Selected topics in distributed computing

Shmuel Zakszaks@cs.technion.ac.il

l'Aquila, Distributed Computing, 12-14/1/09 2

Part 1: Lower bounds and

impossibility results

Part 2: On two fundamental notions:

snapshot in a network,

cost of synchronization

Part 3: Self stabilization

A new approach to fault toleranceA unified approach to transient

failures by formally incorporating them into the design model

shared memory, synchronous

clock synchronization

Part 3.1: Clock synchronization (of Gouda and Herman)

Program at each processor p :

choose a neighbor x of p ; if t(p) = t(x) then t(p):= t(p)+1;

Stabilizing if all start with the same time, however …

Is it stabilizing? no! (may even deadlock)

if t(p) = t(x) then t(p):= t(p)+1;

choose a neighbor x of p ;

Is it stabilizing?

if t(p) < t(x) then t(p):= t(x);

Is it stabilizing? yes!

Is it stabilizing? no!

choose a neighbor x of p

if t(p) = t(x) then t(p):= t(p)+1; if t(p) < t(x) then t(p):=

fairly;

Is it stabilizing? yes!

f (s) =n×range(s) +top(s)

range(s) =max clock - min clock

top(s) =# max clocks

f (s) =5 ×3+2 =17

n=5, range(S)=7-4=3, top(S)=2

n=5, range(S1)=7-7=0, top(S1)=5

1f (s ) =5 ×0 +5 =5 < 17 =f (s)

n=5, range(S2)=8-7=1, top(S2)=2

2f (s ) =5 ×1+2 =7 < 17 =f (s)

2 3s s

n=5, range(S3)=9-8=1, top(S3)=2

3 2f (s ) =5 ×1+2 =7 =7 =f (s )

2. s s' f (s' ) f (s)

1. f (s) =n×range(s) +top(s) n

3. Eventually s ... s' s.t.

f (s' ) <f (s)

hw: prove.

Use for:

correctness (especially termination) + complexity

distributed control ring of (finite-state machine) processes for each machine define privileges (when a

transition is enabled) nodes are influenced only by neighbors legitimate states in each legitimate state there is at least one

privileged machine in each legitimate state each possible move

will bring the system again to a legitimate state

Part 3.2: Mutual exclusion (Dijkstra’s 1st algorithm)

A system is self-stabilizing if , regardless of the initial state and regardless of the privilege selected each time for the next move, at least one privilege will always be present and the system is guaranteed to find itself in a legitimate state after a finite number of moves.

non-stablestates

stablestates

Finite number of transitions

Token ring. N machines, denoted 0 through N-1. A machine is priviliged (“has token”) or

not priviliged. daemon/scheduler determines which of

the priviliged machines will make the next move

Legitimate States: all states with exactly one privileged machine.

In Dijkstra’s model:

Notes to consider:

scheduler (daemon) : points at each time to one of the privileged

machines, or point at each time to one of the machines. centralized scheduler, distributed schedulerfairness: Do we need fairness, and of what kind? Unconditional Fairness: Each machine gets

pointed to infinitely often. Type of fairness: - each machine is pointed at infinitely often, - round robin, or - other.

N processors 0,1,…,N-1 Solution with k-state Machines machine P0

if x[0] = x[N-1] then x[0] := x[0]+1 mod k

machine Pi ( i= 1,2,…,N-1)

if x[i] ≠ x[i-1] then x[i] := x[i-1]

Dijkstra’s 1st algorithm

N=4, K=3

State : 0,1,2

machine P0: if x[0] = x[N-1] then x[0] := x[0]+1 mod k

machine Pi: if x[i] ≠ x[i-1] then x[i] := x[i-1]

So far:

Etc…

Example 1

0 11 0

Etc…

Example 2

1Etc…

Example 3

Machine Number Privileged Non-privileged

0 X[0] = X[N-1] X[0] X[N-1] 1,…,N-1 X[i] X[i-1] X[i] = X[i-1]

Informally:“If machine is privileged, then make it non-privileged”.

Legitimate States: exactly one privileged machine

Legitimate States

0 0 0 0 0 ……

1 0 0 0 0

1 1 0 0 0 0 0 K–1 K–1 K–1……

1 1 1 1 1 0 K-1 K–1 K–1 K–1

2 1 1 1 1 … 2 2 2 2 2 … K–1 K–1 K–1 K–1 K–1

Goal: Show that from an arbitrary state, the system stabilizes to a legitimate state in a finite number of steps.

Intuition:

0 1 2 3 4

0 2 4 1 01 2 4 1 01 2 4 1 12 2 4 1 1

2 2 2 1 1

0 1 2 3 4

0 3 2 1 01 3 2 1 01 1 2 1 01 1 2 2 0

1 1 2 2 2

N=5, K = 5N=5, K = 4

0 1 2 3 4

0 1 2 1 0 0 0 2 1 01 0 2 1 01 0 2 1 11 0 2 2 11 0 0 2 1

0 1 2 3 4

1 1 0 2 12 1 0 2 12 1 0 2 22 1 0 0 22 1 1 0 22 2 1 0 20 2 1 0 20 2 1 0 00 2 1 1 00 2 2 1 00 0 2 1 0

N=5, K = 3

hw: can you get to this configuration?

hw: which configurations have the same property? (Check also for all other examples.)

Theorem:

Assuming a centralized scheduler: starting with any initial configuration, Dijkstra’s 1st algorithm stabilizes iff K > N-2 .

Lemma 2: (liveness) At every configuration, at least one machine is privileged.

Lemma 3: Starting from any configuration, P0 will eventually make a move.

Corollary: Starting at any configuration, P0 will

make an infinite number of steps.(this still does not guarantee stabilization.)

Lemma 1: If all states are equal then the system is stabilized.

Definition: Pi is called special in a configuration if

Lemma 4: If P0 is special in a configuration, then the system will eventually stabilize.

j i x[j ] x[i]

Lemma 5: If in a configuration one of the states is missing, then the system will eventually stabilize.

Theorem:

Proof:

Case 1: K< N-1 Case 2: K = N-1

Case 3: K = N Case 4: K > N

Theorem:

Proof:

( e.g., N=4, k=2)

Case 1. k < N-1

Theorem:

Proof:

Case 4. k > N

… Lemma 5

Theorem:

Proof:

Case 3. k = N

… Lemma 5Either one state is missing,

or all states are distinct. … Lemma 4

Theorem:

Proof:

Case 2. k = N - 1

… Lemma 5Either one state is missing,

or one state occurs twice.

… Lemma 4Either P0 is special,

or x[0] = x[i] i < N-

i = N-1

P0 is not enabled, and after any other move there is a mising state, and … Lemma 5.

If Pi, i 0 makes a move, there is missing state, and done by Lemma 5. If P0 makes a move, we get back to the previous case.

So, one state occurs twice.

Theorem:

Assuming a distrubted scheduler: starting with any initial configuration, Dijkstra’s 1st algorithm stabilizes iff K > N-1 .

hw: prove

S = a configuration (a system state)

r(S) = number of maximal runs of equal states Example: S = 2222011000, f(S) = 4 r(S) + 1 if first(S) = last(S)f(S) = r(S) otherwise.

Note: S is legitimate iff f(S) = 2.

Use of a potential function

Show: If K > N and f(S) > 2, then f(S) never increases and is eventually decreased.

Recall: A common technique to prove termination and correctness

Hw: 1. finish the proof.

2. use f to analyze complexity

Applications

Distributed data structures Digital and analog circuits Genetic algorithms Network protocols Sensor networks

Some general comments on self stabilization

is a special kind of fault tolerance guarantees an automatic recovery from

a transient failure as a design goal is a too strong

property and thus either too difficult to achieve or achieved at the expense of other goals

Self stabilization

Uniform protocol : same program at each processor

Theorem: There is no uniform protocol that solves the mutual exclusion problem (under either a centralized or a distributed scheduler)

Back to Dijkstra’s algorithm

N processors 0,1,…,N-1 Solution with 3-state Machines P0:

if x[0] +1=x[1] then x[0]:=x[0]-1 PN-1 :

if (x[N-2] =x[0]) (x[N-1]≠ x[0]+1) then x[N-1]:=x[0]+1;

Pi ) i= 1,2,…,N-2( :if (x[i]+1=x[i-1]) (x[i]+1=x[i+1])

then x[i] := x[i]+1;

Part 3.3: Mutual exclusion (Dijkstra’s 3rd algorithm)

State : 0,1,2

P0: if x[0] +1=x[1] then x[0]:=x[0]-1

p2p3 0

PN-1 :

if (x[N-2] =x[0]) (x[N-1]≠ x[0]+1) then x[N-1]:=x[0]+1;

p2p3 0

Pi ) i= 1,2,…,N-2( :if (x[i]+1=x[i-1]) (x[i]+1=x[i+1])

then x[i] := x[i]+1;2

p2p3 0

Pi ) i= 1,2,…,N-2( :if (x[i]+1=x[i-1]) (x[i]+1=x[i+1])

then x[i] := x[i]+1;2

p2p3 0

Pi ) i= 1,2,…,N-2( :if (x[i]+1=x[i-1]) (x[i]+1=x[i+1])

then x[i] := x[i]+1;2

p2p3 0

p2p3 1

If all are in the same state?

if )x[N-2] =x[0]( & )x[N-1]≠ x[0]+1( then x[N-1]:=x[0]+1;

if )x[i]+1=x[i-1]( v )x[i]+1=x[i+1]( then x[i] := x[i]+1;

if x[0] +1=x[1] then x[0]:=x[0]-1

[0] [1] [2] ... [ 1]x x x x n

For two neighbors

[ ] [ 1]

x i x i

[0] [1] [2] ... [ 1]

x x x x n

s: 2 1 2 0 1 1 2 2

f)s( = number of + 2 x number of

f)s( = 4 + 2 x 1 = 6

Lemma 1: If there is a unique arrow in a configuration, then it moves back and forth infinitely.

Corollary: It suffices to show that from any configuration we eventually reach a configuration with a unique arrow.

Lemma 2: (liveness) Starting from any initial configuration, at least one process is priveleged.

Lemma 3: Starting from any configuration, eventually .

Proof: if during an execution - .

else - f=0, but then there is no arrow, and in

the next step there is a unique arrow, and

then apply Lemma 1 .

Lemma 4: Between every two successive moves of PN-1 there is at least one move of P0.Proof: recall: the program for PN-1 :

if (x[N-2] =x[0]) (x[N-1]≠ x[0]+1)

then x[N-1]:=x[0]+1; After PN-1 makes a step x[N-1]=x[0]+1,

And this condition makes PN-1 non-privileged.

This condition can become true only after P0 makes a

step [ P0: if x[0] +1=x[1] then

x[0]:=x[0]-1 ]

Lemma 5: A sequence of moves in which P0 does not move is finite.

(therefore – impossible by Lemma 2)Proof: by Lemma 4 – in such a sequence

PN-1 makes at most one step. so, from some point only Pi makes a steps, .

1 2i n

steps 1,2,3,4,5. 1,2 – no change in arrows 3,4,5 – decrease no. of arrows..

Lemma 5: A sequence of moves in which P0 does not move is finite.

(therefore – impossible by Lemma 2)Proof (cont.): from some point only steps 1 and 2.

Follows from topology .

Theorem: Starting from any initial configuration, the system eventually stabilizes.

Proof: by Lemma 5 P0 makes infinite no. of moves.

P0 … P0 … P0 … P0 … P0 … …

Proof (cont.)

P0 … P0 … P0 … P0 … P0 … …

phases

“there exists a rightmost arrow, and it points to the right”

Falsified in each phase.P0 … P0 … P0 … P0 … P0 … …

F T F T F T F T F T

This is done only in steps 3, 4, 6 6 - ok so, assume only 3 and 4. in each phase: they have Δf=-3. 0 and 7 Δf<=2. others Δf=0. so, each phase f is decreased by at least

Complexity?

)following Chernoy, Shalom and Z.(

hw: 2 25 131 stabilization time 36 18

• E. W. Dijkstra, Self-stabilizing systems in spite of distributed control, CACM, 17,11, 1974, pp. 643-644.

• E. W. Dijkstra, A belated proof of self-stabilization, Distributed Computing, 1, 1, 1986, pp. 5-6.

• M. G. Gouda and T. Herman, Stabilizing Unison, Department of Computer Science, 1990.

• V. Chernoy, M. Shalom and S. Zaks, A Self-stabilizing algorithm with tight bounds for mutual exclusion on a ring, DISC 2008.

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