Time and Global State

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Time and Global State

• Jenhui Chen
• Office number 5990
• http//www.csie.cgu.edu.tw/jhchen

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Introduction

• Time is a quantity we often want to measure
  accurately.
• Algorithms that depend upon clock synchronization
  have been developed for several problems in
  distribution.

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Skew between computer clocks in a distributed
system
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Clock synchronization using a time server
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An example synchronization subnet in an NTP
implementation
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Figure 10.4Messages exchanged between a pair of
NTP peers
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Figure 10.5Events occurring at three processes
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Figure 10.6Lamport timestamps for the events
shown in Figure 10.5
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Figure 10.7Vector timestamps for the events
shown in Figure 10.5
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Figure 10.8Detecting global properties
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Figure 10.9Cuts
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Figure 10.10Chandy and Lamports snapshot
algorithm
Marker receiving rule for process pi On pis
receipt of a marker message over channel c if
(pi has not yet recorded its state) it records
its process state now records the state of c as
the empty set turns on recording of messages
arriving over other incoming channels else
pi records the state of c as the set of messages
it has received over c since it saved its
state. end if Marker sending rule for process
pi After pi has recorded its state, for each
outgoing channel c pi sends one marker message
over c (before it sends any other message over
c).
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Figure 10.11Two processes and their initial
states
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Figure 10.12The execution of the processes in
Figure 10.11
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Figure 10.13Reachability between states in the
snapshot algorithm
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Figure 10.14Vector timestamps and variable
values for the execution of Figure 10.9
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Figure 10.15The lattice of global states for the
execution of Figure 10.14
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Figure 10.16Algorithms to evaluate possibly f
and definitely f
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Figure 10.17Evaluating definitely f