Synchronization

1
Synchronization

• CSCI 4900/6900

2
Causality

• Lamports algorithm leads to all events being
  ordered irrespective of whether they are
  dependent on each other
• Example Electronic bulletin board
• Users may post new articles or respond to
  existing ones
• No dependence between different threads
• Lamports algorithm imposes order even between
  independent events
• CAUSALITY Two events are ordered only if one
  occurs after the process knows about the other

3
Vector Timestamps for Causality

• Each event is assigned a vector timestamp
• If VT(a) lt VT (b) then a is said to causally
  precede b
• Each process maintains a vector Pi maintains Vi
• Vii Number of events occurred at Pi
• Vij k then Pi knows that k events have
  occurred at Pj
• When Pi sends message to Pj, it piggybacks Vi
• Pj knows about how many events have occurred at
  Pi
• Pj also knows how many events have occurred at
  other processes
• Pj adjusts its local vector by setting Vjk
  maxVjk, vtk
• Increments Vji by 1
• Delivers a message r only if vt(r) j Vkj
  1 and vt(r) i lt Vki for all i ltgt j

4
Global State

• Local states of each processes together with
  messages in transit
• Global states is needed for detecting termination
  conditions/deadlocks
• Distributed snapshot algorithm (Chandy Lamport)
• Reflects state where distributed system might
  have been
• Always represents a consistent global state
• If it is recorded that a process received a
  message we should have also recorded that the
  message has been sent
• Distributed snapshots via Cuts

5
Cuts for Global State

1. A consistent cut
2. An inconsistent cut

6
Distributed Snapshots Algorithm

• Assumption Processes are connected via
  uni-directional point-to-point links (channels)
• Any process (p) may initiate algorithm
• P records its local state and sends a marker to
  along each outgoing channel
• Marker indicates that receiver should participate
  in the snapshot algorithm
• When process Q receives a marker
• It saves its local state, if it has already not
  done so, and sends marker along each outgoing
  channel
• If Q had recorded local state, it records the
  state of the channel
• DONE to indicate that process has done its part

7
Global Snapshots

1. Organization of a process and channels for a
   distributed snapshot

8
Global State (3)

• Process Q receives a marker for the first time
  and records its local state
• Q records all incoming message
• Q receives a marker for its incoming channel and
  finishes recording the state of the incoming
  channel

9
Termination Detection

• Detecting the termination of a distributed
  computation
• Initiator starts the process by sending a marker
  to all processes
• Each process records its state
• On completing its part of global snapshots, it
  checks whether there are no messages in transit
• If no transit messages process sends DONE to
  initiator
• Else sends CONTINUE
• Initiator has to restart the detection algorithm