A SEMINAR ON EARLIEST DEALINE FIRST ALGORITHM

Presented by Miss. SNEHAL D. PATIL Final Year Computer Science And Engineering

Guided By Prof P.B.NIRANJANE Project & Seminar Guide Computer Science And Engineering

FLOW OF EDF

CONTENTINTRODUCTIONEVOLUTION OF EDFDEADLINE

EXAMPLEADVANTAGESDISADVANTAGES

1.2.3.4.5.6.7.8.9.

CONCLUSIONREFERENCES

INTRODUCTIONConventional Algorithms

FCFS-first come first serve.SCAN-arm moves in one direction.C-SCAN-after servicing last transaction arm returns to start position.SSTF-selects transaction closest to the current arm position .

Diadvantage

• Strictly data consistent • This results in poor performance of conventional algorithms in real-time

systems.

The idea of EDF was published in 1973 in an article of Liu and Layland.

EDF is dynamic. Deadline based scheduling algorithm.The priority of a job is inversely proportional to its

deadline.

EVOLUTION OF EDFEDF- Earliest Deadline First

DEADLINE is a Time by which execution of the transaction should be completed,after the transaction is released.

Deadline= Arrival time+ slack factor*AET, Average execution time(AET)=1.5*block sizeAssumptionSlack factor=2Transmission factor : Read=0.6,Seek factor=0.3

DEADLINE

Flow of EDF

Step 1:Calculate deadline of each transaction present in a queue

Step 2:Schedule deadline

Step 3:Serves transaction with earliest deadline

EXAMPLECase 1:Deadlines of transactions are not close to each other.Table 1:

EDF scheduled the deadlines by giving priority to earliest deadline first. T0, T2, T1, T4, T3, T5

Transaction id

arrival time(At)

Block location(ai)

Block size(bs)

Start block(Si)

End block(Li)

Average execution time (AET)

Deadline (Di)

Transfer time(TT)

T0 0 0 0 0 0 0 0 0T1 1 2 3 2 4 4.5 10 1.8T2 0 11 2 11 12 3 6 1.2T3 3 8 4 8 11 6 15 2.4T4 3 5 3 5 7 4.5 12 1.8T5 5 13 5 13 17 7.5 20 3.0

Case 1(C0NTI.)

Cj,i i=0 i=1 i=2 i=3 i=4 i=5

J=0 ------- 2.4 4.5 4.8 3.3 6.9

J=1 1.2 -------- 3.3 3.6 2.1 5.7

J=2 3.6 4.8 ------- 3.6 3.9 3.3

J=3 3.3 4.5 1.2 -------- 3.6 3.6

J=4 2.1 3.3 2.4 2.7 --------- 4.8

J=5 5.1 6.3 3.0 5.1 5.4 -------

Servise tableCj,i is execution time required to ith transaction ater servicing jth transaction.

Case 2: Deadlines of transactions in overloaded condition

Table 2:

EDF scheduled the deadlines by giving priority to earliest deadline first. T0, T1, T3, T5, T4, T2

transaction id (Tid)

arrival time(At)

Block location(ai)

Block size(Bs)

Start block(Si)

end block(Li)

Average execution time(AET)

deadline(Di)

Transfer time(TT)

T0 0 0 0 0 0 0 0 0

T1 0 7 2 7 8 3 6 1.2

T2 2 7 3 17 19 4.5 11 1.8

T3 1 1 2 1 2 3 7 1.2

T4 1 4 3 4 6 4.5 10 1.8

T5 0 13 3 13 15 4.5 9 1.8

Service table:

EDF gives poor performance in overloaded condition

Case 2(conti.)

Cj,i i=0 i=1 i=2 i=3 i=4 i=5

J=0 ------- 2.1 5.1 0.3 1.3 3.9

J=1 2.4 -------- 4.5 3.3 3 3.3

J=2 5.7 4.8 ------- 6.6 6.3 3.6

J=3 o.6 2.7 6.3 -------- 2.4 7.3

J=4 1.8 1.5 5.1 2.7 --------- 3.9

J=5 4.5 3.6 2.4 5.4 4.5 -------

Case 3: Deadlines of transactions are sameTable 3:

In the above eg. Deadlines of transaction T3 and T4 are same, T0, T1 T3 T4So deadlines will not be scheduled according to earliest deadline Here EDF fails.

Transaction id

arrival time(At)

Block location(ai)

Block size(Bs)

Start block(Si)

End block(Li)

Average execution time (AET)

deadline(Di)

Transfer time(TT)

T0 0 0 0 0 0 0 0 0

T1 1 2 3 2 4 4.5 10 1.8

T2 0 7 6 7 12 9 18 3.6

T3 3 5 3 5 7 4.5 12 1.8

T4 0 2 4 2 5 6 12 2.4

Service tableCase 3(conti.)

Ci,j i=0 i=1 i=2 i=3 i=4

J=0 ----------- 2.4 5.7 3.3 3

J=1 1.2 ----------- 4.5 2.1 3

J=2 3.6 4.8 ------------ 3.9 5.4

J=3 2.1 3.3 3.6 -------- 3.9

J=4 1.5 2.7 4.2 1.8 -------

Advantages• The processor can be fully utilized and almost all deadlines can

be met.• Timing constraints are given consideration.• Real Time transactions are easily scheduled as timing

constraints are considered.Disadvantages• Transactions having same deadline can not be scheduled.• Disk head movement increases.• No consideration where data is present on disk. • The performance of Earliest Deadline steeply degrades in an

overloaded system.

AdvantagesDisadvantages

Conclusion

This paper presented the EDF scheduling technique, considering just the simple case of aperiodic transactions. In this some conventional algorithms , its disadvantage, the evolution of EDF and deadline was discussed . After this presented the flow of EDF algorithm, together with an example considering three cases and some results regarding the feasibility analysis .

References

(1) J. Stankovic, M. Spuri, K. Ramamritham, and G. Buttazzo, Deadline Scheduling For Real-Time Systems: EDF and Related Algorithms, Kluwer Academic Publishers, Boston, 1998

(2) C. L. Liu and J. W. Layland. Scheduling Algorithms for Multiprogramming in a hard-real-time environment. Journal of the ACM, 20(1), 1973

(3)Buttazzo G, Spuri M, Sensini F. Scuola Normale Superiore, Pisa, Italy, ‘‘Value vs. deadline scheduling in overload conditions, 16thIEEE real-time systems symposium (RTSS’95),1995

(4)Jain R, Hughes CJ, Adve SV. Soft real-time scheduling on simultaneous multithreaded processors. In: Proceedings of the 23rd IEEEinternational real-time systems symposium, 2002.

(5)Jeffay K, Martel CU. On non-preemptive scheduling of periodic and sporadic tasks. Proceedings of the 12th IEEE real-time systems

symposium. San Antonio, Texas: IEEE Computer Society Press; 1991

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