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Logical Clocks (addendum)
Logical Clocks
In the class of Feb 3 the question was asked about having different numbers for different events.
Example 1
Assume that P3 has a loop that is executed n times. At the termination of the loop a message is sent to P1. The value of n is arbitrary. A part of P3’s code could look something like this:3.1: i = 0;
3.2: while (i < n)
3.3: i = i+1;
3.4: send (i,P2);
Assume that n = 2; The timing diagram would look something like what appears on the next page.
Example 1
a
b
P1 P2 P3
c
d
e
f
g
h
i
j
p
j is the event that statement 3.1 executes; k is the event that 3.2executes; l is the event that 3.3 executes; m is the event that 3.2 executes; n is the event that 3.3 executes; o is the event that 3.2 executes;p is the event that statement 3.4 executes
k
l
m
1 12
3
4
3
8
9
10
l
n
2345
7
6o
Example 1
a
b
P1 P2 P3
c
d
e
f
g
h
i
j
Assume n is 1 instead of 2. Event j is associated with statement 3.1;Event k is associated with statement 3.2; Event l is associated withStatement 3.3; Event m is associated with statement 3.2; Event n is associatedwith statement 3.4
k
l
m
1 12
3
4
3
6
7
8
l
n
2345
Example 2 In the previous examples we had statements like
“recv(x, P3)” – basically it was made explicit where a message was to be received from.
However, this doesn’t have to be the case. Essentially we can have “recv(x,?)” where the process is waiting for any process.
Assume that P2 has a code segment that looks like the following:3.1: while(;;) {3.2: recv(x,?);3.3 Process message}
P1 and P3 are processes that send messages to P2.
Example
a
b
P1 P2 P3
c
d
e
f
g
ij
m
n
o
Event e represents the execution of 3.1; Event f represents the execution of 3.2; event g represents the execution of event 3.3; event h represents the execution of 3.1; event i represents the execution of 3.2; event j represents the execution of 3.3; event k represents the execution of 3.1;Event l represents the execution of 3.2
1 112
3503
4 4
5152
51h 5
kl
5354
Lots of processing;Assume that k is 50
Example
P1 may be on a much faster machine then P3 and so both its messages may arrive at P2 before P3’s messages
Example
a
b
P1 P2 P3
c
d
e
f
g
ij
j
k
l
Event e represents the execution of 3.1; Event f represents the execution of 3.2; event g represents the execution of event 3.3; event h represents the execution of 3.1; event i represents the execution of 3.2; event j represents the execution of 3.3; event k represents the execution of 3.1;Event l represents the execution of 3.2
1 112
3503
4 4
67
51h
kl
851
5
