Implementation of Lamp ort's Scalar clocks and Singhal-Kshemkalyani’s VC Algorithms

1. Implementation of Lamport's Scalar clocks and Singhal-Kshemkalyani’s VC Algorithms Kent State University Computer Science Department Saleh Alnaeli Advanced Operating System. Spring 2010

2. Goals • Implementing both of the algorithms • study their behavior under some different arguments • Processes Number • Messages Number • Involved processes Number in the computation Note: This presentation assumes that you have a back ground about Logical Clocks and some of its related algorithms (Scalar, Vector, S-K).

3. Lamport's Scalar clocks • was proposed by Lamport 1978 • to totally order events in distributed system • each process Pi has a logical clock Ci (represented as integer value) • consistency condition • consistency: if ab, then C(a) C(b) • if event a happens before event b, then the clock value (timestamp) of a should be less than the clock value of b • strong consistency: consistency and • if C(a)C(b) then ab • scalar clocks are not strongly consistent: • if ab, then C(a)< C(b) but • C(a)< C(b), then not necessarily ab

4. Implementation of Scalar Clocks • Rule1: before executing event update Ci so, Ci:= Ci+ d (d>0) • Rule2: attach timestamp of the send event to the transmitted message when received, timestamp of receive event is computed as follows: Ci:= max(Ci , Cmsg) and then execute R1 • It is implemented in C++ and was verified in different ways: • Checking its consistency using a function compares the new value of previous one locally and with the sender in receive event • Results were compared with vector clock application

5. Generating the computations • Computations were entered from input text file • Generated manually and using a computation generator developed in C++ randomly (random sender and receiver) • Each event is constructed according the following scheme: EventType,SenderID,ReceiverID such that: EventType is 1 for internal event, 2 for send event and 3 for receive event. • Example:3,7,8 means an event to receive a SMS was sent by Pcocess 7 to 8 // Also • order of the events can be changed in the InputFile just make sure the receive event is preceeded by send event • SMS not found or lost for receive without send event. • Sending to process it self is an internal event. Example 2,5,5

6. Singhal-Kshemkalyani’s Algorithm for vector clock S-K • Considered as an efficient implementation of vector clocks. • instead of sending the whole vector only need to send elements that changed. And same update rules are used for the recipient process. • maintain two vectors : • LS[1..n] – “last sent” • LU[1..n] • needs to send with the message only the elements that meet the condition: {(x,vti[x])| LSi[j] < LUi[x]} • The sent vector contains the processes’ Id’s and Clock values of changed processes.

7. S-K Implementation • It is implemented in C++ and was verified in different ways: • Results were compared with others generated by a combined Scalar and vector clock application. • Known examples and random computations were used. • Computations were entered from input text file • Computations were generated using a computation generator developed in C++.

8. Events construction scheme • similar to scalar events format with extra field: • EventType,SenderID,ReceiverID,EventId such that: EventType is 1 for internal event, 2 for send event and 3 for receive event. EventId is number of the event when the message has been sent • Example:3,7,8,4 means an event to receive a SMS was sent by Process 7 to 8 and the event was the fourth send event • order of the events can be changed in the InputFile just make sure the receive event is preceeded by send event • SMS not found or lost for receive without send event. • Sending to process it self is an internal event. Example 2,5,5,4

9. Performance Evaluation • Lamport’s Scalar Clock algorithm: • There were not enough area to study (trivial) • S-K algorithms • Performance metrics evaluated include • Stamps Memory size used in units (1 unit=32 bytes) • Conditions of varying • Processes Number, messages Number, and number of the involved processes in the computation. • It’s expected that SK in the worst case will perform as VC

10. S-K: Simulation Parameters Expectations: In the worst case of S-K will be Vector clock’s work.

11. # processes vs. #messages • events by sequence1 with 2500 messages and 50 lost • figure1 shows that S-K out performance regular VC even with changing the No of processes and involved processes as well Not Sufficient and not satisfied

12. #Messages vs. #involved processes • Events were generated randomly with sequence 2 (randomly picking sender and receiver) • After sending, message is directly received to got more updates in locals V. • Constant # of processes 50 • Changing # of involved processes (10-50)

13. Figure2 shows surprising results

14. Table1 and table2 S-K and VC respectively Messages Number # involved processes Used memory in units

15. Verifying S-K efficiency equation • S-K original paper states that their technique can be beneficial if n<N.b/(log2N+b) Such that: n=avr of entries in Ti, b=bits in a sequence number, log2N=bits needed to code N process ids. • It doesn’t work with my simulation !!! • I have calculated n value in (2500,40 and 110548,30 ) and I compared it with their equation but did not work!!! • Mine is : • When of involved processes gets close to 70% and #of messages gets close to 20N, then S-K becomes inefficient.

16. Conclusion • The sequence of the events plays big role in determining the efficiency of S-K • Number of the involved processes in the computation can affect S-K performance • For low # of messages, S-K seems fine. • When # of involved processes is about 70% and #of messages close to 20N then S-K becomes a weak. • Efficiency equation is not applicable in my experiment .

17. Difficulties • The most difficult issue was generating a computation that can be used for adequate results. • It’s Difficult to predict the order of receiving the messages which make it difficult to generate a computation close to reality.

18. References • Original S-K paper • Logical clock, Adv OS course slides. • Prof. Mikhail Nesterenko (Acknowledge) • http://deneb.cs.kent.edu/~mikhail/classes/aos.s10/ • S-k implementation-Manas Hardas. KSU. (Acknowledge)