Posix Library ImplementationBased F. Muellers PaperLanguage ApplicationLanguage InterfaceC Language ApplicationPosix thread libraryUnix KernelUnix librariesUser LevelKernel Level1Pthread_join and Pthread_detach functionPthread_join(tid, .) : executing threads blocks waiting for the thread tid to exit.Pthread_detach(tid,.): request reclaiming of the resources of the thread tid, if it has completed. ( This is more to take some implementation variations).2Inter-Process Communication and SynchronizationB. Ramamurthy3IntroductionAn important and fundamental feature in modern operating systems is concurrent execution of processes/threads. This feature is essential for the realization of multiprogramming, multiprocessing, distributed systems, and client-server model of computation.Concurrency encompasses many design issues including communication and synchronization among processes, sharing of and contention for resources.In this discussion we will look at the various design issues/problems and the wide variety of solutions available.4Topics for discussionThe principles of concurrencyInteractions among processesMutual exclusion problemMutual exclusion- solutionsSoftware approaches (Dekkers and Petersons)Hardware support (test and set atomic operation)OS solution (semaphores)PL solution (monitors)Distributed OS solution ( message passing)Reader/writer problemDining Philosophers Problem5Principles of ConcurrencyInterleaving and overlapping the execution of processes.Consider two processes P1 and P2 executing the function echo: input (in, keyboard); out = in; output (out, display);6.Concurrency (contd.)P1 invokes echo, after it inputs into in , gets interrupted (switched). P2 invokes echo, inputs into in and completes the execution and exits. When P1 returns in is overwritten and gone. Result: first ch is lost and second ch is written twice.This type of situation is even more probable in multiprocessing systems where real concurrency is realizable thru multiple processes executing on multiple processors.Solution: Controlled access to shared resourceProtect the shared resource : in buffer; “critical resource”one process/shared code. “critical region”7Interactions among processesIn a multi-process application these are the various degrees of interaction:1. Competing processes: Processes themselves do not share anything. But OS has to share the system resources among these processes “competing” for system resources such as disk, printer. Co-operating processes : Results of one or more processes may be needed for another process. 2. Co-operation by sharing : Example: Sharing of an IO buffer. Concept of critical section. (indirect)3. Co-operation by communication : Example: typically no data sharing, but co-ordination thru synchronization becomes essential in certain applications. (direct)8Interactions .(contd.)Among the three kinds of interactions indicated by 1, 2 and 3 above:1 is at the system level: potential problems : deadlock and starvation.2 is at the process level : significant problem is in realizing mutual exclusion.3 is more a synchronization problem.We will study mutual exclusion and synchronization here, and defer deadlock, and starvation for a later time.9Race ConditionRace condition: The situation where several processes access and manipulate shared data concurrently. The final value of the shared data depends upon which process finishes last.To prevent race conditions, concurrent processes must be synchronized.10Mutual exclusion problemSuccessful use of concurrency among processes requires the ability to define critical sections and enforce mutual exclusion.Critical section : is that part of the process code that affects the shared resource.Mutual exclusion: in the use of a shared resource is provided by making its access mutually exclusive among the processes that share the resource.This is also known as the Critical Section (CS) problem.11Mutual exclusionAny facility that provides mutual exclusion should meet these requirements: 1. No assumption regarding the relative speeds of the processes.2. A process is in its CS for a finite time only.3. Only one process allowed in the CS.4. Process requesting access to CS should not wait indefinitely.5. A process waiting to enter CS cannot be blocking a process in CS or any other processes.12Software Solutions: Algorithm 1Process 0.while turn != 0 do nothing;/ busy waitingturn = 1;.Problems : Strict alternation, Busy WaitingProcess 1.while turn != 1 do nothing;/ busy waitingturn = 0;.13Algorithm 2PROCESS 0.flag0 = TRUE;while flag1 do nothing;flag0 = FALSE;PROBLEM : Potential for deadlock, if one of the processes fail within CS.PROCESS 1.flag1 = TRUE;while flag0 do nothing;flag1 = FALSE;14Algorithm 3Combined shared variables of algorithms 1 and 2.Process Pido flag i:= true;turn = j;while (flag j and turn = j) ;critical sectionflag i = false;remainder section while (1);Solves the critical-section problem for two processes.15Synchronization HardwareTest and modify the content of a word atomically.boolean TestAndSet(boolean &target) boolean rv = target;target = true;return rv;16Mutual Exclusion with Test-and-SetShared data: boolean lock = false;Process Pido while (TestAndSet(lock) ;critical sectionlock = false;remainder section17Synchronization Hardware Atomically s variables.void S &a, boolean &b) boolean temp = a;a = b;b = temp;18Mutual Exclusion with SwapShared data (initialized to false): boolean lock;Process Pido key = true;while (key = true) S);critical sectionlock = false;remainder section19Semaphores Think about a semaphore as a classAttributes: semaphore value, Functions: init, wait, signalSupport provided by OSConsidered an OS resource, a limited number available: a limited number of instances (objects) of semaphore class is allowed.Can easily implement mutual exclusion among any number of processes.20Critical Section of n ProcessesShared data: Semaphore mutex; /initially mutex = 1Process Pi: do mutex.wait(); critical section mutex.signal(); remainder section while (1); 21Semaphore ImplementationDefine a semaphore as a class:class Semaphore int value; / semaphore value ProcessQueue L; / process queue /operationswait()signal()In addition, two simple utility operations:block() suspends the process that invokes it.Wakeup() resumes the execution of a blocked process P.22Semantics of wait and signalSemaphore operations now defined as S.wait():S.value-;if (S.value 0) add this process to S.L;block(); / block a processS.signal(): S.value+;if (S.value = 0) remove a process P from S.L;wakeup(); / wake a process23 Semaphores for CSSemaphore is initialized to 1. The first process that executes a wait() will be able to immediately enter the critical section (CS). (S.wait() makes S value zero.) Now other processes wanting to enter the CS will each execute the wait() thus decrementing the value of S, and will get blocked on S. (If at any time value of S is negative, its absolute value gives the number of processes waiting blocked. )When a process in CS departs, it executes S.signal() which increments the value of S, and will wake up any one of the processes blocked. The queue could be FIFO or priority queue.24Two Types of SemaphoresCounting semaphore integer value can range over an unrestricted domain.Binary semaphore integer value can range only between 0 and 1; can be simpler to implement. ex: nachosCan implement a counting semaphore using a binary semaphore.25Semaphore for SynchronizationExecute B in Pj only after A executed in PiUse semaphore flag initialized to 0Code:PiPj Aflag.wait()flag.signal()B26Classical Problems of SynchronizationBounded-Buffer ProblemReaders and Writers ProblemDining-Philosophers Problem27Producer/Consumer problemProducerrepeatproduce item v;bin = v;in = in + 1;forever;Consumerrepeatwhile (in = out) nop;w = bout;out = out + 1;consume w;forever;28Solution for P/C using Semaphores Producerrepeatproduce item v;MUTEX.wait();bin = v;in = in + 1;MUTEX.signal();forever;What if Producer is slow or late?Consumerrepeatwhile (in = out) nop;MUTEX.wait();w = bout;out = out + 1;MUTEX.signal();consume w;forever;Ans: Consumer will busy-wait at the while statement.29P/C: improved solution Producerrepeatproduce item v;MUTEX.wait();bin = v;in = in + 1;MUTEX.signal();AVAIL.signal();forever;What will be the initial values of MUTEX and AVAIL?ConsumerrepeatAVAIL.wait();MUTEX.wait();w = bout;out = out + 1;MUTEX.signal();consume w;forever;ANS: Initially MUTEX = 1, AVAIL = 0.30P/C problem: Bounded bufferProducerrepeatproduce item v;while(in+1)%n = out) NOP;bin = v;in = ( in + 1)% n;forever;How to enforce bufsize?Consumerrepeatwhile (in = out) NOP;w = bout;out = (out + 1)%n;consume w;forever;ANS: Using another counting semaphore.31P/C: Bounded Buffer solutionProducerrepeatproduce item v;BUFSIZE.wait();MUTEX.wait();bin = v;in = (in + 1)%n;MUTEX.signal();AVAIL.signal();forever;What is the initial value of BUFSIZE?ConsumerrepeatAVAIL.wait();MUTEX.wait();w = bout;out = (out + 1)%n;MUTEX.signal();BUFSIZE.signal();consume w;forever;ANS: size of the bounded buffer.32Semaphores - commentsIntuitively easy to use.wait() and signal() are to be implemented as atomic operations.Difficulties: signal() and wait() may be exchanged inadvertently by the programmer. This may result in deadlock or violation of mutual exclusion.signal() and wait() may be left out.Related wait() and signal() may be scattered all over the code among the processes.33MonitorsMonitor is a predecessor of the “class” concept.Initially it was implemented as a programming language construct and more recently as library. The latter made the monitor facility available for general use with any PL.Monitor consists of procedures, initialization sequences, and local data. Local data is accessible only thru monitors procedures. Only one process can be executing in a monitor at a time. Other process that need the monitor wait suspended.34Monitorsmonitor monitor-nameshared variable declarationsprocedure body P1 () . . .procedure body P2 () . . . procedure body Pn () . . . initialization code35MonitorsTo allow a process to wait within the monitor, a condition variable must be declared, ascondition x, y;Condition variable can only be used with the operations wait and signal.The operationx.wait();means that the process invoking this operation is suspended until another process invokesx.signal();The x.signal operation resumes exactly one suspended process. If no process is suspended, then the signal operation has no effect.36Schematic View of a Monitor37Monitor With Condition Variables38Message passingBoth synchronization and communication requirements are taken care of by this mechanism. More over, this mechanism yields to synchronization methods among distributed processes.Basic primitives are: send (destination, message);receive ( source, message);39 Issues in message passingSend and receive: could be blocking or non-blocking:Blocking send: when a process sends a message it blocks until the message is received at the destination.Non-blocking send: After sending a message the sender proceeds with its processing without waiting for it to reach the destination.Blocking receive: When a process executes a receive it waits blocked until the receive is completed and the required message is received. Non-blocking receive: The process executing the receive proceeds without waiting for the message(!).Blocking Receive/non-blocking send is a common combination.40Reader/Writer problemData is shared among a number of processes.Any number of reader processes could be accessing the shared data concurrently.But when a writer process wants to access, only that process must be accessing the shared data. No reader should be present.Solution 1 : Readers have priority; If a reader is in CS any number of readers could enter irrespective of any writer waiting to enter CS.Solution 2: If a writer wants CS as soon as the CS is available writer enters it.41Reader/writer: Priority ReadersWriter:ForCS.wait();CS;ForCS.signal();Reader:ES.wait();NumRdr = NumRdr + 1;if NumRdr = 1 ForCS.wait();ES.signal();CS;ES.wait();NumRdr = NumRdr -1;If NumRdr = 0 ForCS.signal();ES.signal();42Dining Philosophers Examplemonitor dp enum thinking, hungry, eating state5;condition self5;void pickup(int i) / following slidesvoid putdown(int i) / following slidesvoid test(int i) / following slidesvoid init() for (int i = 0; i 5; i+)statei = thinking;43Dining Philosophersvoid pickup(int i) statei = hungry;testi;if (statei != eating)selfi.wait();void putdown(int i) statei = thinking;/ test left and right neighborstest(i+4) % 5);test(i+1) % 5);44Dining Philosophersvoid test(int i) if ( (state(I + 4) % 5 != eating) & (statei = hungry) & (state(i + 1) % 5 != eating) statei = eating;selfi.signal();45SummaryWe looked at various ways/levels of realizing synchronization among concurrent processes.Synchronization at the kernel level is usually solved using hardware mechanisms such as interrupt priority levels, basic hardware lock, using non-preemptive kernel (older BSDs), using special signals.46