Local-lag and Timewarp:
Providing Consistency for
Replicated Continuous Applications
by
Martin Mauve, Jurgen Vogel, Volker Hilt and Wolfgang Effelberg
Irwin Chiu Hau
Computer Science
McGill University
Winter 2004
Comp763: Modern Computer Games
Overview
•
•
•
•
•
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Consistency
Local-lag
Timewarp
Experiments
Dead Reckoning
Conclusion
Comp763: Modern Computer Games
Terminology
A Replicated Application is an
•application that has its local states maintained by multiple
application instances.
Domain Name Servers
Source:
Irwin Chiu Hau
Comp763: Modern Computer Games
Terminology
A Replicated Discreet Application is a
•replicated application that has its states changing in response
to user operations
Microsoft Hearts
Source:
Irwin Chiu Hau
Comp763: Modern Computer Games
Terminology
A Replicated Continuous Application is a
•replicated application that has its states changing in response
to time passage
UT2003
Source:
http://www.unrealtournament.com
Comp763: Modern Computer Games
Overview
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•
•
•
•
•
Consistency
Local-lag
Timewarp
Experiments
Dead Reckoning
Conclusion
Comp763: Modern Computer Games
Consistency
•What is CONSISTENCY?
•Why is it so important?
•Consistency in the Discrete Domain
•Consistency in the Continuous Domain
Comp763: Modern Computer Games
What is CONSISTENCY?
Definition from www.dictionary.com:
1.
Agreement or logical coherence among things or parts
2.
Reliability or uniformity of successive results or events
From what we have seen / heard so far
Acronyms:
•
“Dead Man Shooting”
Comp763: Modern Computer Games
Why is consistency so Important?
In games, CONSISTENCY could give us:
•
Better aim
•
Confidence
In games, INCONSISTENCY could make us:
•
Fall into pits
•
See “Dead Man Shooting”
•
FRUSTRATED
America’s Army
Source:
http://www.americasarmy.com
Comp763: Modern Computer Games
Consistency is Important
Consistency must be present in:
•
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Replicated Discreet Applications
Replicated Continuous Applications
This paper investigates how consistency can
be established!
Comp763: Modern Computer Games
Consistency in the Discreet Domain
Consistency can be established by:
•
•
Finding the correct sequence of all operations that are
dependent of each other
Making sure that every site executes the dependent
operations in that particular sequential order
Comp763: Modern Computer Games
Consistency in the Discreet Domain
Example: Hearts (card game)
Player 1
Player 2
Drop Ace of Spades
Drop Two of Spades
send
send
Inventory:
Ace of Hearts
Two of Hearts
Player 1 dropped Ace of Spades
Player 1 dropped Two of Spades
Player 1’s Inventory:
Ace of Hearts
Two of Hearts
Comp763: Modern Computer Games
Consistency in the Continuous Domain
Consistency can be not established by only:
•
Reusing approaches for the discreet domain
Timing is critical !!
Comp763: Modern Computer Games
Consistency in the Continuous Domain
Example: Train simulation
User 1
User 2
Train approaches a switch
Train approaches a switch
Pull switch
Train turns right
send
Train passes the switch
Switch pulled
Inconsistency
Train goes straight
Comp763: Modern Computer Games
Consistency in the Continuous Domain
Solving consistency in continuous domain requires
in addition:
•
That the operations look as they had been executed at the
CORRECT POINT OF TIME
But first, we need to define what is:
•
•
Consistency
Correctness
Comp763: Modern Computer Games
More Terminology
Definition of CONSISTENCY and CORRECTNESS
Let the reception function Ri be defined as
Ri(t, oj, t0, t*) = false oj, t0, t* received by i after t
true otherwise
Interpreted as:
Ri return false if a given operation has been received
by site i after time t, true otherwise
oj, t0, t* :
Operation that
has been issued at site i at a time t0, with
t* being the time the operation is supposed to be executed
Comp763: Modern Computer Games
More Terminology
Definition of CONSISTENCY in the continuous domain
So, a replicated continuous application ensures consistency iff
For all t, i, j | for all t* ≤ t, ow,t0,t* є O
Ri(t, ow, t0, t* ) Λ Rj(t, ow, t0, t*) => (si,j = sj,t)
Which means, at any time the state at any two sites i and j must be
the same, if both sites have received all the operation that are
supposed to be executed by time t
Comp763: Modern Computer Games
More Terminology
Definition of CORRECTNESS
in the continuous domain
•
•
At any time t the state at any site i
must be the same as the state of the
virtual perfect site P
Provided that i has received all
operations that are supposed to be
executed before t
A Perfect Virtual Site P is a site that
receives all operations by the time they
should be executed
Source: Local-lag and Timewarp
Comp763: Modern Computer Games
Overview
•
•
•
•
•
•
Consistency
Local-lag
Timewarp
Experiments
Dead Reckoning
Conclusion
Comp763: Modern Computer Games
Local-lag
One solution to solve short term inconsistencies,
is to make use of the trade-off
“Response time vs Short-term inconsistencies”
•To lower responsiveness of an application
•To minimize short-term consistencies
This concept is called LOCAL-LAG
Comp763: Modern Computer Games
Responsiveness vs Inconsistency
case : t0 = t*
At any site, operations are
executed immediately once
they have been received
That leads to responsiveness
Source: Local-lag and Timewarp
Comp763: Modern Computer Games
Responsiveness vs Inconsistency
case : t0 < t*
The operations are executed
simultaneously by all the sites
after a certain delay
This minimizes inconsistency
If response time exceed a
certain threshold, players will
notice lag
Source: Local-lag and Timewarp
Comp763: Modern Computer Games
Responsiveness vs Inconsistency
Source: Local-lag and Timewarp
Comp763: Modern Computer Games
Determining a value for Local-lag
•Choose the minimal value for local-lag
A number that would significantly reduce short-term
inconsistencies ( 1ms for LAN, to 150ms for world-wide )
•Choose the highest acceptable response time
A number that will not be noticeable by game players ( 80-100ms )
•Choose a value for local-lag
Choose wisely between the values from above
Comp763: Modern Computer Games
Overview
•Consistency
•Local-lag
•Timewarp
•Experiments
•Dead Reckoning
•Conclusion
Comp763: Modern Computer Games
Timewarp
• Local-lag does not eliminate all inconsistencies
• We need a mechanism that repairs the state caused
by these issues
This Mechanism is called TIMEWARP:
• Maintains a list of operations and a list of states
• Repairs the lists
Comp763: Modern Computer Games
Timewarp
Timewarp ensures CORRECTNESS:
• Proved by induction: At any site, all states that are
saved in the list of states are the same as the ones
computed by the virtual perfect site P
Complexity:
• Claimed to be O(n2), where n is the number of
participants in a session
Comp763: Modern Computer Games
Overview
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•
•
•
•
•
Consistency
Local-lag
Timewarp
Experiments
Dead Reckoning
Conclusion
Comp763: Modern Computer Games
Experiments
A time warp game has been developed to:
•
•
Verify the theoretical observations
Show that local-lag and timewarp can be used in real
applications
Timewarp game
Source: Local-lag and Timewarp
Players control a spaceship:
Accelerate, decelerate, turn
and shoot
Comp763: Modern Computer Games
Experiments
Results
Simulated
Local lag
Network Delay
0 ms
0-120 ms
160 ms
40-120 ms
--140-240 ms
Half of SND
> 290 ms
Half of SND
Notice
local lag
no
yes
-------
Comp763: Modern Computer Games
Notice odd
behaviour
no
no
no
no
yes
Experiments
Number of events:
Increases linearly with number of users
Increases linearly with network delay
Time warp duration: O(n2) where n is the number of players
Source: Local-lag and Timewarp
Comp763: Modern Computer Games
Overview
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•
•
•
•
•
Consistency
Local-lag
Timewarp
Experiments
Dead Reckoning
Conclusion
Comp763: Modern Computer Games
Dead-Reckoning
vs
Local-lag / Timewarp
Both provide consistency
Dead-Reckoning (prediction of objects’ behaviour)
• Does not provide correctness
• Complexity O(n)
Timewarp
• Provides correctness
• Complexity O(n2)
Comp763: Modern Computer Games
Overview
•
•
•
•
•
•
Consistency
Local-lag
Timewarp
Experiments
Dead Reckoning
Conclusion
Comp763: Modern Computer Games
Conclusion
Things to remember:
Local-lag minimizes short-term inconsistencies by:
• Reducing the responsiveness of an application
Timewarp is an algorithm
• Guarantees consistency
• Provides correctness
Comp763: Modern Computer Games
Questions ?
Comp763: Modern Computer Games