Friday, September 11, 2015

Queuing - Single Queue Works, But Why Doesn't Everywhere Do That?

Queuing theory utilizes applied mathematics to deal with the phenomenon of waiting -- arising from the use of mathematical analysis to improve production processes. So why doesn't McDonald's utilize this approach? Customer what times over 90 seconds can be problematic. But perceived wait time is more critical - like page loads in your web browser. If the UI/UX designer has come up with a novel way of loading content, a user will wait out the progress bar. Or, if the content is so compelling (think, your bank account, or cat videos).

But it helps to think of getting your french fry fix take-out as involving a series of work stations, each with a separate task. And each task takes time (e.g. ordering food, instructing workers, retrieving hot fries, putting into packaging food, payment). These stations are generally attended in sequence, and each station takes some time to process one customer. The sequence of stations is a pipeline. But some steps take longer than others -- so building in wait time at certain points actually serves to move the production process along without bottlenecks. McDonalds provides several queues in parallel, the first for ordering and paying, and the second, an (invisible) station where customers wait while their food is gathered and served. The time it takes to cook the food is accounted for in the time taken to gather the food items.

The same analysis can be applied to packet switching with internetworking, or with automobile assembly. For my masters work, I looked at a supplier to a Japanese auto manufacturer -- with a supply chain represented as a multi-input, multi-stage queuing network. An input order to the supply chain was represented by stochastic variables, for the occurrence time and for the quantity of items to be delivered in each order. I had seen such an approach when learning about the (now sunsetted) wide area network at the central bank, where I was involved with information security. A "star" network topology has a central top level node that all other nodes connect to. "Packets" are passed through the central node. This helped me understand alternate ways of queuing -- something I have carried forward with my efforts at automating workflow in Bluedog's SAAS offering, where 'jobs' have to be passed from one stage to another, based on business rules.

The typical first-come, first-serve system of waiting in line is incredibly inefficient, in terms of both time and space. First, it essentially rewards people for wasting their time: Those who arrive first get the goods, but they also spend more hours of their precious time on Earth standing around and waiting. Second, long lines tend to create congestion and bottlenecks that cause problems for others. Think of the traffic jams that form as cars try to leave a football game, or the long boarding line at an airport that snakes across the walkway, getting in everyone else's way.

Read more here... Danish Researcher Report or read this guy's ideas.

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