We investigate the problem of semi-online scheduling jobs on m identical pa
rallel machines where the jobs arrive in order of decreasing sizes. We pres
ent a complete solution for the preemptive variant of semi-online schedulin
g with decreasing job sizes. We give matching lower and upper bounds on the
competitive ratio for any fixed number m of machines; these bounds tend To
(1 + root3)/2 approximate to 1.36603, as the number of machines goes to in
finity. Our results are also the best possible for randomized algorithms. F
or the non-preemptive variant of semi-online scheduling with decreasing job
sizes, a result of Graham (SIAM J. Appl. Math. 17 (1969) 263-269) yields a
(4/3 - 1/(3m))-competitive deterministic non-preemptive algorithm. For m=2
machines, we prove that this algorithm is the best possible (it is 7/6-com
petitive). For m=3 machines we give a lower bound of(1 + root 37)/6 approxi
mate to 1.18046. Finally, we present a randomized non-preemptive 8/7-compet
itive algorithm for m = 2 machines and prove that this is optimal. (C) 2000
Elsevier Science B.V. All lights reserved.