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Bakery Algorithm in Process Synchronization

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Prerequisite – Critical Section, Process Synchronization, Inter Process Communication The Bakery algorithm is one of the simplest known solutions to the mutual exclusion problem for the general case of N process. Bakery Algorithm is a critical section solution for N processes. The algorithm preserves the first come first serve property.

How does the Bakery Algorithm work?
In the Bakery Algorithm, each process is assigned a number (a ticket) in a lexicographical order. Before entering the critical section, a process receives a ticket number, and the process with the smallest ticket number enters the critical section. If two processes receive the same ticket number, the process with the lower process ID is given priority.

How does the Bakery Algorithm ensure fairness?
The Bakery Algorithm ensures fairness by assigning a unique ticket number to each process based on a lexicographical order. This ensures that processes are served in the order they arrive, which guarantees that all processes will eventually enter the critical section.

  • Before entering its critical section, the process receives a number. Holder of the smallest number enters the critical section.
  • If processes Pi and Pj receive the same number,
if i < j 
Pi is served first; 
else 
Pj is served first.
  • The numbering scheme always generates numbers in increasing order of enumeration; i.e., 1, 2, 3, 3, 3, 3, 4, 5, …

Notation – lexicographical order (ticket #, process id #) – Firstly the ticket number is compared. If same then the process ID is compared next, i.e.-

– (a, b) < (c, d) if a < c or if a = c and b < d
– max(a [0], . . ., a [n-1]) is a number, k, such that k >= a[i]  for i = 0, . . ., n - 1

Shared data – choosing is an array [0..n – 1] of boolean values; & number is an array [0..n – 1] of integer values. Both are initialized to False & Zero respectively. Algorithm Pseudocode –

repeat
    choosing[i] := true;
    number[i] := max(number[0], number[1], ..., number[n - 1])+1;
    choosing[i] := false;
    for j := 0 to n - 1
        do begin
            while choosing[j] do no-op;
            while number[j] != 0
                and (number[j], j) < (number[i], i) do no-op;
        end;

        critical section
    
    number[i] := 0;
    
        remainder section

until false;

Explanation – Firstly the process sets its “choosing” variable to be TRUE indicating its intent to enter critical section. Then it gets assigned the highest ticket number corresponding to other processes. Then the “choosing” variable is set to FALSE indicating that it now has a new ticket number. This is in-fact the most important and confusing part of the algorithm. It is actually a small critical section in itself ! The very purpose of the first three lines is that if a process is modifying its TICKET value then at that time some other process should not be allowed to check its old ticket value which is now obsolete. This is why inside the for loop before checking ticket value we first make sure that all other processes have the “choosing” variable as FALSE. After that we proceed to check the ticket values of processes where process with least ticket number/process id gets inside the critical section. The exit section just resets the ticket value to zero. Code – Here’s the C code implementation of the Bakery Algorithm. Run the following in a UNIX environment – 

C




// Importing the thread library
#include "pthread.h"
 
#include "stdio.h"
 
// Importing POSIX Operating System API library
#include "unistd.h"
 
#include "string.h"
 
// This is a memory barrier instruction.
// Causes compiler to enforce an ordering
// constraint on memory operations.
// This means that operations issued prior
// to the barrier will be performed
// before operations issued after the barrier.
#define MEMBAR __sync_synchronize()
#define THREAD_COUNT 8
 
volatile int tickets[THREAD_COUNT];
volatile int choosing[THREAD_COUNT];
 
// VOLATILE used to prevent the compiler
// from applying any optimizations.
volatile int resource;
 
void lock(int thread)
{
 
    // Before getting the ticket number
    //"choosing" variable is set to be true
    choosing[thread] = 1;
 
    MEMBAR;
    // Memory barrier applied
 
    int max_ticket = 0;
 
    // Finding Maximum ticket value among current threads
    for (int i = 0; i < THREAD_COUNT; ++i) {
 
        int ticket = tickets[i];
        max_ticket = ticket > max_ticket ? ticket : max_ticket;
    }
 
    // Allotting a new ticket value as MAXIMUM + 1
    tickets[thread] = max_ticket + 1;
 
    MEMBAR;
    choosing[thread] = 0;
    MEMBAR;
 
    // The ENTRY Section starts from here
    for (int other = 0; other < THREAD_COUNT; ++other) {
 
        // Applying the bakery algorithm conditions
        while (choosing[other]) {
        }
 
        MEMBAR;
 
        while (tickets[other] != 0 && (tickets[other]
                                           < tickets[thread]
                                       || (tickets[other]
                                               == tickets[thread]
                                           && other < thread))) {
        }
    }
}
 
// EXIT Section
void unlock(int thread)
{
 
    MEMBAR;
    tickets[thread] = 0;
}
 
// The CRITICAL Section
void use_resource(int thread)
{
 
    if (resource != 0) {
        printf("Resource was acquired by %d, but is still in-use by %d!\n",
               thread, resource);
    }
 
    resource = thread;
    printf("%d using resource...\n", thread);
 
    MEMBAR;
    sleep(2);
    resource = 0;
}
 
// A simplified function to show the implementation
void* thread_body(void* arg)
{
 
    long thread = (long)arg;
    lock(thread);
    use_resource(thread);
    unlock(thread);
    return NULL;
}
 
int main(int argc, char** argv)
{
 
    memset((void*)tickets, 0, sizeof(tickets));
    memset((void*)choosing, 0, sizeof(choosing));
    resource = 0;
 
    // Declaring the thread variables
    pthread_t threads[THREAD_COUNT];
 
    for (int i = 0; i < THREAD_COUNT; ++i) {
 
        // Creating a new thread with the function
        //"thread_body" as its thread routine
        pthread_create(&threads[i], NULL, &thread_body, (void*)((long)i));
    }
 
    for (int i = 0; i < THREAD_COUNT; ++i) {
 
        // Reaping the resources used by
        // all threads once their task is completed !
        pthread_join(threads[i], NULL);
    }
 
    return 0;
}


Output: Output

Advantages of Bakery Algorithm:

  • Fairness: The Bakery Algorithm provides fairness, as it ensures that all processes get a fair chance to access the critical section, and no process will be left waiting indefinitely.
  • Easy to Implement: The algorithm is easy to understand and implement, as it uses simple concepts such as turn numbers and flags to ensure mutual exclusion.
  • No Deadlock: The Bakery Algorithm ensures that there is no deadlock situation in the system.
  • No starvation: The algorithm also ensures that there is no starvation of any process, as every process gets a fair chance to enter the critical section.

Disadvantages Bakery Algorithm:

  • Not Scalable: The Bakery Algorithm is not scalable, as the overhead of the algorithm increases with the number of processes in the system.
  • High Time Complexity: The algorithm has a high time complexity, which increases as the number of processes in the system increases. This can result in performance issues in systems with a large number of processes.
  • Busy Waiting: The algorithm requires busy waiting, which can lead to wastage of CPU cycles and increased power consumption.
  • Memory Overhead: The algorithm requires extra memory to store the turn number and flag values, which can lead to increased memory overhead in systems with a large number of processes.

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Last Updated : 05 May, 2023
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