Solutions of a Problem in Concurrent Programming

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Solutions of a Problem in Concurrent Programming

• Proseminar Software Pioneers
• By Prof. Dr. Heike Wehrheim
• Diplo. Inform. Björn Metzler

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Contents

• A brief introduction of Dijkstra and Lamport
• Problem in Concurrent Programming
• Dijkstras Solution
• Semaphores Solution
• Lamports Solution
• Conclusion

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Edsger W. Dijkstra and Leslie Lamport

• Photographs are from google photo

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Problem in Concurrent Programming

• N computers
• one critical section.
• Two requirements
• Communication between computers
• Undividable writing or reading

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Basic Goals of Dijkstras Solution

• Symmetric between N computers
• No After you After you blocking

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Dijkstras Solution
Part I Interger j Li0bifalse Li1if
k?i then Li2 begin citrue Li3
if bk then ki goto Li1
end else . .
Critical Section
1
2
3
N
i

• Code from Solution of a Problem in Concurrent
  Programming Control E.W.Dijkstra

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Dijkstras Solution
Part II Li4begin cifalse for
j1 step 1 until N do if j?i and
not cj then goto Li1 end critical
section citruebitrue . . go
to Li0
Critical Section
i
1
2
3
N
Code from Solution of a Problem in Concurrent
Programming Control E.W.Dijkstra
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Problems in Dijkstras Solution

• When the i-th computer breaks down in the
  critical section, the whole system will be in a
  dead lock

Critical Section
????
i
Hey, we have waited too long!
1
2
3
N
What is he doing for such a long time?
Who knows. Just have to wait here.
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Semaphores Solution

• Semaphore values
• Two indivisible actions
• P- operation
• V- operation
• Random choosing from the waiting list

begin semaphore mutex mutex 1 parbegin
begin L1 P(mutex) critcal
section 1 V(mutex)
remainder of cycle 1
go to L1 end
begin L2 P(mutex)
critical section 2
V(mutex) remainder of
cycle 2 go to L2
end parendend end
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Problems of Semaphores Solution

• We can no forecast or decide which process will
  be removed from the waiting list.
• Explicit control - deadlock

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Lamports Solution

• Begin integer j
• L1 choosingi1
• numberi1maximum(number1,...numberN)
  
• choosingi0
• for j1 step 1 until N do
• begin
• L2 if choosingj?0 then goto L2
• L3 if numberj?0 and (numberj,j)lt
  (numberi,i) then goto L3
• end
• Critical section
• Numberi0
• Noncritical section
• Goto L1
• end

Critical Section
1
2
3
N
i
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Lamports Solution

• Begin integer j
• L1 choosingi1
• numberi1maximum(number1,...numberN)
  
• choosingi0
• for j1 step 1 until N do
• begin
• L2 if choosingj?0 then goto L2
• L3 if numberj?0 and (numberj,j)lt
  (numberi,i) then goto L3
• end
• Critical section
• Numberi0
• Noncritical section
• Goto L1
• end

Critical Section
i
1
2
3
N
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Problems in Lamports Solution

• These computers are no longer symmetrical.
• The number attribute will be too big.

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Conclusion

• Dijkstras Solution
• N computers get equal chances, and avoid after
  you after you blocking.
• Weak point Once one computer breaks down inside
  the CS, that will be a catastrophe.
• Semaphores Solution
• Based on a semaphore value with two undividable
  action P and V.
• Weak point Too reliance upon a single hardware
  component.
• Lamports Solution
• Every computer got its own attribute and dont
  afraid break-down in CS.
• This attribute will be too huge and increase the
  systems load.