- It takes a long time to correct a high-queue state.
- Queue can do enormous damage during this high-queue period.
- The most straightforward way to use the WIP constraint is to block all demand when it reaches its upper limit.
It's preferable to block demands than to allow queues to reach a high-queue state because the damage a high-queue condition can cause is greater than the gains of the demands.
You can block queues in two ways:
- Eject the arriving job from the system.
- Hold back the job in an upstream queue. This approach doesn't reduce demand; it just shifts the location of the queue. In addition, it can generate economic benefits because different queues have different holding costs.