Valve Hardware Day 2006 - Multithreaded Edition

What Is Multithreading?

Before we get into discussion of how to go about multithreading, it may be beneficial for some if we explain what multithreading means. Most people who use computers are now familiar with the term multitasking. As the name implies, this involves running multiple tasks at the same time. This can be done either in the real world or on computers, and depending on what you're doing you may experience an overall increase in productivity by multitasking.

For example, let's say you're cooking dinner and it will consist of three dishes: roasted chicken, mashed potatoes, and green beans. If you were to tackle this task without any multitasking, you would first cook the chicken, then the potatoes, and finally the green beans. Unfortunately, by the time you're finished cooking the green beans, you might discover that the chicken and potatoes are already cold. So you decide to multitask and do all three at once: first you start boiling some water on the stove for the potatoes, while doing that you pull the chicken out of the refrigerator and place it into a pan and start heating the oven. Then you peel the potatoes. By now the water is boiling, so you put the potatoes into the water and let them cook. The oven is also preheated now, so you put the chicken in and let it begin cooking. The beans won't take too long to cook, so just wash them off and set them to the side for now. Eventually the potatoes are finished cooking, but before finishing those you put the green beans in a steamer and put them on the stove. Then you drain the potatoes and mash them up, add butter and whatever else you want, and now both the beans and chicken are done as well. You put everything onto plates, serve it up, and you're finished.

What's interesting to note is that the above description does not actually involve doing two things at once. Instead, you are actually doing portions of each task and then while you're waiting for certain things to complete you work on other tasks. On the classic single processor computer system, the same situation applies: the processor never really does two things at once; it just switches rapidly between various applications giving each of them a portion of the computational power of available. In order to actually do more than one thing at a time, you need more cooks in the kitchen, or else you need more processors. In the case of our example, you might have two people working on dinner, allowing more elaborate dishes to be prepared along with additional courses. Now while one person works on preparing the main three dishes we mentioned above, a second person could work on something like an appetizer and a dessert.

You could potentially even add more people, so you might have five people each preparing a single dish for a five course meal. Slightly trickier would be to have multiple people working on each dish. Rather than doing something mundane like grilled chicken, you could have a chicken dish with various other items to liven it up, along with a sauce. In extremely complex dishes, you could even break down a dish into more steps that various individuals could work on completing. Obviously, more can be accomplished as you add additional people, but you also run the risk of becoming less efficient so that some people might only be busy half the time.

We started with talking about multitasking, but the last example began to get into the concept of multithreading. In computer terminology, a "thread" is basically a portion of a program that needs to be executed. If you have a task that is computationally intensive and it is written as a single threaded application, it can only take advantage of a single processor core. Running two instances of such an application would allow you to use two processor cores, but if you only need to run one instance you need to figure out a way to take advantage of the additional computational power available. Multithreading is what is required, and in essence it involves breaking a task into two or more pieces which can be solved simultaneously.

Where multitasking can be important whether or not you have multiple processor cores available, multithreading really only begins to become important when you have the ability to execute more than one thread at a time. If you have a single core processor, multithreading simply adds additional overhead while the processor spends time switching between threads, and it is often better to run most tasks as a single thread on such systems. It's also worth noting that it becomes much easier to write and debug programming code when it is running as a single threaded application, because you know exactly in what order each task will execute.

We will return to this "cooks in the kitchen" example a bit more when we talk about the various types of threading environments. It's a bit simplistic, but hopefully it gives you a bit better idea about what goes on inside computer programs and what it means to break up a task into threads.