Download Clock Synchronization: Synchronizing Wallclock Time in Distributed Systems and more Study notes Computer Science in PDF only on Docsity!
Maria Hybinette, UGA
CSCI: 4210/
Simulation & Modeling
PDES: Synchronizing Clocks
(Wallclock Time)
Maria Hybinette, UGA 2
Outline
! Clock Synchronization Problem
! Wallclock Time: Definitions
! Synchronization algorithms
! Correcting clock errors
Maria Hybinette, UGA 3
Problems With Unsynchronized Clocks
! Each processor has a local oscillator providing wallclock time values ! At any instant clocks in distinct processors will differ ! Clocks in different processors drift relative to each other Position update generated @ 3. 3.010 3.020 3.030 3.040 3.050 3.060 3.070 3. Wallclock time (processor 2) Wallclock time^ 3.025^ 3.035^ 3.045^ 3.055^ 3.065^ 3.075^ 3.085^ 3. (processor 1) Received @ 3. Message received before it was sent! Dead reckoning? A mechanism to synchronize hardware clocks is needed. Maria Hybinette, UGA 4
Time Definitions
! Historically, time definitions based on
periodic astronomical events
» Solar day: time duration that elapses between event where the sun is at the highest point in the sky » 24 hours per solar day, 3600 seconds per hour » Greenwich mean time: solar time at the Greenwich meridian (observatory near London)
! Today, atomic clocks define standard time
sources
» Time duration based on transitions of cesium atom
Time Definitions
! Historically, time definitions based on
periodic astronomical events
» Solar day: time duration that elapses between event where the sun is at the highest point in the sky » 24 hours per solar day, 3600 seconds per hour » Greenwich mean time: solar time at the Greenwich meridian (observatory near London)
! Today, atomic clocks define standard time
sources
» Time duration based on transitions of cesium atom
Time Definitions (cont.)
! Earth’s rotation slowing down slightly
! Duration of a “day” is increasing
! Slowing down effect not accounted for by
atomic clocks
! Solution: leap seconds are periodically added
to time produced by cesium clock
» Resulting time called Coordinated Universal Time (UTC) » Time standard accepted world wide » Disseminated by standards organizations
- National Institute of Standards and Technology (NIST) in U.S.
- Available via satellites, radio broadcasts, phones
Maria Hybinette, UGA 7
Clock Synchronization Problems
! Ensuring different processors maintain
clocks that are synchronized with each
! = maximum offset allowed between two clocks
! Ensuring that clocks are synchronized with
UTC
» Only an issue if interacting with UTC-based devices Maria Hybinette, UGA 8
Synchronized Wallclock Time
! Notation
» Ci = hardware clock in processor i » T = UTC » Max clock drift ": 1 - "! dCi/dT! 1 + " » Clocks in two processor may drift 2 " per unit UTC
! Processors must periodically resynchronize
their clocks
! How often?
»! = maximum offset allowed between two clocks » Clocks can drift! time units in !/ 2 " time » Resynchronization must be done at least every !/ 2 " time units Maria Hybinette, UGA 9
Clock Synchronization Algorithms
! Centralized, pull algorithms
» Each processor periodically requests current time from a central time server » Server reads local clock and returns current time value
! Centralized, push algorithms
» Central time server periodically sends current time to other processors
! Distributed algorithms
» No central time server » Processors use distributed algorithm to synchronize their local clock
! Basic problem: communication latency time
is unpredictable
Maria Hybinette, UGA 10
Centralized, Pull Algorithm
TS = local time in requestor when request sent I = interrupt service time T = time value provided by server TR = local time in requestor when response received TR = T + L where L is communication latency Estimate L = (TR - TS - I) / 2 Collect several estimates of L, discard outliers, average Latency often symmetric in LANs, typically not in WANs
Time Server
Time Requestor
TS T TR Wallclock time
I
Push and Distributed Algorithms
! Push algorithms
» Server periodically broadcasts time values » Requires assumption of message latency
! Distributed algorithms
» Avoids single point of failure, bottleneck for large systems » A push algorithm
- Each processor periodically sends time values to a subset of the other processors
- Incoming time values: discard outliers, average received values
Network Time Protocol
! Distributed clock service implemented on Internet
! Hierarchical subnetwork of clock servers
» Clock information flows down hierarchy » Servers at level i synchronize with servers at level i+ 1
! Primary servers
» Level 1 of tree » Synchronized with national time servers
! Secondary servers
» Level 2 , 3 , …
