CPSC 668 Distributed Algorithms and Systems

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CPSC 668Distributed Algorithms and Systems

• Fall 2009
• Prof. Jennifer Welch

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Number of R/W Variables

• Bakery algorithm used 2n shared read/write
  variables.
• Tournament tree algorithm used 3n shared
  read/write variables.
• Can we do (asymptotically) better, in terms of
  fewer variables?
• No!

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Lower Bound on Number of Variables

• Theorem (4.19) Any no-deadlock mutual exclusion
  algorithm using read/write variables must use at
  least n shared variables.
• Proof Strategy Show by induction on n there must
  be at least n variables.
• For each n, there is a configuration in which n
  variables are covered means some processor is
  about to write to it.

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Appearing Quiescent

• Two configurations C and D are P-similar if each
  processor in P has same state in C as in D and
  each shared variable has same value in C as in D.
• A configuration is quiescent if all processors
  are in remainder section.
• To make the induction go through, the
  configuration whose existence we prove must
  appear quiescent to a set of processors
• C is P-quiescent if there is a quiescent
  configuration D such that C and D are P-similar

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Warm-Up Lemma

• Before a processor can enter its CS, it must
  write to an uncovered variable.
• Lemma (4.17) If C is pi-quiescent, then there is
  a pi-only schedule ? such that
• pi is in CS in ?(C) and
• during exec(C,?), pi writes to a variable that is
  not covered in C.

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Proof of Warm-Up Lemma (a)

• Since C is pi-quiescent, it looks the same to pi
  as some quiescent D.
• By ND, some pi-only schedule ? exists starting at
  D in which pi enters CS.
• When ? starts at C, pi also enters CS.

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Proof of Warm-up Lemma (b)

• Suppose in contradiction when ? is executed
  starting at C, pi writes to the set of variables
  W but all the variables in W are covered in C.
• Let P be the set of processors covering the
  variables in W.

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Proof of Warm-up Lemma (b)
?1
?2
?
C
E
Q
successively invoke ND
pj-only
pj in CS
overwrites W
Contradiction!
Only difference between C and C' are the writes
by pi, but those values are overwritten in ?1 so
the info is lost.
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Main Result

• For all k between 1 and n, for all quiescent C,
  there exists D s.t.
• D is reachable from C by steps of p0,,pk-1 only
• p0,,pk-1 cover k distinct variables in D
• D is pk,,pn-1-quiescent.

implies desired result when k n
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Proof of Main Result - Basis

• By induction on k.
• Basis k 1. Must show for all quiescent C,
  there exists D s.t.
• D is reachable from C by steps of p0 only
• p0 covers a variable in D
• D looks quiescent to the other procs.
• By warm-up lemma (a), if p0 takes steps alone, it
  eventually writes to some var.
• Desired D is just before p0 's first write.

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Proof of Main Result - Induction

• Assume for k, show for k1.

by ind. hyp.
by warm-up lemma (b)
as in pf. of warm-up lemma
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Proof of Main Result - Induction
k vars.
?0
C
C1
only p0 to pk-1 take steps
any qui. config.
p0 to pk-1 cover W pk to pn-1 qui.
by ind. hyp.
but why is the same set of k vars covered again?
pk-only
?
?
D1'
pk covers x not in W
p0 to pk-1 o'write W, become quiescent
pk in entry looks qui. to rest
k1 vars.
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Proof of Main Result - Fix

• The result of applying ? to D1 might result in a
  different set of k variables, W', being covered
  instead of W.
• If W' includes x, we have not succeeded in
  covering an additional variable.
• To fix this problem, repeatedly apply inductive
  hypothesis to get
• C1,D1,C2,D2,C3,D3,
• Since number of variables is finite, there exist
  i and j such that in Ci and Cj the same set of k
  variables is covered.
• Then apply same argument as before, replacing C1
  and C2 with Ci and Cj.

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Fast Mutual Exclusion

• The read/write mutex algorithms we've seen so far
  require a processor to access f(n) variables in
  the entry section even if no contention.
• It would be nice to have a fast algorithm if no
  competition, a processor enters CS in O(1) steps.
• Even better would be an adaptive algorithm
  performance depends on number of currently
  competing processors, not total number.

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Fast Mutual Exclusion

• Note that multi-writer shared variables are
  required to be fast.
• Combine two mechanisms
• provide fast entry when no contention
• provide no deadlock with there is contention

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Contention Detector Overview

• A doorway mechanism captures a set of processors
  that are concurrently accessing the detector
• Use a race to choose a unique one of the captured
  processors to "win"

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Contention Detector

• Uses two shared variables, door and race.
• Initially door "open", race -1.
• race id
• if door "closed" then return "lose"
• else
• door "closed"
• if race id then return "win"
• else return "lose"

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Analysis of Contention Detector

• Claim At most one processor wins the contention
  detector.
• Why?
• Let K be set of procs. that read "open" from door
  in Line 2.
• Let pj be proc. that writes to race most recently
  before door is first set to "closed".
• No node pi other than pj can win
• If pi is not in K, it loses in Line 2.
• If pi is in K, it writes race before pj does but
  checks again (Line 5) after pj 's write and
  loses.

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Analysis of Contention Detector

• Claim If pi executes the contention detector
  alone, then pi wins.
• Why?
• Trace through the code when there is no
  concurrency.

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Ensuring No Deadlock

• If there is concurrency, it is possible that no
  processor wins the contention detector.
• To ensure progress
• nodes that lose the contention detector
  participate in an n-processor ME alg.
• The winner of the n-processor alg. competes with
  the (potential) winner of the contention detector
  using a 2-processor ME alg.
• Winner of 2-processor alg. can enter CS

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Ensuring No Deadlock
lose
contention detector
n-proc. mutex
win
play role of p0
play role of p1
2-proc. mutex
critical section
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Discussion of Fast Mutex

• Be careful about the exit section contention
  detector needs to be reset properly
• This is a modular presentation doesn't specify
  particular n-proc and 2-proc subroutine mutex
  algorithms
• Not adaptive even if only 2 procs are
  contending, execute the potentially expensive
  n-proc algorithm