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Synchronization II. CSE5306 Lecture Quiz 17 due at 5 PM Tuesday, 7 October 2014. 6.3.5 A Token Ring Algorithm. A token ring network can make a resource mutually exclusive. A token circulates around the ring till a needy process can briefly hold it, while using the resource.

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synchronization ii

Synchronization II

CSE5306 Lecture

Quiz 17 due at 5 PM

Tuesday, 7 October 2014

6 3 5 a token ring algorithm
6.3.5 A Token Ring Algorithm
  • A token ring network can make a resource mutually exclusive.
  • A token circulates around the ring till a needy process can briefly hold it, while using the resource.
  • The protocol is simple, fast, and it never starves a process. When a crashed process cannot acknowledge its token passing message, the sender throws the token over it to the next process.
  • But the token can be lost when its holder crashes, and using a simple expiration timer risks the possibility that the holder actually is still using the token.
6 3 6 comparing the algorithms
6.3.6. Comparing the Algorithms
  • The centralized algorithm is simple and efficient, requiring only 3 messages to enter and exit a critical region. Assuming brief resource usage, access is delayed by only 2 messages. Crashes can bring down system.
  • The decentralized algorithm makes k attempts to send/rcv messages to/from m coordinators. Its access delay is 3mk. It is immune to crashes, but starvation and efficiency are problems.
  • The distributed algorithm sends request and grant messages to its n-1 peers, with access delay 2(n-1). Crashes can bring it down.
  • The token ring algorithm may waste hours sending token-passing messages, while no one is using the resource. Access delay is from 1 to n-1. Crashes can bring down system.
6 4 global positioning nodes
6.4 Global Positioning Nodes
  • Geometric overlay networks organize large numbers of communicating (possibly mobile) nodes by their physical positions:
  • A client may be redirected to a server’s closest replicate, assuming everyone’s position is known.
  • Optimal replica placement minimizes average client-to-replica response time.
  • Position-based routing uses only local information in forwarding messages to waypoints closest to the destination.
global positioning continued
Global Positioning (continued)
  • Contacting three nodes can fix a mobile client’s position, from their measured message latencies, d = √(xi-xp)2+(xi-xp)2, i=1..3
  • If inconsistent distances violate the triangle inequality, use only two (landmark) nodes at a time, and minimize their cumulative error.
  • Or model equal-tension springs from every node to the client.
r u o k
R U O K ?
  • Which mutual exclusion algorithm is most reliable? __
  • Centralized.
  • Decentralized.
  • Distributed.
  • Token ring.
r u o k1
R U O K ?

2. A client contacts three nearby nodes at (1,2),(2,3) and (3,1), measuring messages latencies of 6, 4 and 2 microseconds. What is the client’s position? __

  • (11,3) b. (-11,3) c. (-11,-3) d. (11,-3)
r u o k2
R U O K ?

Match the following algorithms with their definitions below.

3. Centralized algorithm. __

4. Decentralized algorithm. __

5. Distributed algorithm. __

6. Token ring algorithm. __

  • Others grudgingly grant resource access, if my timestamp is earlier than theirs.
  • Many replicated resources and their DHT-based coordinators allocated around a Chord ring.
  • Ask the coordinator process’ permission to use the resource.
  • Message circulates till a needy process briefly holds it, while using the resource.
r u o k3
R U O K ?

Match the following terms with their definitions below.

7. Starved. __

8. Geometric overlay network. __

9. Optimal replica placement. __

10. Position-based routing. __

11. Triangle inequality. __

12. Landmarks. __

  • Uses only local information in forwarding messages to waypoints closest to the destination.
  • Many pairs of nodes, which may be used to triangulate position.
  • A process with reduced access a resource.
  • Side c < a + b.
  • Large numbers of communicating (possibly mobile) nodes organized by their physical positions.
  • Minimizes average client-to-replica response time.
6 5 election algorithms
6.5 Election Algorithms
  • An election algorithm chooses one of many nodes to serve as coordinator.
  • All processes know each other’s node identification numbers, but they do not know which nodes are currently active.
  • Traditionally the bully and ring algorithms were most popular.
  • Wireless environments and large-scale systems call for more sophistication.
6 5 1 traditional election algorithms
6.5.1 Traditional Election Algorithms
  • Bully algorithm:
    • When node P finds its coordinator is unresponsive, it sends an ELECTION message to all higher numbered nodes.
    • Each receiving node answers “OK” and recursively performs steps 1-3.
    • If nobody answers, P sends a victory (COORDINATOR) message and becomes the new coordinator.
  • Crashed nodes also hold elections, when they restart.
  • The name comes from the biggest numbered node always winning.
traditional elections continued
Traditional Elections (continued)
  • Ring Algorithm:
    • When node P finds its coordinator is unresponsive, it circulates an ELECTION message around the ring.
    • Each receiving node responds and adds its number to the circulating message.
    • When P receives its own ELECTION message, it circulates a COORDINATOR message around the ring to inform everyone that it is the new coordinator.
  • If two nodes do this simultaneously, messages circulated by the lower-numbered node will have the last word.
6 5 2 wifi elections
6.5.2 WiFi Elections
  • Traditional elections don’t work in wireless networks, where message passing is unreliable and topologies change. WiFi elections choose the best candidate:
    • A node that receives an ELECTION message for the first time calls the sender its parent.
    • It Acks its parent’s message, telling its battery life and other resources.
    • And it relays the parent’s message to its other neighbors.
    • When other nodes call it their parent and Ack its message, it returns their resource reports to its parent.
  • Eventually the initiator designates the most resourceful node as their new coordinator.
  • Each node joins only one of many concurrent elections.
6 5 3 large scale system elections
6.5.3 Large-Scale System Elections
  • Massive distributed systems elect superpeers:
    • Low-latency access to normal nodes.
    • Even distribution across the overlay network.
    • Predefined ratio to the total number of nodes.
    • Serve more than a fixed number of normal nodes.
  • In structured DHT-based systems, we can identify a superpeer as number 11100000 and its nodes as 111ΦΦΦΦΦ, where Φ=don’t care.
  • In unstructured gossip-based systems, we can randomly distribute tokens. The tokens repel each other, such that each token forces nearby tokens away. Every token eventually settles on a superpeer.
6 6 summary
6.6 Summary
  • Synchronization causes communicating processes to “do the right thing at the right time,” without a globally shared clock.
  • Processes synchronize clocks by exchanging clock values and accounting for propagation delays.
  • Latencies can be estimated from separation distances.
  • Lamport showed that ordering is more important than real time. His timestamps extend to vector timestamps that order events by causality.
  • A coordinator can ensure a resource’s mutual exclusion; i.e., only one process’ access at a time.
  • Election algorithms safely replace crashed coordinators and select superpeers.
r u o k4
R U O K ?

Match the following election algorithms with their definitions below.

13. Bully. __

14. Ring. __

15. WiFi. __

16. Superpeer. __

  • Highest numbered node always wins.
  • Lowest numbered node always has the last word.
  • Initiator chooses most resourceful node as coordinator.
  • Zeroth node identifier in each series marks DHT-based coordinators. Repulsive tokens settle on multiple coordinators in randomly unstructured systems.