Valve Hardware Day 2006 - Multithreaded Edition
by Jarred Walton on November 7, 2006 6:00 AM EST- Posted in
- Trade Shows
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.
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.
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primer - Tuesday, November 7, 2006 - link
it's a nice article. i'm anxious to see what develops out of Valve in the near future.how about using some Athlon FX, Opteron 100/1200 series or higher speed Athlon X2s for crying out loud. i know that here is a performance gap even with the higher AMD models currently, but please show us an attempt at not being as one-sided.
jm20 - Tuesday, November 7, 2006 - link
The tests at 2.0 and 2.4 Ghz for the AMD chips are there, you can plot how it 'should' improve. Here are my scaling projections based on the data given for the 939 platform.AMD Speed single dual
2.0 14.0 27.0
2.2 15.5 29.5
2.4 17.0 32.0
2.6 18.8 35.0
2.8 20.8 38.2
3.0 23.1 41.7
I just hope this will show up correctly :/
Very interesting article, I'm very excited to see an almost linear improvement from single to dual core.
yacoub - Tuesday, November 7, 2006 - link
Your Tested Systems details on page 8 - the last three show a CPU of a Venice 3200+ oc'd to 2.4GHz (hey that's what I run too!), but the headings suggest they should really show Intel chips. :)yacoub - Tuesday, November 7, 2006 - link
Yes please make it only visual and not actually a part of the game, because much like trying too hard to make graphics look realistic, it really just adds more frustration for the player than anything else. It's already annoying in HL2 getting caught up on the edge of prop_physics related objects. The last thing we need is also getting caught or bumped by another player ;)
JarredWalton - Tuesday, November 7, 2006 - link
If the world is more interactive, there might not actually be a need for "prop physics" objects. There's also the potential to make a game with different gameplay mechanics depending on how the character interacts with the world. It sounds like initially Valve will make it a visual enhancement, to make entities look more lifelike in their behavior, but down the road others could potentially do more. Like most of the enhancements, what we want to see is how they can actually change and improve gameplay beyond just being visual.yacoub - Tuesday, November 7, 2006 - link
"You're doing a disservice to the customer if you're not using all of the CPU power."okay so that alone shows the need for dual-core so the customer can offload background tasks to a second core so the game can truly get 100% of a core. ;)
timmiser - Tuesday, November 7, 2006 - link
Background tasks?? I agree with Gabe that I want all of my CPU working the game. I'm sitting here with an Athlon X2 4800+ and I can't run Flight Sim X at an acceptable frame rate all the while I watch the graph that shows CPU#2 at idle while CPU#1 tries to run the entire program by itself!On another note, how about Microsoft develope some type of new API (DirecThread?) that automatically takes care of utilizing multiple cores so that game companies like Valve don't have to employ entire multi-threaded R&D divisions just so their games use both cores.
It seems like we are going back to the caveman days before DirectInput, DirectSound, & DirectX etc. Remember when you had to choose your sound card, joystick, and video card from a list within the game?? Let's not go there again!!
-Tim
JarredWalton - Tuesday, November 7, 2006 - link
That was a direct quote from... Gabe I think. Others might disagree, but I thought it was an interesting take on things. As for background tasks, they usually only need a bit of CPU time (less than 5% in most cases), so unless you really want to encode videos and play games at the same time....yacoub - Tuesday, November 7, 2006 - link
What a worthless attempt at sarcasm:Why not just say something normal like:
It's not omg-witty, but at least that sentence doesn't end with a preposition. @___@
JarredWalton - Tuesday, November 7, 2006 - link
Which obviously ruined the whole article. Seriously yacoub, a little tact with posting would be nice. Rather than stating:You could have just said something normal like:
We are real people, and derogatory adjectives like "worthless" tend to irritate more than help. You may not intend it that way, but imagine for a second someone talking about what you worked on for a couple days last week and describing it as "absolute garbage". The key word in "constructive criticism" is to actually make it constructive.
Now, thanks for the suggestion, and I'm more than happy to change things a bit to appeas people. Like most people, I just appreciate a bit more consideration, even if I'm just doing my job here. :) As the old cliche goes, you can catch more flies with honey than with vinegar. (Well, my grandpa used to say that anyway.)