Dekker’s algorithm in Process Synchronization

05 Jan, 2022
Prerequisite – Process Synchronization, Inter Process Communication To obtain such a mutual exclusion, bounded waiting, and progress there have been several algorithms implemented, one of which is Dekker’s Algorithm. To understand the algorithm let’s understand the solution to the critical section problem first. A process is generally represented as :  
do {
    //entry section
        critical section
    //exit section
        remainder section
} while (TRUE);
The solution to the critical section problem must ensure the following three conditions:  
Mutual ExclusionProgressBounded Waiting
Mutual Exclusion
Progress
Bounded Waiting
One of the solutions for ensuring above all factors is Peterson’s solution.Another one is Dekker’s Solution. Dekker’s algorithm was the first probably-correct solution to the critical section problem. It allows two threads to share a single-use resource without conflict, using only shared memory for communication. It avoids the strict alternation of a naïve turn-taking algorithm, and was one of the first mutual exclusion algorithms to be invented.Although there are many versions of Dekker’s Solution, the final or 5th version is the one that satisfies all of the above conditions and is the most efficient of them all. Note – Dekker’s Solution, mentioned here, ensures mutual exclusion between two processes only, it could be extended to more than two processes with the proper use of arrays and variables.Algorithm – It requires both an array of Boolean values and an integer variable: 
var flag: array [0..1] of boolean;
turn: 0..1;
repeat

        flag[i] := true;
        while flag[j] do
                if turn = j then
                begin
                        flag[i] := false;
                        while turn = j do no-op;
                        flag[i] := true;
                end;

                critical section

        turn := j;
        flag[i] := false;

                remainder section

until false;
First Version of Dekker’s Solution – The idea is to use a common or shared thread number between processes and stop the other process from entering its critical section if the shared thread indicates the former one already running. 
CPP
Python3
Main(){     int thread_number = 1;    startThreads();} Thread1(){    do {         // entry section        // wait until threadnumber is 1        while (threadnumber == 2)            ;         // critical section         // exit section        // give access to the other thread        threadnumber = 2;         // remainder section     } while (completed == false)} Thread2(){     do {         // entry section        // wait until threadnumber is 2        while (threadnumber == 1)            ;         // critical section         // exit section        // give access to the other thread        threadnumber = 1;         // remainder section     } while (completed == false)}
def Thread1():    doWhile=False    while not completed or not doWhile:        doWhile=True        # entry section        # wait until threadnumber is 1        while (threadnumber == 2):            pass         # critical section         # exit section        # give access to the other thread        threadnumber = 2         # remainder section def Thread2():    doWhile=False    while not completed or not doWhile:        doWhile=True        # entry section        # wait until threadnumber is 2        while (threadnumber == 1):            pass         # critical section         # exit section        # give access to the other thread        threadnumber = 1         # remainder section if __name__ == '__main__':     thread_number = 1    startThreads()
The problem arising in the above implementation is lockstep synchronization, i.e each thread depends on the other for its execution. If one of the processes completes, then the second process runs, gives access to the completed one, and waits for its turn, however, the former process is already completed and would never run to return the access back to the latter one. Hence, the second process waits infinitely then.Second Version of Dekker’s Solution – To remove lockstep synchronization, it uses two flags to indicate its current status and updates them accordingly at the entry and exit section. 
CPP
Python3
Main(){     // flags to indicate if each thread is in    // its critical section or not.    boolean thread1 = false;    boolean thread2 = false;     startThreads();} Thread1(){     do {         // entry section        // wait until thread2 is in its critical section        while (thread2 == true)            ;         // indicate thread1 entering its critical section        thread1 = true;         // critical section         // exit section        // indicate thread1 exiting its critical section        thread1 = false;         // remainder section     } while (completed == false)} Thread2(){     do {         // entry section        // wait until thread1 is in its critical section        while (thread1 == true)            ;         // indicate thread2 entering its critical section        thread2 = true;         // critical section         // exit section        // indicate thread2 exiting its critical section        thread2 = false;         // remainder section     } while (completed == false)}
def Thread1():    doWhile=False    while not completed or not doWhile:        doWhile=True        # entry section        # wait until thread2 is in its critical section        while (thread2):            pass         # indicate thread1 entering its critical section        thread1 = True         # critical section         # exit section        # indicate thread1 exiting its critical section        thread1 = False         # remainder section def Thread2():    doWhile=False    while not completed or not doWhile:        doWhile=True        # entry section        # wait until thread1 is in its critical section        while (thread1):            pass         # indicate thread1 entering its critical section        thread2 = True         # critical section         # exit section        # indicate thread2 exiting its critical section        thread2 = False         # remainder section         if __name__ == '__main__':     # flags to indicate if each thread is in    # its critical section or not.    thread1 = False    thread2 = False     startThreads()
The problem arising in the above version is mutual exclusion itself. If threads are preempted (stopped) during flag updation ( i.e during current_thread = true ) then, both the threads enter their critical section once the preempted thread is restarted, also the same can be observed at the start itself, when both the flags are false.Third Version of Dekker’s Solution – To re-ensure mutual exclusion, it sets the flags before the entry section itself. 
C++
Python3
Main(){     // flags to indicate if each thread is in    // queue to enter its critical section    boolean thread1wantstoenter = false;    boolean thread2wantstoenter = false;     startThreads();} Thread1(){     do {         thread1wantstoenter = true;         // entry section        // wait until thread2 wants to enter        // its critical section        while (thread2wantstoenter == true)            ;         // critical section         // exit section        // indicate thread1 has completed        // its critical section        thread1wantstoenter = false;         // remainder section     } while (completed == false)} Thread2(){     do {         thread2wantstoenter = true;         // entry section        // wait until thread1 wants to enter        // its critical section        while (thread1wantstoenter == true)            ;         // critical section         // exit section        // indicate thread2 has completed        // its critical section        thread2wantstoenter = false;         // remainder section     } while (completed == false)}
if __name__=='__main__':    # flags to indicate if each thread is in    # queue to enter its critical section    thread1wantstoenter = False    thread2wantstoenter = False     startThreads()  def Thread1():    doWhile=False    while (completed == False or not doWhile):        doWhile=True        thread1wantstoenter = True         # entry section        # wait until thread2 wants to enter        # its critical section        while (thread2wantstoenter == True):            pass                      # critical section         # exit section        # indicate thread1 has completed        # its critical section        thread1wantstoenter = False         # remainder section  def Thread2():    doWhile=False    while (completed == False or not doWhile) :        doWhile=True        thread2wantstoenter = True         # entry section        # wait until thread1 wants to enter        # its critical section        while (thread1wantstoenter == True):            pass                      # critical section         # exit section        # indicate thread2 has completed        # its critical section        thread2wantstoenter = False         # remainder section
The problem with this version is a deadlock possibility. Both threads could set their flag as true simultaneously and both will wait infinitely later on.Fourth Version of Dekker’s Solution – Uses small time interval to recheck the condition, eliminates deadlock, and ensures mutual exclusion as well. 
CPP
Python3
Main(){     // flags to indicate if each thread is in    // queue to enter its critical section    boolean thread1wantstoenter = false;    boolean thread2wantstoenter = false;     startThreads();} Thread1(){     do {         thread1wantstoenter = true;         while (thread2wantstoenter == true) {             // gives access to other thread            // wait for random amount of time            thread1wantstoenter = false;             thread1wantstoenter = true;        }         // entry section        // wait until thread2 wants to enter        // its critical section         // critical section         // exit section        // indicate thread1 has completed        // its critical section        thread1wantstoenter = false;         // remainder section     } while (completed == false)} Thread2(){     do {         thread2wantstoenter = true;         while (thread1wantstoenter == true) {             // gives access to other thread            // wait for random amount of time            thread2wantstoenter = false;             thread2wantstoenter = true;        }         // entry section        // wait until thread1 wants to enter        // its critical section         // critical section         // exit section        // indicate thread2 has completed        // its critical section        thread2wantstoenter = false;         // remainder section     } while (completed == false)}
if __name__ == '__main__':     # flags to indicate if each thread is in    # queue to enter its critical section    thread1wantstoenter = False    thread2wantstoenter = False     startThreads()  def Thread1():    doWhile=False    while (completed == False or not doWhile):        doWhile=True        thread1wantstoenter = True         while (thread2wantstoenter == True) :             # gives access to other thread            # wait for random amount of time            thread1wantstoenter = False             thread1wantstoenter = True                  # entry section        # wait until thread2 wants to enter        # its critical section         # critical section         # exit section        # indicate thread1 has completed        # its critical section        thread1wantstoenter = False         # remainder section        def Thread2():    doWhile=False    while (completed == False or not doWhile):        doWhile=True        thread2wantstoenter = True         while (thread1wantstoenter == True) :             # gives access to other thread            # wait for random amount of time            thread2wantstoenter = False             thread2wantstoenter = True                  # entry section        # wait until thread1 wants to enter        # its critical section         # critical section         # exit section        # indicate thread2 has completed        # its critical section        thread2wantstoenter = False         # remainder section
The problem with this version is the indefinite postponement. Also, a random amount of time is erratic depending upon the situation in which the algorithm is being implemented, hence not an acceptable solution in business critical systems.Dekker’s Algorithm: Final and completed Solution – -Idea is to use favoured thread notion to determine entry to the critical section. Favoured thread alternates between the thread providing mutual exclusion and avoiding deadlock, indefinite postponement, or lockstep synchronization. 
CPP
Python3
Main(){     // to denote which thread will enter next    int favouredthread = 1;     // flags to indicate if each thread is in    // queue to enter its critical section    boolean thread1wantstoenter = false;    boolean thread2wantstoenter = false;     startThreads();} Thread1(){    do {         thread1wantstoenter = true;         // entry section        // wait until thread2 wants to enter        // its critical section        while (thread2wantstoenter == true) {             // if 2nd thread is more favored            if (favaouredthread == 2) {                 // gives access to other thread                thread1wantstoenter = false;                 // wait until this thread is favored                while (favouredthread == 2)                    ;                 thread1wantstoenter = true;            }        }         // critical section         // favor the 2nd thread        favouredthread = 2;         // exit section        // indicate thread1 has completed        // its critical section        thread1wantstoenter = false;         // remainder section     } while (completed == false)} Thread2(){     do {         thread2wantstoenter = true;         // entry section        // wait until thread1 wants to enter        // its critical section        while (thread1wantstoenter == true) {             // if 1st thread is more favored            if (favaouredthread == 1) {                 // gives access to other thread                thread2wantstoenter = false;                 // wait until this thread is favored                while (favouredthread == 1)                    ;                 thread2wantstoenter = true;            }        }         // critical section         // favour the 1st thread        favouredthread = 1;         // exit section        // indicate thread2 has completed        // its critical section        thread2wantstoenter = false;         // remainder section     } while (completed == false)}
if __name__ == '__main__':     # to denote which thread will enter next    favouredthread = 1     # flags to indicate if each thread is in    # queue to enter its critical section    thread1wantstoenter = False    thread2wantstoenter = False     startThreads()  def Thread1():    doWhile=False    while (completed == False or not doWhile) :        doWhile=True        thread1wantstoenter = True         # entry section        # wait until thread2 wants to enter        # its critical section        while (thread2wantstoenter == True) :             # if 2nd thread is more favored            if (favaouredthread == 2) :                 # gives access to other thread                thread1wantstoenter = False                 # wait until this thread is favored                while (favouredthread == 2):                    pass                                      thread1wantstoenter = True                               # critical section         # favor the 2nd thread        favouredthread = 2         # exit section        # indicate thread1 has completed        # its critical section        thread1wantstoenter = False         # remainder section         def Thread2():    doWhile=False    while (completed == False or not doWhile) :        doWhile=True        thread2wantstoenter = True         # entry section        # wait until thread1 wants to enter        # its critical section        while (thread1wantstoenter == True) :             # if 1st thread is more favored            if (favaouredthread == 1) :                 # gives access to other thread                thread2wantstoenter = False                 # wait until this thread is favored                while (favouredthread == 1):                    pass                                      thread2wantstoenter = True                               # critical section         # favour the 1st thread        favouredthread = 1         # exit section        # indicate thread2 has completed        # its critical section        thread2wantstoenter = False         # remainder section      
This version guarantees a complete solution to the critical solution problem.References – Dekker’s Algorithm -csisdmz.ul.ie Dekker’s algorithm – Wikipedia 
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