1. Timewarp
Elias Muche

2. What It Is An implementation of virtual time
Imaginary Time
Imaginary Ticking
One dimensional
Used to organize and synchronize distributed systems
Analogous to virtual memory in operating systems

3. Major Applications
Used to organize and synchronize distributed systems
Distributed discrete event simulation
Imposes a temporal coordinate system on distributed systems
Concurrency control mechanism for databases

4. Distributed Discrete Event Simulation
Every process represents a simulation object
Virtual time identified by simulation time
Processes schedule events by passing messages

5. Database Concurrency Virtual time derived by real time
Data(tables, records) treated as processes that respond to read/write messages
Each transaction must be completed in a band virtual time
Must not overlap with other transactions’ bands
High order bits of transaction’s virtual time used to calculate real time initiation
Middle order bits used to break ties

7. Virtual Time Systems
Each primitive action has virtual time and virtual space
Messages use stamps
Sender name
Virtual send time
Receiver name
Virtual receive time
Virtual send time must be smaller than the virtual receive time
Virtual Receive time of each event must be smaller than the virtual time of next event for the same process

8. Timewarp Mechanism Some processes allowed to use virtual clocks
Others allowed to fall behind
Messages can be received out of order
This means a process shouldn’t wait for the “next” message
Uses timestamps, virtual receive time for ordering

9. Timewarp Mechanism Continued
Local Control Mechansim
Each process has its own virtual clock variable
The clock changes value in between events
Process “terminates” when input queue is empty

10. Timewarp Mechanism Continued
Global control mechanism
Manages space, flow control, I/O
Global virtual time
All virtual times at time r
All virtual send times of sent messages(non-processed)

11. Implementations: Rollback
Handles fault tolerance
Look ahead rollback
Each process executes normally
If process causes synchronization conflict, rolls back to pre-conflict point in time
Process then executes along revised path

15. Performance

16. Performance Cont.

17. Summary Timewarp used to synchronize distributed systems
Uses virtual time and space coordinates
Lookahead rollback is a common implementation
Timewarp consists of local and global mechanisms
Performance:
Number of rollbacks increase as number of processors increase
Execution time decreases as number of processors increase

18. Questions?