Process is the activity of executing a program in CPU. A program that is alive. Conceptually, each process has its own CPU.All processes, if possible, run concurrently. Physical concurrency = Parallelism: Requires multi CPULogical concurrency = time-shared CPUProcesses compete or cooperate. Example. A program makes a call r = f(x).What if it doesn’t want to wait till the result r is returned?Want to be able to do this:■ r = f(x) ■ do something else while the function isn’t returned■ use intermediate results; wait if f(x) is not complete.These all amount to: ■ Make the function call and then do the other works while the function call is still pending (process)■ Retrieving values from processes and functions executingindependently the rest of the process (communication)■ Waiting for things to complete (synchronization)Conway’s model for parent-child processes:The Petri-netdisplays parallel (asynchronous) processes in actions. Sometimes, cobegin-coend construct features wellA;cobeginB; C;coend;D;Conway suggests a more general model: A; L: C;fork(L); join(X);B;await-join(X);D;The “join” synchronizes the two processes and then kills one. Only then the remaining process proceeds to D. ●ACBDIn UNIX, Conway’s model is somewhat preserved. A;if (fork() = = 0) {B; // child processexit();}else {C; // parent processWait(); //wait till B calls exit }D; // parent processIn UNIX, ……pid = fork (); // creates a child with a pid = 0 // parent process receives the pid of the childNote. fork() function is expensive in set-up time and memory. Alternatively, a thread construct is desired. This isalso known as a light-weight process. A thread is a schedulable part of a process. Multithreaded operations using Pthread library in C is oftenused to generate and work with thread. But this library is not thread-safe (no guarantee that concurrent threads will work properly).Java language supports threads as one of its essential features. Java library provides a Thread class that is rich in collections. Example. public class HelloThread extends Thread { public void run() { System.out.println("Hello from a thread!"); } public static void main(String args[]) { (new HelloThread()).start(); }} Thread.start starts a new thread. A Model of concurrent processes(From Operating Systems Theory by Coffman & Denning)Central commodity is a task T. Initiation of task T is the event Tand the termination is the event T(.)t time occurrence of an event.The duration of a task T is )T(t)T(t  and is assumed non-zero.Our tasks are uninterpreted. A set of n tasks comprises a task-set: ]T,...,T,T[n21 under a partial-order. The pair),(C  is a task-system.TT means T is completed before Tis allowed to begin. A task-system Cis independent if   Tasks are performed on a physical system comprising a number of resources. Collectively, these resources describe the system state Ss . The function of tasks is to promote and migrate the system from allowed state to state)Ss,s(ss .Task initiation events correspond to: a. Acquisition or assignment of resourcesb. Initialization of resource statesc. Reading input valuesTask termination events correspond toa. Release of resourcesb. Writing of output valuesc. Saving resource states, etc. Graphical representation of a task system is a precedence graph with its vertices over and edges)T,T( if only if TTand there exists no Tsuch that TTT. A typical example:Some observations:a. A task with no successor is a terminal taskb. A task with no predecessor is an initial taskc. If a task is neither a successor nor a predecessor to any task is an independent taskd. T,Tare concurrent if they are independent to each other.e. A nonterminal task is at a level k in C if the shortest path from the initial task to this vertex is of length k.An execution sequence  in an n-task system ),(C  is any string n221a...aa where each ia is either an initiation event or a termination event subject to the following conditions:a. Each Tand T of T will appear only once in .1T2T3T4T5T6Tb. if Tai then at some 1ik  Takc. if Tai then at some ik  Takd. In , no unmatched initiation/termination event will appear on its own.Some valid sequences for our graph:1665544332211TTTTTTTTTTTT6624554332112TTTTTTTTTTTTA task Tis live in an execution sequence i21a...aa if for some Taj where ij  is invoked but its termination Takis yet to be posted where ik .A partial execution sequence is a prefix of an execution sequence. A thread is a partial execution sequence where every initiation action is matched with its termination, and every termination action is preceded by an initiation action. At theconclusion of a thread, there is no live task lingering.A typical thread is supported by dummy actions, if necessary. For instance, dummydummythreadTˆT is a valid string. Atask-system is closed if it is bracketed bya starting and an ending task. We’d be dealing with closed systems.As a task sequence  unfolds over resource set  the system migrates from state to state yielding a string of states starting the initial state 0s. If  then n2i10s...s...ss where each ia transforms the system i1iss . Sometimes, i1iss  if ia is only a trivial action like reading a memory location.Suppose, 1C and 2Care closed task systems with no tasks incommon. a. 21C.C is a concatenation of the task systems, the end task of 1C now precedes the first task of 2C.b. 21C||CC  is a parallel system such that if )nm(221c..ccc is inC and then )CC( ,21with m221a..aa and n221b...bb such thatstartT2T3T4T5TendT■ 1c is either 1a or 1b■ i21a...aa and j21b...bb are the elements of ji21c...cc and )nm(2ji , then 1jic is either 1ia or 1jb. c. ...C.C.C.CCk is a k-fold looping system when the task sequence C repeats k-times. At a programming level, following models are encouraged:For asynchronous (potentially parallel) construct involving independent processes 21C||C :: cobegin1C;2C;coend;This is also abstracted as a parallel construct)C,C(P21 Compare this to the sequential (concatenated) construct.21C.C = )C,C(S21:: 21C;CExcept