Concurrent Processes In Operating Systems

Concurrent Processes In direct SystemsThe scheduleming technique, to spend interrupts to simulate the cooccurring slaying of any(prenominal)(prenominal) political platforms on Atlas computers was kn bear as multi programme. It was pi unmatchableered by Tom Kilburn and David Howarth.Multi program in early days was d i exploitation assembly level lyric. Slightest steal in programs could gear up program unpredictable hence examen them was hard-fought besides the assembly level language had no apprehensionual foundation. direct outlines designed utilize this multi computer computer programming techniques grew in truth huge and unpredictable their designers talk about softw be crisis. This created an urgent research and development deprivation for co-occurrent programming techniques.Computer scientists took the initial step towards understanding the issues related to coinciding programming during mid 1960s, they discovered fundamental concepts, expressed them by programming nonation, included them in programming languages and utilize these languages to write the exemplar direct systems. These resembling concepts were then applied to any form of check computing. mental institution of Concurrent lickes in operating systemsProcesses played a key utilisation in shaping early operating systems. They were gener ally run in a morosely sequential regularise. Multiprogramming existed only the work ates did not hardly run coincidingly instead a beat based utensil was employ in which a limited amount of time was abandoned to distributively serve. Even in those days the coverors speed was fast nice to give and illusion that the multiple summonses were running concurrently. They were called as timeharing or multiprogramming operating systems (November 1961, called CTSS Compatible Time-Sharing System in addition Multics the predecessors of UNIX real by MIT)These lineament operating systems were very popular and were seen as a b reak finished during those measure. The major(ip) draw covert was complexity of the system design which made it difficult to make it more than than versatile and flexible so that a wiz all nominate OS could be built. Also the imaging sharing done by these impactes was primitive or inefficient and it but showed in that respect was a dig of core for research and development.Work on these operating systems made elbow room for concurrent fulfiles. Most of the original concepts related to concurrency were developed during this period. These innovative compositions and concepts went on become the prefatory principles on which todays operating systems and concurrent applications argon designed. (A major excogitate under taken by IBM in this direction was in 1964 the OS/360 for their modernistic mainframes system 360)To build reliable concurrent dish outes understanding and developing grassroots concepts for concurrency was fundamental let us talk about concurrency an d some of its basic programming concepts.ConcurrencyIn computer science, concurrency is a home of systems in which salwaysal computations atomic build 18 executing coincidentally, and potentially interacting with each opposite. WikipediaLet us consider a real life standard a ho utilize project much(prenominal) as the building of a manse will lead some work to go on in parallel with different works. In principle, a project standardized building a sign of the zodiac does not require any concurrent activity, but a plummy feature of such a project is that the whole labour tin female genitals be blameless in shorter time by allowing non-homogeneous wedge shape tasks to be carried out concurrently. thither is no reason any painter give noticenot paint the house from outside (weather permitting), while the p give wayerer is busy in the upstair rooms and the joiner is fitting the kitchen units downstairs. there argon however some constraints on concurrency which is pos sible. The brick layer will normally have to watch until the foundation of the house had been layered before he could begin the task of building the walls. The divers(a) tasks involved in such a project weed ordinarily be regarded as indep haltent of one an separate, but the computer programing of the tasks is limit by notions of a task A must be completed before task B commode beginA image example is that of a railway ne twork. A number of trains making journeys at heart a railway network, and by contrast with the previous example, when they start and they end is generally independent of roughly of the otherwise journeys. Where the journeys interact though is at places where routes cross or use common sections of surmount for pcts of journeys. We can in this example regard the movement of trains as programs in execution, and the sections of track as the visions which these programs may or may not have to trade with other programs. Hence the two trains run concurre ntly in cause their routes interact sharing the same resources without interrupting each other similar to concurrent do workes in operating systems.So as discussed earlier we understand that changees atomic number 18 of the essence(predicate) to implement concurrency so let us discuss the work as a concept which will introduce us to the close important concept for concurrency i.e. moveFundamental conceptsProcessA process is a running program OS keeps track of running programs in form of processes and their info. A process is made of multiple threads.ThreadsThe need to write concurrent applications introduced threads. In other words, threads ar processes that apportion a single address lacuna. Each thread has its own program counter and stack.Threads ar often called lightweight processes as N threads have 1 scalawag table, 1 address space and 1 PID while N processes have N page tables, N address spaces and N PIDs.Therefore, a sequence of executing instruction manual is called a thread that runs independently of other threads and yet can shargon data with other threads directly. A thread is contained wrong a process. There can exist multiple threads deep down a process that share resources like memory, while different processes do not share these resources.A simple thread exampleThere are two classes defined in this example tropely SimpleThread which is a subclass of the Thread class and TwoThreads class.class SimpleThread extends Threadpublic SimpleThread(String str)super(str)public vacuum run()for (int i = 0 i System.out.println(i + + getName())Trysleep((int)(Math.random() * 1000))catch (InterruptedException e) System.out.println(DONE + getName())The method SimpleThread() is a constructor which sets the Threads name used later in the program. The attain takes place in the run() method which contains a for loop that iterates ten times that displays the iteration number and the name of the Thread, then sleeps for a random separation of up to a second.The TwoThreads class provides a main() method that creates two SimpleThread threads named London and briskYork.class TwoThreadspublic static void main (String args) newly SimpleThread(London).start()new SimpleThread(NewYork).start()The main() method also starts each thread immediately hobby its construction by calling the start() method.Following concepts are mostly used at the thread level and also the issues discussed are encountered while implementing concurrency. later onmath circumstanceA accelerate civilize occurs when multiple processes gravel and finagle the same data concurrently, and the outcome of the execution depends on the particular coiffe in which the access takes place.http//www.topbits.com/ function- consideration.htmlIt is not so easy to hear race condition during program execution if it is observed that the cling to of dual-lane versatiles is unpredictable, it may be caused because of race condition. In concurrent programming there are more than one legal possible thread executions hence order of thread execution cannot be predicted.Race condition may buzz off uncertain results. Outcome of race condition may occur after a long time. In order to prevent unpredictable results because of race condition, following methods are used- vernacular exclusionMutual exclusion (often lessen to mutex) algorithms are used in concurrent programming to block the simultaneous use of a common resource, such as a world(a) variable, by pieces of computer commandment called tiny sections. (Wikipedia)-Critical Region (CR)A part of enroll that is always punish under correlative exclusion is called a deprecative region. Due to this, the compiler instead of the programmer is supposed to full stop that the resource is neither being used nor referred to outside its lively regions. composition programming, critical section resides when signalises are used. CRs are needed plainly if the data is writeable.It consists of two part sVariables These must be accessed under mutual exclusion.New language demesnement It identifies a critical section that has access to variables.There are two processes namely A and B that contain critical regions i.e. the code where dual-lane data is readable and writable.-SemaphoresSemaphores are mechanisms which protect critical sections and can be used to implement condition synchronization. Semaphore encapsulates the shared variable and utilise semaphore, only allowed set of surgical procedures can be carried out. It can suspend or wake processes.The two operations performed using semaphores are wait and signal, also known as P and V respectively. When a process performs P operation it notifies semaphore that it wants to use the shared resource, if the semaphore is free the process gains access to the shared variable and semaphore is decremented by one else the process is delayed.If V operation is performed, then the process notifies the semaphore that it has finished using shared variable and semaphore value is developmented by one.By using semaphores, we attempt to void other multi-programming problem of Starvation.There are two kinds of SemaphoresBinary semaphores Control access to a single resource, taking the value of 0 (resource is in use) or 1 (resource is available).Counting semaphores Control access to multiple resources, thus assuming a range of nonnegative values.-LocksThe most common way to implement mutex is using lock away(predicate)s. A lock can be either locked or unguaranteed. The concept is analogues to locks we use in our doors a person enters the room, locks the door and starts working and leaves the room after finishing the job, if other person wants to enter the room when one person is already inside, he has to wait until the door gets unlocked.Subtasks in a parallel program are often called threads. Smaller, lightweight versions of threads are known as fibres, which are used by some parallel computer architecture and bigger versions are called as processes. Many times threads need to change the value of shared variable, instruction interleaving mingled with programs could be in any order For example, consider the following programThread AThread B1A - sympathise variable X1B Read variable X2A Increment value of X by 12B Increment value of X by 13A issue back to variable X3B Write back to variable XAs we can see in the example both the threads are carrying out same steps which are to read the shared variable, increment its value and write back its value to the same variable. It is clear how live it is to manage these instructions in correct order, for instance if instruction 1A is executed surrounded by 1B and 3B it will generate an incorrect return. If locks are used by one thread, another thread cannot read, write the shared variable. Following example explains usage of locksThread AThread B1A Lock variable X1B Lock variable X2A Read variable X2B Read variable X3A Increment value of X by 13B Increment value of X by 14A Write back to variable X4B Write back to variable X5A Unlock variable X5B Unlock variable XWhichever thread locks the variable first, uses that variable exclusively, any other thread will not be able to gain access to shared variable until it is unlocked over again. Locks are useful for correct execution but on the other hand they disinclined down the program.-MonitorsA reminder is a mutual exclusion enforcing synchronization construct. Monitors provide more structure than conditional critical regions and can be implemented as efficiently as semaphores. Monitors are supported by a programming language rather than by the operating system. They were introduced in Concurrent Pascal and are used as the synchronization mechanism in the Java language. A observe consists of code and data. All of the data and some of the code can be cliquish to the monitor, accessible only to the code that is part of the monitor. Monitor has a single lock that mu st be acquired by the task to execute monitor code i.e. mutual exclusion is provided by making sure that execution of procedures in the same monitor are not overlapped.Active task is the term used for the task which owns the monitor lock. There cannot be more than one active task in the monitor.The monitors lock can be acquired by a task through one of several monitor come ups. It gives up the lock either by blocking a condition variable or by returning from a monitor method.A condition variable is a queue or event queue that is part of the monitor. Two monitor methods called as wait and notify can only be accessed by a condition variable queue. The behaviour of a monitor is known by the relative foregoingities and scheduling of several(a) types of queues.The monitor locks are acquired by the processes in the monitor queues. The queues may be combined in some implementations. The tasks compete for the lock when the monitor lock becomes free.Condition Variable In order to make su re that processes do not enter a busy waiting state, they should notify some events to each other this facility is provided by Monitors with the help of condition variables. If a monitor function wants to proceed by making a condition true then it has to wait for the corresponding condition variable. When a process waits, it gives up the lock and is taken out from set of runnable processes. When a process makes condition true then it notifies a waiting process using condition variable.The methods mentioned above are used to prevent race condition but they might result into serious problems like stand and famishment let us have a look at these problems one at a time as we go further.standstillDeadlock refers to a specific condition where two or more processes are each waiting for each other to release a resource, or more than two processes are waiting for resources in a circular fibril.Conditions for deadlock to occur1 Mutual exclusion Mutual exclusion means only one process can use a resource at a time.2 Hold and wait A process may hold a allocated resource while awaiting assignment of other resource.3 No pre-emption A resource can be released voluntarily by the process holding it. One process cannot use resource forcefully held by another process. A process that receives such resources cannot be break off until it is finished using the resource.4 Circular wait A closed chain of processes exists, such that each process holds a resource required by another process in the chain.Deadlock occurs only when circular wait condition is not solvable and circular wait is not resolvable if first three conditions hold hence all four conditions taken together constitute necessary and sufficient condition for deadlock.In the plot above we can see that process P1 holds resource R1 and communicates for resource R2 held by process P2 , and process P2 is craveing for resource R1.Methods to handle Deadlock1. Deadlock streakDeadlock prevention is to ensure that one of th e four necessary conditions for deadlock can never hold in following waysI1. Mutual exclusion allocate one resource to only one process at a time.2. Hold and wait It requires a process to request and be allocated its resources before it begins its execution, or allow process to request a resource only when process has none. This may lead to low resource utilization. It also may give rise to starvation problem, a process may be held for a long time waiting for all its required resources. The application need to be aware of all the resources it requires, if it needs redundant resources it releases all the resources held and then requests for all those it needs.3. No pre-emption If a process is holding some resources and requests for another resource held by some other process that cannot be allocated to it, then it releases all the resources currently held.The state of pre-empted resource has to be saved and later restored.4. Circular wait To make this condition fail, we can impose a total ordering on all resources types. It is also required that each process requests resources in strict increase order. Resources from the same resource type have to be communicate together.2. Deadlock avoidanceIn deadlock avoidance, the system checks if granting a request is safe or not .The system needs additional prior information regarding overall potential use of each resource for each process i.e. maximum requirement of each resource has to be tell in advance by each process.3. Deadlock detectionIt is important to know if there exists a deadlock situation in the system hence an algorithm is needed to periodically check existence deadlock. recovery from deadlockTo recover from deadlock, the process can be terminated or we can pre-empt the resource. In terminating processes method we can terminate all the processes at once or terminate one process and then again check for deadlock.Similarly there are mechanisms like fair scheduling that can be used to avoid starvation of r esources.- sporting schedulingFair scheduling is to allow multiple processes to fairly share the resources. The main idea is to ensure each thread gets equal CPU time and to minimize resource starvation.- archetypical in first out (FIFO)FIFO or First Come, First Served (FCFS) is the simplest scheduling algorithm that queues processes in the order they arrive in the ready queue.Scheduling overhead is minimal because context switches occur only when process terminates and re-organization of the process queue is not required.In this scheme, completion of both process is possible, hence no starvation.-Shortest remaining timeWith this scheduling scheme, processes with least affect time are arranged as the next process in the queue. To achieve this, prior knowledge of completion time is required. call up a scenario where a shorter process arrives when another process is running, in this theatrical role the current process is stopped and is divided into two parts. This results in addit ional context switching overhead.-Fixed priority pre-emptive schedulingThe operating system gives a fixed priority rank to every process, and the processes are arranged in the ready queue based on their priority this results in richly priority processes interrupting lower priority processes.Waiting and response times are inversely proportional to priority of the process.If there are more high priority processes than low priority processes, it may result into starvation of the last mentioned processes.-Round-robin schedulingIn this scheduling algorithm, each process is allotted a fixed time unit. There could be extra overhead if time unit per process allotted is very small. Round robin has better average response time than rest of the scheduling algorithms. There cannot be starvation since processes are queued based on any priority.Also there are some desired Properties of Concurrent Programs these properties will ensure a reliable concurrent program.There are some geniusistics that a concurrent program must possess. They can be either a golosh or a liveness property.Safety properties assert that nothing full-grown will ever happen during a program execution.Examples of safety property are Mutual exclusion No deadlock Partial nicetyA safety property is a condition that is true at all points in the execution of a program.Liveness properties assert that something close will eventually happen during a program execution.Examples include justice (weak) Reliable conference Total correctnessCommunicating sequential processCommunicating sequential process was introduced in a paper scripted by C. A. R. Hoare in 1978. In this paper he described how various sequential processes could run in parallel irrespective of the central processor (i.e. it can be a single core or multi-core processor). CSP is an integration of two terms, Communication and Sequential process.A communication is an event that is described by a pair C, V, where C is the name of the channel o n which communication takes place and V is the value of the message which passes through this channel by C .A. R. Hoare.In a Sequential Process new process cannot be started until the preceding process has completed.As CSP was more of a programming language so most of the syntax and notations were inherited from ALGOL 60 programming language. Most of the notations were single character instead of English words. For example,? and represents input and output respectively.CSP inherits the concept of Co routines over old programming structures such as subroutines. The structure of Co routines is comprised of COPY (copies character from output of one process to the input of second process), SQUASH is used to replace specified character with other characters, DISASSEMBLE, ASSEMBLE and REFORMAT.-OCCAMOne of the renowned implementation of CSP is occam. It is named after William of Ockam. It is a strict procedural language. It was developed at INMOS. Occam2 programming language is used in m ost of the software developing companies across the world. It is an extension of occam1 which lacks multi-dimension arrays, functions and other data type support. Occam2 came into existence in 1987s. The latest version is occam2.1 which was developed in 1994. BYTESIN operator, fixed-length array returned from procedures, named data types etc. were some of the new features of occame2.1. the compiler designed for occam2.1 named KRoC (Kent Retargetable occam Compiler) is used to create machine code from different microprocessors. Occam-pi is the name of the new occam variant which is influenced by pi-calculus. It is implemented by newer versions of KRoC.JCSPJava programming language also implements the concept of CSP by JCSP. JCSP is a complete programming implementation of CSP i.e. it does not contain deep mathematical algebra. JCSP is used to avoid race condition, deadlock, live lock and starvation programmatically via java programs. The main benefit of JCSP is that most of the alge braic part is already developed and stored in libraries so the programmer does not require strong mathematical skills. To invoke a method he needs to import these inbuilt libraries.Concurrency Test Tools figure a concurrent application is very challenging task. Maintaining interaction between concurrently executing threads is very difficult task for programmer. It is very difficult to understand the nature of threads from one run of a program as they are nondeterministic. As result, it becomes very difficult for testing and debugging. So it is good idea to invest in techniques which can avoid this conditions aid in the process of development. We are exploring these ideas with tools for concurrency.cheatThis is one of the important tools, created by Microsoft Research, which is used to test multi move code systematically. CHESS facilitates both model checking and dynamic abridgment. It has the potential to detect race conditions, livelocks, hangs, deadlocks and data corruption issue s. Concurrency errors are detected by investigating thread schedules and interleaving and for this it chooses a specialized scheduler on which it repeatedly runs regular unit test. The specialized scheduler creates specific thread interleaving.CHESS controls state space explosion using iterative context bounding which puts a limitation on number of thread switching. This supports scientifically experimented concept that most of the concurrency bugs can be revealed with less number of thread switches. This concept is removed better than traditional model checking.CHESS uses Goldilocks lockset algorithm to detect deadlock and race condition. For reporting a livelock, it is anticipated that programmes terminate and bring out fairness for all threads.THE INTEL THREAD tick offSimilar to CHESS, INTEL THREAD CHECKER is used for detecting problems in concurrency like data races and deadlock and it also draws out erroneous synchronization. The thread checker makes use of source code or t he compiled binary for making memory references and to monitor WIN32 synchronization primitive. At the time of execution, information given by the compiled binary is used for constructing incomplete order of execution this step is followed by happens before analysis of the incomplete order obtained.For improving efficiency and implementation, it is better to remember latest access to shared variable than to remember all accesses. The disadvantage of this tool is it cannot find all bugs while analysing long-running applications.RACERXUnlike first two dynamic analysis tools we have discussed above, RACERX is a static analysis tool. It is not required to gab the entire source code rather user gives table which contains specification of APIs which are useful in gaining and releasing locks. Using such small sized tables proves to be advantageous because they lessen the overhead of write entire source code.The working of RACERX is carried out in several phases. In the first phase, R ACERX builds a Control Flow Graph once it has iterated through each source code file. CFG consists of information about function calls, use of pointers, shared memory and other data. When building CFG is done, calls to these APIs are marked.This first phase is followed by analysis phase which involves checking race condition and deadlock.The last phase is post processing errors reported, the purpose is to prioritize errors by their moment and harmfulness.CHORDThis tool is used for Java language, it is context sensitive static analysis tool. Its flow insensitive nature makes it more scalable than other static tools with the disadvantage of low accuracy. It also deals with the distinguishing synchronization primitives available in Java.ZINGZING, a pure model checker tool, verifies the design of multi threaded programs. It has the ability to model concurrent state machines using its own language that describes complex states and transition. It assures the design quality by verifying a ssumptions and confirming the mien or absence of some conditions.KISSMicrosoft Research developed another model checker tool, named KISS (Keep It Simple and Sequential) for concurrent C programs. It converts a concurrent C program into a sequential program that features the operation of interleaving and controls non-determinism. Thereafter, the analysis is performed by a sequential model checker.While using this tool, the programmer is expected to justify the validation of concurrency assumptions.Introduction of multi-core processors increased the grandness on concurrency by many folds.Concurrency and multicore processorMulti core processorsThe computer exertion is undergoing a paradigm shift. Chip manufacturing businesss are shifting development resources away from single-processor chips to a new generation of multi-processor chips known as multicores.Multiple processors are manufactured by placing them on the same die. Hence they share the same circuit. A die is a small block of semiconducting material, on which a given functional circuit is fabricated.A) Single loadB) Multi CoreWhy were they introduced?As we grow further in terms of processing power the hardware industry faces three main challengesPower derive of power consumed by processors has been increasing as more and more decently processors have been introduced to the market. The environment cost and the energy needs have compelled the manufacturer as well as organisations to reconsider their strategies to an extent where change in way the processors are manufactured or operate was inevitable.Processors can be overclocked or underclocked. Overclocking a processor increases the number of instructions it can execute but at the same time increases the power consumption also overclocking a processor does not guarantee a performance utility as there are many other factors to consider.Increasing the number of processors per core (quad or eight) will further improve the power to performance ratio.Me mory clock Memory clock has not improved like the CPU clock hence adding a limitation on the processor performance. Often the instruction to be fetched must be retrieved from relatively slow memory, causing the CPU to stall while waiting for the instruction to be returned. So instead of building faster CPUs underclock it and have more number of cores with their own dedicated memories to have more instructions executed in the same given time.Also the clock speed in itself system grow infinitely due to fundamental physics it has hit a wall. Chips melt above 5GHz of clock speed.Many possibilities are opened by placing two or more powerful computing cores on a single processor.True concurrent applications can be developed only on multicore processors. On single core processors concurrent applications can clog the processor degrading the performance of the application. On multi-core systems, since each core has its own cache, the operating system has sufficient resources to handle mos t compute intensive tasks in parallel.What are the effects of the hardware shift on concurrent programming?The free lunch of performance in terms of ever faster processors is over- Microsoft C++ guru Herb Sutter.For past five decades the ever increasing clock speed has carried the software industry through its progress but now the time has come for the software engineers to face the challenge everlasting(a) directly at them which they have managed to ignore so far. Also as more and more cores are added to hardware the gap between the hardware potential and the s