Bakery algorithm

6.1 Silberschatz, Galvin and GagneOperating System Concepts – 8th
Edition
n Process Critical Section Problem
 Consider a system of n processes (P0, P1 ... Pn-1).
 Each process has a segment of code called a
critical section in which the process may change
shared data.
 When one process is executing its critical section,
no other process is allowed to execute in its critical
section.
 The critical section problem is to design a protocol
to serialize executions of critical sections.

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

Edition
Bakery Algorithm
Notations
 (ticket #, process id #)
(a,b) < (c,d) if a < c or
if a == c and b < d
 max (a0,…, an-1) is a number, k, such
that k ≥ ai for i = 0, …, n–1

Edition
Bakery Algorithm
Data Structures
 boolean choosing[n];
 int number[n];
These data structures are initialized to false and 0,
respectively

Edition
Bakery Algorithm
 Structure of Pi
do {
choosing[i] = true;
number[i] = max(number[0],number[1],…,number [n – 1]) + 1;
choosing[i] = false;
for (j = 0; j < n; j++) {
while (choosing[j]) ;
while ( (number[j] != 0) && ((number[j], j) < (number[i], i)) ) ;
}
Critical Section
number[i] = 0;
Remainder section
} while (1);

Edition
Bakery Algorithm
Process Number
P0 3
P1 0
P2 7
P3 4
P4 8
P0 P2 P3 P4
(3,0) < (3,0) (3,0) < (7,2) (3,0) < (4,3) (3,0) < (8,4)
Number[1] = 0 Number[1] = 0 Number[1] = 0 Number[1] = 0
(7,2) < (3,0) (7,2) < (7,2) (7,2) < (4,3) (7,2) < (8,4)
(4,3) < (3,0) (4,3) < (7,2) (4,3) < (4,3) (4,3) < (8,4)
(8,4) < (3,0) (8,4) < (7,2) (8,4) < (4,3) (8,4) < (8,4)
1 3 2 4

Edition
Bakery Algorithm
 P1 not interested to get into its critical section ⇒ number[1] is 0
 P2, P3, and P4 wait for P0
 P0 gets into its CS, get out, and sets its number to 0
 P3 get into its CS and P2 and P4 wait for it to get out of its CS
 P2 gets into its CS and P4 waits for it to get out
 P4 gets into its CS
 Sequence of execution of processes:
<P0, P3, P2, P4>

Edition
Bakery Algorithm
 Meets all three requirements:
 Mutual Exclusion:
(number[j], j) < (number[i], i) cannot be true for both Pi and Pj
 Progress:
 Decision takes complete execution of the ‘for loop’ by one
process
 No process in its ‘Remainder Section’ (with its number set to
0) participates in the decision making
 Bounded-waiting:
At most one entry by each process (n-1 processes) and then
a requesting process enters its critical section (First-Come-
First-Serve)

Edition
Synchronization Hardware
Many systems provide hardware support for critical section code
Uniprocessors – could disable interrupts
Currently running code would execute without preemption
Generally too inefficient on multiprocessor systems
Modern machines provide special atomic hardware instructions
 Atomic = non-interruptable
Either test memory word and set value
Or swap contents of two memory words

Bakery algorithm

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Bakery algorithm