CS603 Process Synchronization

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CS603Process Synchronization

• February 11, 2002

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Synchronization Basics

• Problem Shared Resources
• Generally data
• But could be others
• Approaches
• Model as sequential process
• Model as parallel process
• Models assume global state!

P1
P3
P2
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Global State

• Plausible in sequential system
• But expensive
• Solution Send only needed portions of state
• What about Parallel model?
• Distributed Virtual Memory!
• With all the advantages and problems
• What about non-memory resources?
• Question What is global state?

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Alternative Mutual Exclusion

• Control access to shared resources
• Only one process allowed to use resource at a
  time
• Doesnt require global state
• Sequential simulation can be viewed as mutual
  exclusion on entire state
• But we can do better
• Control only when resource in contention

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Mutual Exclusion TechniquesCentralized Approach

• Request for resource sent to coordinator
• When available, responds go ahead
• When done, requestor sends release to
  coordinator
• Problems
• Doesnt scale
• Single point of failure
• No way to distinguish failure from resource in use

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Evaluating Mutual Exclusion Techniques

• Does it guarantee mutual exclusion?
• Does it prevent starvation?
• If a process requests a resource, it is
  guaranteed to eventually get it
• Assumes resource use bounded
• Is it fair?
• What do we mean by fair?
• Does it scale?
• Does it handle failures?

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Mutual ExclusionToken Passing

• Token associated with resource
• Initially held by first process
• When resource not in use, token passed to next
  process
• Problems
• Potential delay
• Resource need and path of token unrelated
• Communication cost
• Failure causing loss of token

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Mutual ExclusionToken Passing

• Does it guarantee mutual exclusion?
• Yes
• Does it prevent starvation?
• Yes
• Is it fair?
• Everyone gets their turn
• But not first-come, first-served
• Does it scale?
• Yes
• Does it handle failures?
• No

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Timestamp-based request(Lamport 78)

• When resource needed broadcast request
• Name of resource needed
• Process ID of requestor
• Time at requestor
• On receiving a request message
• If using resource, queue request
• If waiting for resource, compare request
  timestamp with your own request timestamp
• Lowest wins if other is lower, send OK,
  else queue
• If not waiting for or using resource, send OK
• When OK received from all other processes, use
  resource

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Timestamp-based request(Lamport 78)

• Does it guarantee mutual exclusion?
• Yes
• Does it prevent starvation?
• Yes
• Is it fair?
• First-come, first-servedWhere first defined by
  clock synchronization
• Does it scale?
• Requires all processes to respond to any request
• Does it handle failures?
• No single point of failure
• Instead, n points of failure!

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Comparison of Approaches
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Whats next?

• Fault tolerant solutions
• Colored Ticket Algorithm
• Multiple resources
• Dining philosophers problem