Programmer Musings

More Human Multitasking

Isn't it funny how you sometimes run into the same concept everywhere at once? A couple of weeks ago, I wrote a piece (Another View of Human Multitasking) refuting some of the conclusions in a Joel Spolsky article on human multitasking. This week, I stumbled across another article, Creating Passionate Users: Your brain on multitasking, that makes pretty much the same points as Joel's essay. Interestingly for me, this essay points to some original research backing up her claims.

As I suspected, the research is specifically related to pre-emptive modes of multitasking. As I stated in my earlier essay, we've known for years that pre-emptive multitasking is not the fastest way to solve problems on a computer either. If the tasks are cpu-bound, every time-slice incurs the task switch overhead. The reasons we use pre-emptive multitasking in computers have little to do with overall processing speed.

As I said in my previous essay, switching tasks when you are blocked is the only way to get more work done in a given amount of time with a multitasking system. Just like a computer, a human can get more done with lower priority tasks that you turn to when the main task is blocked. That way, even when you can't progress on the main task, you can still make progress on something. This is what I have always meant when I say that I multitask well.

Interrupts

One area I did not touch in the previous essay, was the concept of interrupts. When an interrupt comes in, there is a forced task switch just like with pre-emptive multitasking. Unlike a computer, humans cannot store their mental state on the stack and come back to it. An interrupt pretty much makes you lose all of the dynamic state you've built up. Anything you've written down or committed to more long-term storage is retained of course. But, the part that you are working on right now is lost unless you explicitly take time to save it.

This explains why phone calls or random meetings can really ruin your flow. While in flow, it feels to me like I have more of the problem and solution space in my head at one time. It almost feels like I can see a large portion of the problem space spread out in front of me. When an interruption occurs, all of that understanding and feel for the problem space vanishes. There's no time or place to store it. So, once the interrupt is handled, we have to start from scratch slowly building up that information all over again.