Load-limited order release in job shop scheduling systems

This report examines the practice of using work load limits to control the release of orders to a job shop. Whenever the inventory of work at a work centre exceeds some critical value (its ‘load limit’), further release of orders which are routed to that work centre are blocked from entering the shop. After the inventory is ‘worked off’, release of work to the shop gateways is again permitted. Load-limited order release is intuitively appealing because it appears to be a method for reducing system inventory and flow times. The practice of load limiting order release is becoming popularized by some of the recent production planning software products now on the market. A notable example is OPT. In this report, analytical results for an M/M/1 queueing model, along with existing simulation studies of multi-machine job shops are interpreted to form a theory about the effects of using load limits. The major finding here is the proposition that system flow time, inventory, and order tardiness all deteriorate to the extent that load limits introduce idle time into the schedule. Based on the arguments presented here, a very cautious approach toward the use of input control schemes for anywhere but gateway work centres would be advised. The conclusions drawn here are to a great extent arrived at by interpreting the research results of others, so there is a clear need for further research which tests these assertions in a more direct and controlled way.