# Assigning identical operators to different machines

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1 EINDHOVEN UNIVERSITY OF TECHNOLOGY Department of Mathematics and Computing Science Memorandum CaSaR Assigning identical operators to different machines A.G. de Kok J. van der Wal Eindhoven, November 1993 The Netherlands

2 Einqhoven University of Technology Department of Mathematics and Computing Science Probability theory, statistics, operations research and systems theory P.O. Box MB Eindhoven - The Netherlands Secretariat: Telephone: Dommel building ISSN

3 ASSIGNING IDENTICAL OPERATORS TO DIFFERENT MACHINES Jan van der Wal and Ton G. de Kok, Eindhoven University oftechnology ABSTRACT This paper presents a new approximation for the response times in a production system with M machines and s operators, where each machine has its own arrival stream ofjobs, where no machine can work without the constant attention of an operator and where the operators can serve only one machine at a time. The arrival streams are Poisson with equal rates and all jobs have the same service time distribution. Several strategies are considered for the operators. Extensive simulations show that for the case of exponential service times the differences between these strategies are usually small. The approximation is also quite good for non-exponential service times. 1. INTRODUCTION We will consider a production system with M machines attended by s operators with s < M, so the number of operators is smaller than the number of machines. Machine m receives a Poisson arrival stream ofjobs with rate Am' The streams for different machines are independent. No machine can work without the constant attention of an operator. An operator can serve only one machine at a time. All machines are different, so a job can only be realized on the machine where it arrived. When controlling such a system the main problem is to avoid as much as possible states in which jobs are waiting while operators are idle. Clearly this may always happen asa job can arrive at a machine that is already busy when one or more operators are idle. So in this system the operators will be less efficient than in an MIG Is system. We will restrict our attention to the symmetric case with identical arrival rates, Am =" for all m, and the same service time distribution on all machines. For the case of exponential service times Durlinger and Geurts [1] give an approximation based on aggregation which is excellent if an operator who has completed a job randomly chooses an idle and waiting machine. In this paper we present another approximation based on the asymptotic behaviour of the system which suggests a straightforward extension to the case of non-exponential service times. Further we study a number of strategies for the operator problem: which job to

4 - 2- serve next. The differences are mostly small. The asymptotic approximation turns out to be very good for both low and high utilizations and practically all interesting strategies. Throughout this paper the mean service time is taken to be 1 and the service times are assumed to be exponential except in section 7. Section 2 introduces the six operator strategies that we will compare. In section 3 the approximation of Durlinger and Geurts is given. Section 4 contains the asymptotic approximation. Section 5 presents the results from the simulations. In section 6 we discuss the ergodicity conditions for the Shortest Queue strategy. Section 7 gives some results for non-exponential service times. 2 THE OPERATOR STRATEGIES Whenever a job is completed an operator is "freed". Then it has to be decided where he should go to next. As we stated before, this system will always be less efficient than the MIG Is system and it is important to avoid as much as possible unbalanced states. (A state is called unbalanced if the difference in queue lenghts is large.) In an unbalanced state it is more likely that in the near future a state will be reached where jobs are waiting while one or more operators are idle. So the better strategies are the ones that try to balance the queue lengths. In this paper we will compare six strategies. 1. Random Machine (RM). The operator selects at random one of the idle and waiting machines. All machines have equal weights. 2. Random Job (RJ). The operator selects at random one of the idle and waiting machines. The machines have weights equal to the number of jobs waiting. So the operators give some preference to the longer queues. 3. Longest Queue (LQ). The operator goes to (one of) the longest queue(s). One may show that this strategy is optimal, see [3]. 4. First Corne First Served (FCFS). The operator goes to the job which arrived first (considering only the jobs waiting at idle machines). 5. Longest Queue Exhaustive Service (LQES). An operator always completes all jobs at a machine before he becomes free. Then he goes, as in LQ, to the longest queue. For comparison we also consider 6. Shortest Queue (SQ). After completing a job the operator goes to the idle and waiting machine with the shortest queue. This strategy maximizes the average response time, see [3]. Ifwe denote the mean waiting time (response time) for strategy RM by W RM (SRM), etc., then we expect the following partial ordering of the waiting times:

5 - 3 - WLQ < W RJ < W RM < WSQ ' and WLQ < WpCPS < WSQ' WLQ < WLQES < WsQ. The inequality W RJ < W RM is intuitively clear. Giving preference to the longer queue in some random way should help. The order for the strategies FCFS and LQES and their position relative to RJ and RM is unclear. Further one may expect that the differences are small for low loads but they might become considerable for high loads. The results from the simulations in section 5 will confirm this. For all these strategies (except FCFS) the state ofthe system in the completely symmetric case is fully characterized by the state vector (i 1,...,iM; n 1,..,,nM), with i m = 1 if machine m is busy and i m = 0 if machine m is idle, and n m the number ofjobs waiting (not in service!) at machine m. For none of these systems we have been able to obtain a simple expresssion for the equilibrium distribution. 3. AGGREGATION BASED ON THE BOSE EINSTEIN ASSUMPTION The approximation ofdurlinger and Geurts [1] for the completely symmetric exponential case is based on the assumption that the system behaves almost according to the Bose Einstein rule if an operator who just finished his job picks the next machine at random. Their approach is as follows. Assume that the margimal distribution of the jobs at the various queues given that the total number of jobs in the system equals k follows the Bose-Einstein probability distribution. I.e., all possible distributions of k jobs over M queues are equally likely, so the probability p(k],..., k M ; k) of finding k m jobs in queue m, m = 1,..., M, satisfies p(k)....,k M ;k)=l/ k +M -1] [ M-1 (1) Remark Note that this result is exact if the number of operators is equal to the number of machines, in which case each machine has its own operator and thus all machines are independent. Then, as one easily sees, the probability of finding microstate (k 1,..., km) is equal to (1 - p)m pk with k = U m This assumption (for s < M) makes it possible to aggregate the microstates into macrostates. The macrostates are characterized by the total number of jobs waiting and in service in the system.

6 -4- For this system the rate y(k - 1, k) by which transitions from k - 1 to k are made equals AM, the total arrival rate. The rate by which transitions are made from k to k - 1 is equal to the expected number of active machines. This number is approximated using equation (1). The probability p(l ; k) that lout of the M queues are empty is given by p(/;k)= [ M}[ 1 M-I-l k-l] / [k+m-~ M-l J (2) If 1queues are empty M - I are nonempty and then the rate from k tot k - 1 is equal to min(s, M -I). So the mean rate y (k,k - 1) for transitions from k to k - 1 is given by M-I Y(k,k - 1) = L p (l ; k)min(s, M -I) (3) l=o This birth and death process for the macrostates is easily solved numerically. In the sequel we call this approximation AGGR(egation). 4. AN APPROXIMATION BASED ON THE ASYMPTOTIC BEHAVIOUR In this section we derive an approximation for the mean response time based on the asymptotic behaviour of the system. First, let us think a little about the asymptotic behaviour of the system if M becomes large. If the number of operators s and their utilization p remain constant while M tends to infinity then the arrival rate per machine A. = sp/m tends to zero and the total number of jobs in the system converges in distribution to the number in the M 1M Is queue with arrival rate sp and mean service time 1. Further, if s = M then each queue behaves as an M 1M 11 queue with arrival rate p and mean service time 1. These considerations suggest that a convex combination of the response times in the M 1M 11 and M IMis queues might work well. So we suggest an approximation for the mean waiting time W(s,M, p) of the following form: W(s,M, p) =a.(s,m, p) WI (p) + (1- a.(s,m, p)) Ws(p), (4) where We (p) is the mean waiting time in an M IM Ic queue with mean service time 1. The problem is to find a suitable function a.(s,m, p). A primitive attempt is to plot the ideal factor a. for a number of simulation results. We started with a testbed of 88 simulations with M equal to 4, 8 and 16 with s running from 2 to M - 1 and utilizations 0.7,0.8,0.9 and 0.95 for which the strategies RM and RJ were simulated. For this set we found excellent results for a function of the form 1 a.(s,m, p) =(s 1M) p(1-p) + q(l-s/m) + r, with approximately p =0.88, q =0.35 and r =0.02. (5)

7 - 5 - This function yields a maximum absolute relative error in the mean response time for this testbed of 10 percent and an average absolute error of less than 2 percent. However we have no understanding for this function at all and when we extended the testbed with the 44 simulations for the utilizations 0.3 and 0.5 the quality of the approximation dropped considerably. Being not very pleased with this function we looked for a more sophisticated way of obtaining a.. Let us again think about what happens ifm and s are large. Ifs is large, then for utilization p on the average s(l- p) operators are free. So for large s there will "always" be a free operator. Now look at one machine. For that machine the arrival rate is equal to sp/m. As there is always an operator available this queue will behave (approximately) as an M 1M 11 with arrival rate sp/m and mean service time 1. So the mean waiting time for each machine is approximately equal to W}(sp/M)=(sp/M)/(l-sp/M). Further Ws(p) goes to 0 if s tends to infinity, so for large s we should have W} (sp/m) a.(s,m, p) = -W-}-(P-)- (6) The approximation (4) with a. given by (6), further denoted by ASYMP, will tum out to be very good, even for p= SIMULATION RESULTS We have run extensive simulations for the testbed of 132 cases: M equal to 4,8 and 16, s running from 2 to M -1 and operator utilizations 0.3, 0.5, 0.7, 0.8, 0.9 and Tabels 5.1 to 5.6 give for each of the six strategies the results per utilization. In all simulations the standard deviation of the point estimate for S is around 2 promille of the mean. The simulations for utilization 0.95 require per case and per strategy already a couple of hours on a Sun Sparc workstation.

8 - 6- Response times for p =0.30 M s SQ RM RJ LQES FCFS LQ AGGR ASYMP Tabe15.1

9 - 7 - Response times for p =0.50 M s SQ RM RJ LQES FCFS LQ AGGR ASYMP Tabe15.2

10 - 8 - Response times for p =0.70 M s SQ RM RJ LQES FCFS LQ AGGR ASYMP Tabe15.3

11 - 9 - Response times for p = 0.80 M s SQ RM RJ LQES FCFS LQ AGGR ASYMP Tabel5.4

12 - 10- Response times for p =0.90 M s SQ RM RJ LQES FCFS LQ AGGR ASYMP Tabe15.5

13 Response times for p = 0.95 M s SQ RM RJ LQES FCFS LQ AGGR ASYMP Tabel5.6 From these results we can draw a number ofconclusions. First of all we see from tabel 5.6 that the SQ strategy does not use the capacity efficiently when p is large and the number of operators is close to the number of machines. In three cases (indicated by 00) the system appears to be no longer ergodic. We will come back to this in section 6. The other 5 strategies are compared in Tabels These tables give for p~0.7 the relative differences between the optimal strategy LQ and the strategies: RM, RJ, LQES and FCFS. For p equal to 0.3 and 0.5 the differences are negligible.

14 - 12- Relative percentage difference with LQ policy for p = 0.70 M s RM RJ LQES FCFS Tabe15.7

15 - 13- Relative percentage difference with LQ policy for p =0.80 M s RM RJ LQES FCFS Tabe15.8

16 - 14- Relative percentage difference with LQ policy for p = 0.90 M s RM RJ LQES FCFS Tabe15.9

17 Relative percentage difference with LQ policy for p = 0.95 M s RM RJ LQES FCFS Tabel5.10 Mean relative percentage difference with LQ policy P RM RJ LQES FCFS TabeI5.11 Due to the limited accuracy of the simulations some negative differences are found in spite ofthe optimality of the LQ policy.

18 - 16- We see that the differences increase with p. Also note that the differences between the 5 strategies and the two approximations are always very small if p ~ 0.8. The differences increase if p is increased and are larger if s is close to M if M is not too large. These results are summarized in tabels Mean and maximal relative differences for p ~ 0.90 Percentage difference with AGGR RM RJ LQES FCFS LQ Mean Maximal Percentage difference with ASYMP RM RJ LQES FCFS LQ Mean Maximal Tabe15.12 Mean and maximal relative differences for p = 0.95 Percentage difference with AGGR RM RJ LQES FCFS LQ Mean Maximal Percentage difference with ASYMP RM RJ LQES FCFS LQ Mean Maximal TabeI5.13

19 Mean and maximal relative differences for all p Percentage difference with AGGR RM RJ LQES FCFS LQ Mean Maximal Percentage difference with ASYMP RM RJ LQES FCFS LQ Mean Maximal Tabel5.14 So for p = 0.95 the Bose-Einstein aggregation gives large errors except for strategy RM. For the other 4 strategies the asymptotic approximation is better and even quite good. 6. ERGODICITY Tabel 5.6 indicated that p < 1 is not the ergodicity condition for the shortest queue policy as one might have expected. We do not know how to obtain the exact ergodicity condition. The following reasoning gives some insight in the problem. Let us consider the case s = M -1. And let us assume a preemptive resume policy is used with priority to the machine with the lower number, so machine 1 has priority over all other machines, machine 2 has priority over 3 upto M, etc. Since there are M - 1 operators there is always an operator available for any of the machines 1 upto M - 1. So these machines all behave as M 1M 11 queues with arrival rate and utilization a:= sp/m = (M-I)p/M. The fraction of time an operator is available for machine M is just the fraction of time that at least one of the first M -1 machine queues is empty, which is equal to 1 - am - I. The arrival rate to machine M equals a as well. So the system will be ergodic if a<1-a M - 1 (5.1) The ergodicity conditions for M = 4,8 and 16 are given in tabel 6.1.

20 Ergodicity condition preemptive priorities M=4 M=8 M=16 Pmax Table 6.1 If preemptions are not allowed the behaviour of the system is more complicated. But it seems clear that in that case as well p < 1 will not be sufficient either. In the case of the shortest queue policy there are no preemptions and no priorities. But once one of the machine queues has become long the operators are going to give priority to the other (shorter!) machine queues. So with one long queue the system behaves as the nonpreemptive version of the system described above. Intuitively this explains to us why P < 1 is not sufficient. Our conjecture is that p < 1 is sufficient for the policies LQ, FCFS, LQES and RJ and not for RM and SQ. 7. NON-EXPONENTIAL SERVICE TIMES The asymptotic approximation given in section 4 suggests a number of straightforward generalizations for non-exponential service times. Note that in (4) we can replace the waiting times for the M 1M 11 and M IMis queues by the waiting time in the MIG 11 and one of the approximations for the waiting time in the MIG Is queues, see e.g. Tijms [2]. Then, as one easily verifies, the same ex can be used as in (6) for the exponential service times. Tables 7.1 and 7.2 give the simulation results for Erlang-2 and Erlang-4 distributed service times. We compare the results for the strategies RM, RJ and LQ with the asymptotic approach. For the MIG Is queue we used the approximation given in equations (4.196) and (4.200) in Tijms[2]: with WM IG Is =[(1-P)Ys + P(1+c 2 )12] W M 1M Is, Y= (1-c )-- + c 2 -, s+1 s and c 2 the squared coefficient of variation (1/2 for Erlang-2 and 1/4 for Erlang-4).

21 Response times for Erlang-2 service times M S P RM RJ LQ ASYMP Tabel7.1 Response times for Erlang-4 service times M S P RM RJ LQ ASYMP Tabel7.2 As we see the result are just as good as in the exponential case. The Asymptotic approximation is quite close to the results for the Random Job policy. A more primitive approach is to simply multiply the waiting times for the exponential case by the expected residual service time divided by the mean service time. Ifwe do this with the simulation results for the exponential service time then the results are between 0 and 2 percent below the simulation results for the Erlang service times.

22 - 20- It might be worthwhile to find good approximations for the differences between the various strategies and the differences with the approximations. References [1] DURLINGER, P.PJ. and J.H.J. GEURTS, Determining work order throughput times in dual constrained production departments, Eindhoven University of Technology, Department of Industrial Engineering, Working paper BDKlKBS/89-13, December [2] TIJMS, H.C., Stochastic Modelling and Analysis: A Computational Approach. Wiley, Chichester, 1986, edition [3] WAL, J. VAN DER, The longest queue policy is optimal for the assignment of identical operators to different machines, Eindhoven University of Technology, Department of Mathematics and Computing Science, Memorandum CaSaR, 1994.

23 List of COSOR-memoranda Number Month Author Title January P. v.d. Laan Subset selection for the best of two populations: C. v. Beden Tables of the expected subset size January R.J.G. Wilms Characterizations of shift-invariant distributions J.G.F. Thiemann based on summation modulo one February Jan Beirlant Asymptotic confidence intervals for the length of the John H.J. Einmahl shortt under random censoring February E. Balas One machine scheduling with delayed precedence J. K. Lenstra constraints A. Vazacopoulos March A.A. Stoorvogel The discrete time minimum entropy H00 control J.H.A. Ludlage problem March H.J.C. Huijberts Controlled invariance of nonlinear systems: C.H. Moog nonexact forms speak louder than exact forms March Marinus Veldhorst A linear time algorithm to schedule trees with communication delays optimally on two machines March Stan van Hoesel A class of strong valid inequalities for the discrete Antoon Kolen lot-sizing and scheduling problem March F.P.A. Coolen Bayesian decision theory with imprecise prior probabilities applied to replacement problems March A.W.J. Kolen Sensitivity analysis of list scheduling heuristics A.H.G. Rinnooy Kan C.P.M. van Hoesel A.P.M. Wagelmans March A.A. Stoorvogel Squaring-down and the problems of almost-zeros J.H.A. Ludlage for continuous-time systems April Paul van der Laan The efficiency of subset selection of an c:-best uniform population relative to selection of the best one April R.J.G. Wilms On the limiting distribution offractional parts ofextreme order statistics

24 Number Month Author Title May L.C.G.J.M. Habets On the Genericity of Stabilizability for Time-Day Systems June P. van der Laan Subset selection with a generalized selection goal C. van Eeden based on a loss function June A.A. Stoorvogel The Discrete-time H oo Control Problem with A. Saberi Strictly Proper Measurement Feedback B.M. Chen June J. Beirlant Maximal type test statistics based on conditional J.H.J. Einmahl processes July F.P.A. Coolen Decision making with imprecise probabilities July J.A. Hoogeveen Three, four, five, six or the Complexity of J.K. Lenstra Scheduling with Communication Delays B. Veltman July J.A. Hoogeveen Preemptive scheduling in a two-stage multiprocessor flo J.K. Lenstra shop is NP-hard B. Veltman July P. van der Laan Some generalized subset selection procedures C. van Eeden July R.J.G. Wilms Infinite divisible and stable distributions modulo July J.H.J. Einmahl Tail processes under heavy random censorship with F.H. Ruymgaart applications August F.W. Steutel Probabilistic methods in problems of applied analysis August A.A. Stoorvogel Stabilizing solutions of the H 00 algebraic Riccati equation August R. Perelaer The Bus Driver Scheduling Problem of the Amsterdam J.K. Lenstra Transit Company M. Savelsbergh F. Soumis August J.M. van den Akker Facet Inducing Inequalities for Single-Machine C.P.M. van Hoesel Scheduling Problems M.W.P. Savelsbergh September H.J.C. Huijberts Dynamic disturbance decoupling of nonlinear systems H. Nijmeijer and linearization

25 Number Month September Author L.C.G.J.M. Habets Title Testing Reachability and Stabilizability of Systems over Polynomial Rings using Grobner Bases September Z. Liu J.A.C. Resing Duality and Equivalencies in Closed Tandem Queueing Networks November A.G. de Kok J. van der Wal Assigning identical operators to different machines

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