Time This powerpoint presentation has been adapted from: 1)bina/cse486/.../TimeGlobalState...
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Transcript of Time This powerpoint presentation has been adapted from: 1)bina/cse486/.../TimeGlobalState...
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Time
This powerpoint presentation has been adapted from:1) www.cse.buffalo.edu/~bina/cse486/.../
TimeGlobalStatesApr20.ppt
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Contents
Introduction Clocks, events and process states Synchronizing physical clocks Logical time & logical clocks
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Contents
Introduction Clocks, events and process states Synchronizing physical clocks Logical time & logical clocks
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Time
Why time is important in distributed systems?A quantity that needs to be measured
accurately• to know at what time of day a particular event
occurred at a particular computer• is important for auditing purposes• to maintain the consistency of distributed data
Introduction
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Contents
Introduction Clocks, events and process states Synchronizing physical clocks Logical time & logical clocks
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Time
How to timestamp events in terms of their execution?Consider the following notations
• ϸ– A collection of N processes pi, i = 1,2, .. N (p executes on a single
processor)
• si
– The state of pi (each p has a state, the state changed when it is executed)
• Actions of pi
– Operations that transform pi’s state (p executes with a series of actions.
– Send or receive message between pi
Clocks, events and process states
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Time
e• Event: occurrence of a single action
Relation between the events(i)• The series of events for a single process p• e.g., e i e` : e occurs before e` at pi
history(pi ) = hi = <ei0, ei
1, ei2, …>
• The series of event e in process pi
Clocks, events and process states
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Time
How to timestamp the events (e)?Clock in computer:
• Each computer has its own physical clock• Is a device that counts oscillations occurring in a
crystal at a definite frequency• The OS reads the node’s hardware time: Hi(t)
» The counts of oscillation since an original point• Then, scale it and add offset to produce software
clock, Ci(t) = Hi(t)+» To timestamp of an event
Clocks, events and process states
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Time
Computer clocks tend not to be in a perfect agreement.Clock drift
• Clocks count time at different rates
Clock skew• The instantaneous (direct) difference between the readings of any
two clocks
Why? Oscillators are subject to physical variations • Frequency, temperature
Clocks, events and process states
Network
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Time
How to synchronize computer clocks?Use external source that provide highly accurate
timeUse atomic oscillatorUsed by International Atomic Time
Clocks, events and process states
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Time
What is the international standard for time keeping?Coordinated Universal Time (UTC)
• Based on atomic time• Time is coordinated with astronomical time• UTC signal is broadcasted from land-based radio station to
satellites covering many parts of the world
Clocks, events and process states
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Contents
Introduction Clocks, events and process states Synchronizing physical clocks Logical time & logical clocks
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Time
How to know at what time of the day an event occurs? Two types of synchronization:
• External• Internal
Notations:• Ci : pi’s clock• I : an interval of real time
Synchronizing physical clocks
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Time
External synchronizationFor a synchronization bound D > 0, and for a source S of UTC
time, |S(t)-Ci(t)| < D, for i = 1, 2, … N and for all real times t in IClocks Ci are accurate to within the bound D
Internal synchronizationFor a synchronization bound D > 0, |Ci(t)-Cj(t)| < D for i, j =1,2, …
N, and for all real times t in IClocks Ci agree within the bound D
Clocks that are internally synchronized are not necessarily externally synchronized If the system ϸ is externally synchronized with a bound D, then the same
system is internally synchronized with a bound of 2D.
Synchronizing physical clocks
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Time
Synchronization in a synchronous system there will be a minimum time of transmission where no other processes
executed at the same time and no other network traffic existed. Maximum time of transmission in synchronous mode is always set (i.e.,
time out is applied) Protocol
• Sender: send M(t)• Receiver: set time to t + Ttrans
Bounds are known in synchronous system• min < Ttrans < max (constant)
Optimum transmission time, • Ttrans = (min+max) / 2• Receiver’s clock = t + (min+max) / 2
Synchronizing physical clocks
t t+maxt +Ttranst + min
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Time
The common practice in distributed system is asynchronous;The factors lead to message delay are not bounded
(no upper bound max)The Internet is asynchronousTtrans = min + x, where x ≥ 0, the value of x is not
knownMethods for synchronizing clock in asynchronous
distributed systems:• Cristian’s method• The Berkeley algorithms• The Network Time Protocol
Synchronizing physical clocks
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Time
Cristian’s method of synchronizing clocksUse time serverProtocol
• mr – process p requests the time from server • mt – process receives the time (t is inserted in the
message)• Tround - process p records the round-trip (send request-get
reply)• Estimated time: t + Tround/2
Synchronizing physical clocks
mr
mtp Time server,S
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Time
Cristian’s method of synchronizing clocksAccuracy analysis
• If the minimum delay of a message transmission is min, then accuracy: (Tround/2 – min)
Synchronizing physical clocks
t
t +Tround-min
t +Tround/2
t + min
t +Tround
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Time and Global State
The Berkeley algorithmsInternal synchronization using a coordinator
computer (master). Other computers that their clocks need to be synchronized is known as slaves1. The master polls the slaves’ clocks2. The master estimates the slaves’ clocks by round-
trip time3. The master averages the slaves’ clock values4. The master sends back to the slaves the amount
that the slaves’ clocks should adjust5. Slave adjust its clock
Synchronizing physical clocks
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Time
The Network Time Protocol (NTP)An architecture for a time service and a protocol to
distribute time information over the InternetAims:
• External synchronization– Enable clients across the Internet to be synchronized accurately to
UTC• Reliability
– Can survive lengthy losses of connectivity» Redundant server & redundant path between servers
• Scalability– Enable clients to resynchronize sufficiently frequently to offset the
rates of drift found in most computers• Security
– Protect against interference with the time service
Synchronizing physical clocks
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Time
NTP service is provided by servers located across the InternetPrimary servers are connected directly to a time
source, e.g. radio clock receiving UTCSecondary servers are synchronized to primary
servers
Synchronizing physical clocks
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Time
Network Time Protocol Architecture
Synchronizing physical clocks
1
2
3
2
3 3
Note: Arrows denote synchronization control, numbers denote strata.
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Source: Wikipedia
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Time
3 modes of NTP synchronization:Multicast mode
• Intended for use on a high speed LAN• One or more servers multicast the time to other servers
in the LAN• Low accuracy but sufficient
Procedure-call mode• Similar to Christian’s• Higher accuracy than multicast
Symmetric mode• Use by the servers that supply time information in LANs• The highest accuracy
Synchronizing physical clocks
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Contents
Introduction Clocks, events and process states Synchronizing physical clocks Logical time & logical clocks
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Time
Events in a single process is ordered uniquely by times shown in the local clock
Clock is unable to be synchronized perfectly. Hence physical time is not accurate to determine the occurrence of events
Logical time and logical clocks
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Time
Happened-before relationIt is also sometimes known as the relation of
causal ordering or potential causal ordering.
Logical time and logical clocks
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Time
Logical clocks- a simple mechanism to capture HB order numerically. Known as Lamport timestamps algorithm (it is a software counter).LC1
• Li is incremented before each event is issued at process pi : Li :=Li+1
LC2: (a) When a process pi sends a message m, it
adjoin the value t = Li on m(b) On receiving (m,t), a process Pj computes Lj :=
max(Lj, t) and then applies LC1 before timestamping the event receive(m)
Logical time and logical clocks
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Time
Totally ordered logical clocks algorithmAssumption
Ti : local timestamp of e that is an event occurring at pi
Tj : local timestamp of e` that is an event occurring at pj
The timestamps of two events e and e` are (Ti, i), (Tj, j)When (Ti, i) < (Tj, j), if and only if Ti < Tj , or Ti = Tj and i < j
Logical time and logical clocks
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Time
Vector Clocks algorithmEach process pi keeps a vector clock Vi
VC1: Initially, Vi[j]=0, for i, j = 1,2…, NVC2: Just before pi timestamps an event, it sets
Vi[i] := Vi[i] +1VC3: pi includes the value t= Vi in every message it
sendsVC4: When pi receives a timestamp t in a message,
it sets Vi[j] :=max(Vi[j], t[j]), for j=1,2…,N
Logical time and logical clocks
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End of the Chapter…
