Advantages and Disadvantages of Coroutines

In fact, you may be wondering why you would ever need to program coroutines that don't communicate with each other. For example, why not just run hatter after alice in the usual way? What advantage is there in running the two jobs simultaneously?

Even if you are running on a computer with only one processor (CPU), then there may be a performance advantage.

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Roughly speaking, you can characterize a process in terms of how it uses system resources in three ways: whether it is CPU-intensive (e.g., does lots of number crunching), I/O-intensive (does a lot of reading or writing to the disk), or interactive (requires user intervention).

We already know from Chapter 1 that it makes no sense to run an interactive job in the background. But apart from that, the more two or more processes differ with respect to these three criteria, the more advantage there is in running them simultaneously. For example, a number-crunching statistical calculation would do well when running at the same time as a long, I/O-intensive database query.

On the other hand, if two processes use resources in similar ways, it may even be less efficient to run them at the same time as it would be to run them sequentially. Why? Basically, because under such circumstances, the operating system often has to "time-slice" the resource(s) in contention.

For example, if both processes are "disk hogs," the operating system may enter a mode where it constantly switches control of the disk back and forth between the two competing processes; the system ends up spending at least as much time doing the switching as it does on the processes themselves. This phenomenon is known as thrashing; at its most severe, it can cause a system to come to a virtual standstill. Thrashing is a common problem; system administrators and operating system designers both spend lots of time trying to minimize it.