In general mutexes are smaller and faster than semaphores

22 Example Multi threaded matrix calculation

23 Summary Race Condition Critical Region Mutex Semaphore