LPT rule: Whenever a machine becomes free for assignment, assign that job whose. processing time is the largest among those jobs not yet assigned.

Transcription

1 LPT rule Whenever a machine becomes free for assignment, assign that job whose processing time is the largest among those jobs not yet assigned. Example m1 m2 m3 J3 Ji J1 J2 J3 J4 J5 J J1 J6 J2 J5 J4 Optimal schedule p i = 20 p i / m = 20 / 3 = 6.67 Optimal schedule length must be an integer. Thm Let TS be a set of n independent jobs and m be the number of machines. We have t LPT (TS, m) / t opt (TS, m) 4/3 1/(3m), where t LPT (TS, m) and t opt (TS, m) denote the Cmax of the LPT schedule and the optimal schedule, respectively. Moreover, the bound is achievable for every m. CS 537 Scheduling Algorithms 25

2 m = 2 3, 3, 2, 2, 2 LPT OPT m = 3 5, 5, 4, 4, 3, 3, 3 LPT OPT General m 2m+1 jobs P 1 = P 2 = 2m - 1 P 3 = P 4 = 2m 2 P 5 = P 6 = 2m 3 P 2m-1 = P 2m = P 2m+1 = m CS 537 Scheduling Algorithms 26

3 For m even, we have 3m-1 4m-1 LPT J 1 J 2m J 2m+1 J 2 J 2m-1 J 3 J 2m-2 J 4 J 2m-3 J m-1 J m+2 J m J m+1 3m OPT J m-1 J 1 J 2m-2 J 2 J 2m-3 J 3 J 2m-4 J 4 J 2m-5 J m J 2m-1 J 2m J 2m+1 CS 537 Scheduling Algorithms 27

4 Proof The theorem is trivially true for m = 1. Assume m 2. The proof is by contradiction. 1. Assume TS is the smallest (in terms of number of jobs) set of jobs that violates the bounds. Assume the jobs are ordered in descending order of their processing times, i.e. p 1 p 2 p n. Let s i (S LPT ) and f i (S LPT ) denote the starting time and finishing time of J i in S LPT, respectively. Note that s i (S LPT ) s i+1 (S LPT ) for 1 i n. 2. Claim 1 1. f n (S LPT ) = t LPT (TS,m) 2. f i (S LPT ) < t LPT (TS,m) 1 i n-1 Proof Suppose not. Let Jr(r<n) be such that f r (S LPT ) = t LPT (TS,m). Consider TS = {J 1,, J r }. Clearly t LPT (TS,m) = t LPT (TS,m). On the other hand, t OPT (TS,m) t OPT (TS,m). Therefore, t LPT (TS,m) / t OPT (TS,m) t LPT (TS,m) / t OPT (TS,m) > 4/3 1/3m Therefore TS is a smaller set of jobs that violates the bound. Contradiction! CS 537 Scheduling Algorithms 28

5 Claim 2 In an optimal schedule for TS, no machine can execute more than 2 jobs. Proof t OPT (TS, m) 1/m (j= 1 to n) p j S n (S LPT ) 1/m (j= 1 to n-1) p j t LPT (TS, m) = S n (S LPT ) + p n 1/m (j= 1 to n-1) p j + p n = 1/m (j= 1 to n) p j + (1 1/m) p n Therefore, t LPT (TS, m) / t OPT (TS, m) [1/m p j (j= 1 to n) + (1 1/m) p n ] / t OPT (TS, m) 1 + [(1 1/m) p n / t OPT (TS, m)] Since, t LPT (TS, m) / t OPT (TS, m) > 4/3 1/(3m), we have 1 + (1 1/m) p n / t OPT (TS, m) > 4/3 1/(3m) or t OPT (TS, m) < 3 p n Since P n is the smallest processing time, the optimal schedule can have at most 2 jobs on each machine. CS 537 Scheduling Algorithms 29

6 We will transform the optimal schedule in such a way that the schedule length (makespan) will never be increased. Finally, it is shown that the transformed schedule is isomorphic to the LPT schedule. since we assume that t LPT (TS, m) / t OPT (TS, m) > 4/3 1/3m > 1 Type I operation M k > M k No change in schedule length Type II operation M k M l > CS 537 Scheduling Algorithms 30

7 M k M l No change in schedule length Type III Operation M k M l > > M k M l No change in schedule length Apply Types I, II, and III operations exhaustively until none can be applied. We obtain the following type of schedule (by rearranging machines if necessary) CS 537 Scheduling Algorithms 31

8 Pk 1 Pk 2 Pk s t t Pk 1 Pk 2 Pk s 1 2 t Since no Type I operation can be applied, we have l l 1 l t Since no Type II operation can be applied, we have Pk s 1 Since no Type III operation can be applied, we have t t-1 1 Therefore Pk 1 Pk 2 Pk s l t t 1 The transformed schedule looks very much like the LPT schedule. In fact, they can be different only in the assignment of the second layer jobs. CS 537 Scheduling Algorithms 32

9 Consider 2 machines, both of which have 2 jobs assigned p p p q q q p q q p And p > q + q LPT rule would assign p to machine q. But this means that there is an optimal schedule with 1 machine having 3 jobs. to claim 2. Hence, we conclude that the transformed schedule is isomorphic to S LPT. Therefore t LPT (TS,m) = t OPT (TS,m). To show that at most a finite number of Type I, II, or III operations can be applied. Define Ij(S) to be the last time t such that machine j becomes idle in S. Let J(S) = (1 i < j m) I i (s) I j (s) 1. If S is obtained from S by a Type I operation, then J(S ) = J(S) CS 537 Scheduling Algorithms 33

10 2. If S is obtained from S by a Type II or III operation, then J(S ) < J(S) Between any Type II or III operations, there are at most m Type I operation that can be applied. Only a finite number of Type II or III operations can be applied because of conditions (2) and because there are only a finite number of schedules. P prmp Cmax P2 prmp, prec Cmax P prmp, intree Cmax P prmp, outtree Cmax ** All the above can be solved in polynomial time ** P prmp, prec Cmax is NP-hard Pm prmp, prec Cmax is open for fixed m > 2 CS 537 Scheduling Algorithms 34

11 P prmp Cmax can be solved by McNaughton s Rule 1. Compute t = max { max {}, 1/m ( i = 1 to n) } t is a lower bound for the optimal makespan 2. Schedule jobs one by one (wrap around if needed) J 4 J 1 J 2 J 2 J 3 J 4 t CS 537 Scheduling Algorithms 35