Oct 222009

When working on high performance supercomputers, network latency and bandwidth are of utmost importance. If messages cannot be sent quickly enough between compute nodes, a supercomputer may actually perform more poorly than a standard server or workstation. So much time is spent waiting for input from other locations that nothing is actually accomplished. These days, more time is spent optimizing latency than bandwidth since most clusters have all the bandwidth they need (2-4GB/sec).

I’ve discovered that an analogy can be made with traffic on the highway. Computers keep getting faster, and now maybe traffic can too. I promise you’ll notice this the next time you head out on the road.

Every time something happens on the highway, each car must individually react. Humans are driving the cars, so you’ll see at least 0.5 seconds latency before any cars react to an event. And for those shaving/sleeping/talking on the phone, the reaction time will be dangerously long (if they even notice what happened).

I’m convinced that this simple fact causes most traffic jams and significantly slows down busy traffic. Let’s take a local stop light as an example:

The light stays green for 30 seconds. You’d think the number of cars which pass through will be determined by their speed, but reaction time is really the determining factor. Yesterday, the light had already turned red by the time the car in front of me moved at all. Each car waits until the car in front has started moving, which adds a significant time delay for each car. Based on my measurements last night, there were about 1.5 seconds added for each car.

Now imagine that all cars started moving as soon as the light turned green. Suddenly, that 30 seconds of latency would be gone. With traffic moving at 10mph, up to 30 compact cars would pass through during a 30-second green light. But with 1.5 seconds latency between each one, at most 13 cars will make it through!

This means we’re operating at less than 50% efficiency. How many of the traffic jams you’ve been in have been caused by an actual road hazard? How many were just “phantom” jams caused by cars bunching up and introducing latency into the flow of traffic?

These are the sorts of tactics advanced drivers consider, as you can drive much more efficiently when you are aware of all traffic in front of you. Start matching speeds with vehicles 2 or 3 cars ahead of you and see how differently everything works. I promise it will make the BRAKE-GAS-BRAKE-GAS drivers look very ridiculous.

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