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Distributed Synchronization. In single CPU systems Semaphores and monitors Essentially shared memory solutions How about distributed synchronization? Relevant information is scattered Processes make decisions based on local information

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distributed synchronization
Distributed Synchronization
  • In single CPU systems
    • Semaphores and monitors
    • Essentially shared memory solutions
  • How about distributed synchronization?
    • Relevant information is scattered
    • Processes make decisions based on local information
    • A single point of failure in a system should be avoided
    • No common clock or other precise global time source exists

ICSS741 - Time and Coordination

what is needed
What Is Needed
  • In order to coordinate events in a distributed system
    • We may need to know the time at which a particular event took place
    • We may need to determine the order in which two events took place, or should take place, without respect to the time they actually occur
  • Synchronization requires either
    • Global time
    • Global ordering

ICSS741 - Time and Coordination

time synchronization
Time Synchronization
  • Is it possible to synchronize all clocks to produce a single, unambiguous time standard?
  • Time synchronization need not be absolute
    • What usually matters is that processes agree on the order in which events occur (not necessarily the time at which they occur)

ICSS741 - Time and Coordination

time and coordination
Time and Coordination
  • Basically two problems with time
    • External synchronization
      • Synchronize clocks with an authoritative external source of time
    • Internal synchronization
      • The internal consistency of the clocks is what matters, not whether they are close to the real time

ICSS741 - Time and Coordination

astronomical time
Astronomical Time
  • Since the 17th century time has been measured astronomically
    • The event of the sun reaching the highest point in the sky is called the transit of the sun
    • The interval between two consecutive transits of the sun is called a solar day
  • In the 1940s, it was established that the earth’s rotation is not constant
    • The earth is spinning slower
    • 300 million years ago there were about 400 days per year

ICSS741 - Time and Coordination

atomic time
Atomic Time
  • The atomic clock was invented in 1948
    • One second is the time it takes the cesium 133 atom to make 9,192,631,770 transitions
    • Currently about 50 cesium-133 clocks exist
    • Periodically they are averaged to produce international atomic time (TAI)
    • The Bureau International de l’Heure (BIH) maintains the official clock

ICSS741 - Time and Coordination

leap seconds
Leap Seconds
  • Currently about 86,400 TAI seconds is about 3msec less than a mean solar day
    • Not a problem until noon becomes 6am
  • BIH solves the problem by inserting leap seconds to compensate for the difference
    • Leap seconds are added whenever the discrepancy grows to 800 msec
    • Power companies will increase their frequencies to compensate
  • UTC (Universal coordinated time) is the result

ICSS741 - Time and Coordination

obtaining accurate time
Obtaining Accurate Time
  • UTC is an international standard for the current time
    • WWV shortwave radio from Fort Collins (accuracy 0.1 – 10 milliseconds)
    • GEOS satellites (0.1 milliseconds)
    • GPS satellites (1 millisecond)

ICSS741 - Time and Coordination

physical clocks
Physical Clocks
  • Computer each contain their own physical clocks
    • Timer might be a better word…
    • Utilize crystal that oscillate at a known frequency
    • A count of the oscillations is maintained
    • Software typically takes this count, divides it down, and stores it as a number in a register
  • Most systems provide date/time from the counter
  • Ordering events, in a single machine, with such a clock is easy
    • Provided the clock resolution is fine enough

ICSS741 - Time and Coordination

clock drift
Clock Drift
  • Crystal-based clocks are subject to drifting
    • the change in the offset between the clock and a nominal perfect reference clock per unit of time measured by the reference clock
  • Typical drift rates
    • Quartz crystals – 10-6 (about a difference of one second every 1,000,000 seconds or 11.6 days)
    • Atomic clocks – 10-13

ICSS741 - Time and Coordination

external synchronization
External Synchronization
  • Lets say you have access to a UTC time source
  • Assume the machine has a timer that causes an interrupt H times a second
    • Current clock value is C
    • When UTC time is t, the value of the clock on machine p is Cp(t)
    • Ideally Cp(t)=t for all p and t (dC/dt should be 1)

ICSS741 - Time and Coordination

maximum drift rate
Maximum Drift Rate

dC/dt > 1

dC/dt = 1

Fast clock

Perfect clock

dC/dt < 1

Slow clock

ICSS741 - Time and Coordination

synchronizing physical time
Synchronizing Physical Time
  • What exactly does it mean to synchronize two clocks?
    • Clocks inherently suffer from drifting
    • Assuming clocks can always be precisely synchronized in unrealistic
    • Define an acceptable range for the difference in time reported by two clocks (clock skew)
  • A distributed physical clock synchronization service defines, and maintains, a maximum skew throughout the system.

ICSS741 - Time and Coordination

the basic algorithm
The Basic Algorithm
  • A wants to read B’s clock
    • A sends a request to B
    • B records its current clock value
    • The clock value is sent back to A
    • B’s clock value is adjusted to reflect travel time
    • B’s clock value can now be compared to A’s
  • Step 4 is difficult to implement accurately

ICSS741 - Time and Coordination

interesting question
Interesting Question
  • So you have to adjust your time
    • Your clock is slow – move it ahead
    • Your clock is fast – move it back?
  • Implementations
    • Slow down your clock so it will continually move towards the real time
    • Speed up your clock so it will move towards the real time
    • Just move your clock ahead to the real time

ICSS741 - Time and Coordination

cristian s algorithm
Cristian’s Algorithm
  • One machine knows the true time
  • Periodically each machine sends a request for the current time

T0

Request

Time

I, interrupt handling time

CUTC

T1

Measured with the same clock

ICSS741 - Time and Coordination

transit time
Transit Time
  • Estimating propagation time
    • ( T1 – T0 ) / 2
    • ( T1 – T0 – I ) / 2
    • If minimum possible propagation delay is known, the estimate can be made better
  • Accuracy can be improved by taking several measurements
    • Any measurement in which T1 – T0 exceeds a threshold is discarded (congestion)

ICSS741 - Time and Coordination

icmp timestamp request reply
ICMP Timestamp Request/Reply

Type (17 or 18)

Code (0)

Checksum

Identifier

Sequence Number

32-bit originate timestamp

32-bit receive timestamp

32-bit transmit timestamp

Same clock so difference is accurate

rtt

ICSS741 - Time and Coordination

the berkeley algorithm
The Berkeley Algorithm
  • Time server is active, and polls each machine periodically for its time
    • Based on the answers, an average time is computed
    • A fault-tolerant average is used
    • Machines are then told to slow down, or speed up their clocks
  • Suitable for systems where no UTC source is available

ICSS741 - Time and Coordination

berkeley algorithm
Berkeley Algorithm

740

740

Current Time = 720

Move clock forward 7

Current Time = 740

Adjusted TimeA = 730

Adjusted TimeB = 742

Average = 737

Current Time = 737

720

737

+7

Network delay = 10

Network delay = 5

ICSS741 - Time and Coordination

network time protocol
Network Time Protocol
  • NTP is used to synchronize the time of a computer client to another server or reference time source
  • Client accuracies are typically within a millisecond on LANs and up to a few tens of milliseconds on WANs
  • NTP configurations utilize multiple redundant servers and diverse network paths in order to achieve high accuracy and reliability
  • Configurations can use authentication to prevent accidental or malicious protocol attacks

ICSS741 - Time and Coordination

ntp strata
NTP Strata

Primary Servers (stratum 1 ) sync to UTC source

Secondary Servers (stratum 2 ) sync to primary servers

Workstations

ICSS741 - Time and Coordination

usna ntp time servers
USNA NTP Time Servers

ICSS741 - Time and Coordination

rules of engagement
Rules of Engagement
  • Clients should avoid using the primary servers whenever possible
    • In most cases the accuracy of the NTP secondary (stratum 2) servers is only slightly degraded relative to the primary servers
    • As a group, the secondary servers may be just as reliable

ICSS741 - Time and Coordination

when to use a primary
When to Use a Primary
  • As a general rule
    • The secondary server provides synchronization to a sizable population of other servers and clients
    • The server operates with at least two and preferably three other secondary servers in a common synchronization subnet
    • The administration(s) that operates these servers coordinates other servers within the region, in order to reduce the resources required outside that region.
  • In order to ensure reliability, clients should spread their use over many different servers

ICSS741 - Time and Coordination

ntp servers
NTP Servers
  • http://www.ntp.org (home page for NTP)
  • List of Primary Servers (100)
    • http://www.eecis.udel.edu/~mills/ntp/clock1.htm
  • List of Secondary Servers (110)
    • http://www.eecis.udel.edu/~mills/ntp/clock2.htm
  • Our server
    • timehost.cs.rit.edu

ICSS741 - Time and Coordination

synchronization modes
Synchronization Modes
  • Servers synchronize in one of three modes
    • Multicast
      • Used on high speed LANs
      • Servers periodically broadcast their time
      • Low accuracies, but efficient
    • Procedure-call
      • Similar to the operation of Cristian’s algorithm
    • Symmetric
      • Used by master servers
      • Pairs of servers exchange information
      • Timing data is retained in order to improve accuracy

ICSS741 - Time and Coordination

ntp design goals
NTP Design Goals
  • The four primary design goals of NTP are
    • Allow accurate UTC synchronization
    • Enable survival despite significant losses of connectivity
    • Allow frequent resynchronization
    • Protect against malicious or accidental interference

ICSS741 - Time and Coordination

accurate synchronization
Accurate Synchronization
  • NTP provides the following information relative to the primary server
    • Clock offset
      • Difference between the two clocks
    • Round-trip delay
      • Total transmission time for the messages
    • Dispersion
      • Offsets are predicted
      • Dispersion is a measure of how much the prediction differs from what what reported
      • Large dispersion values indicate inaccuracy

ICSS741 - Time and Coordination

logical clocks
Logical Clocks
  • Since physical clocks cannot be perfectly synchronized across a distributed system
    • Physical time cannot be used to determine the order in which events occur
  • Logical clocks can be used to order events within a distributed system
  • The essence of a logical clock is the happens-before relationship

ICSS741 - Time and Coordination

happens before
Happens-Before
  • The happens-before relationship is denoted a b
    • If a and b are events in the same process, and a occurs before b, then ab
    • If a is the event of a message being sent by one process, and b is the event of the message being received by another process, then ab
    • If a b, and bc, then ac
  • Any two events that are not in a happen-before relationship are concurrent

ICSS741 - Time and Coordination

events
Events

ICSS741 - Time and Coordination

lamport s logical clock
Lamport’s Logical Clock
  • To obtain logical ordering, timestamps that are independent of physical clocks are used
  • Lamport clocks follow these rules
    • Each process increments it clock between every two consecutive events
    • If a sends a message to b, the message includes T(a). Upon receipt, b sets its clock to the greater of T(a)+1 and the current clock

ICSS741 - Time and Coordination

lamport s algorithm
Lamport’s Algorithm

0

8

16

24

32

40

48

61

69

77

85

0

10

20

30

40

50

60

70

80

90

100

0

8

16

24

32

40

48

56

64

72

80

0

10

20

30

40

50

60

70

80

90

100

0

1

2

3

4

5

6

7

8

70

71

0

1

2

3

4

5

6

7

8

9

10

a

a

b

b

c

c

d

d

e

e

f

f

A(6)  F(10)  B(24)  C(50)  D(60)  E(64)

A(6) B(24)  C(50)  D(60)  E(64)  F(71)

ICSS741 - Time and Coordination

partial ordering
Partial Ordering
  • If a  b then L(a) < L(b)
  • Note that
    • L(d) < L(e)
  • Does not imply that
    • d  e
    • Since d and e might be concurrent
  • Plus L(a) might equal L(b)

ICSS741 - Time and Coordination

example
Example

ICSS741 - Time and Coordination

total ordering
Total Ordering
  • Between every event the clock must tick at least once
  • Since events cannot happen at the same time, attach the process number to the low-order end of the time, separated by a decimal point
  • Now
    • If a happens before b in the same process, C(a) < C(b)
    • If a and b represent the sending an receiving of a message, C(a) < C(b)
    • For all events a and b, C(a) is not equal to C(b)

ICSS741 - Time and Coordination

vector clocks
Vector Clocks
  • Vector clocks were designed to overcome the shortcomings of Lamport’s clocks
    • A vector clock is an array of times
  • The rules:
    • Initially, Vi[j]=0, for i,j = 1,2 …, N
    • Just before pi timestamps an event, it increments Vi[i]
    • piincludes the value t = Vi in every message it sends
    • When pi receives a timestamp in as message, it takes the component-wise maximum of the two vector timestamps

ICSS741 - Time and Coordination

example1
Example

ICSS741 - Time and Coordination

comparing timestamps
Comparing Timestamps
  • Vector timestamps are compared as follows
    • V = V’ iff V[j] = V’[j] for j=1,2,…,N
    • V <= V’ iff V[j] <= V’[j] for j=1,2…,N
    • V < V’ iff V<=V’ and V != V’
  • So what?
    • If V(e) < V(e’) then ee’
    • c and e are concurrent since neither V(c) <= V(e) nor V(e)<=V(c)

ICSS741 - Time and Coordination

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