Section 15.3. What Makes a Computer Fast?

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15.3. What Makes a Computer Fast?

Computers are getting faster all the timeMoore's Law promises us that. But that doesn't help us to compare computers that are all of the same Moore's Law generation. How do you compare advertisements in the paper and figure out which of the computers listed is really the fastest?

Simply being fast is only one criterion for picking a computer, of course. There are issues of cost, how much disk space you need, what kind of special features you need, and so on. But in this section we'll explicitly deal with what do the various factors in the computer ads (see Figure 15.2 for some examples) mean in terms of computer speed.

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Figure 15.2. Sample computer advertisements.

15.3.1. Clock Rates and Actual Computation

When computer ads list that they have a "Some-brand Processor 2.8 Ghz" or "Other-brand Processor 3.0 Ghz," what they're talking about is the clock rate. The processor is the smarts of your computerit's the part that makes decisions and does computation. It does all this computing work at a particular pace. Imagine a drill sergeant shouting, "Go! Go! Go! Go!" That's what the clock rate ishow fast does the drill sergeant shout "Go!"? A clock rate of 2.8 Ghz means that the clock pulses (the drill sergeant shouts "Go!") 2.8 billion times per second.

That doesn't mean that the processor actually does something useful with every "Go!" Some computations have several steps to them, so it may take several pulses of the clock to complete a single useful computation. But in general, a faster clock rate implies faster computation. Certainly, for the same kind of processor, a faster clock rate implies faster computation.

Is there really any difference between 2.8 Ghz and 3.0 Ghz? Or is 1.0 Ghz with processor X about the same as 2.0 Ghz with processor Y? Those are much tougher questions. It's not really that much different than arguing over Dodge versus Ford trucks. Most processors have their advocates and their critics. Some will argue that processor X can do a particular search in very few clock pulses because of how well it's designed, so it's clearly faster even at a slower clock rate. Others will say that Processor Y is still faster overall because its average number of clock pulses per computation is so lowand how common is that particular search that X does so fast, anyway? It's almost like arguing about whose religion is better.

The real answer is to try some realistic work on the computer that you're considering. Does it feel fast enough? Check reviews in computer magazinesthey often use realistic tasks (like sorting in Excel and scrolling in Word) to test the speed of computers.

15.3.2. Storage: What Makes a Computer Slow?

The speed of your processor is only one factor in what makes a computer fast or slow. Probably a bigger factor is where the processor goes to get the data that it works with. Where are your pictures when your computer goes to work on them? That's a much more complex question.

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You can think about your storage as being in a hierarchy, from the fastest to the slowest.

• Your fastest storage is your cache memory. Cache is memory that is physically located on the same silicon chip (or very, very close to that) as your processor. Your processor takes care of putting as much as possible in the cache and leaving it there as long as it's needed. Cache is accessed far faster than anything else on your computer. The more cache memory that you have, the more things the computer can access very quickly. But cache (of course) is also your most expensive storage.

• Your RAM storage (whether it's called SDRAM or any other kind of RAM) is your computer's main memory. RAM (an acronym for Random Access Memory) of 256 Mb (megabytes) means 256 million bytes of information. It's where your programs reside when they're executing, and it's where your data is that your computer is directly acting upon. Things are in your RAM storage before they're loaded into the cache. RAM is less expensive than cache memory, and is probably your best investment in terms of making your computer faster.

• Your hard disk is where you store all your files. Your program that you're executing now in RAM started out as an .exe (executable) file on your hard disk. All your digital pictures, digital music, word processing files, spreadsheet files, etc., are stored on your hard disk. Your hard disk is your slowest storage, but it's also your largest. A hard disk of 40 Gb means that you can store 40 billion bytes on it. That's a lot of spaceand that's pretty small these days!

Movement between levels in the hierarchy means a huge speed difference. Some experts have said that if the speed of access of cache memory is like reaching for a paper clip on your desk, then getting something off the hard disk means traveling to Alpha Centaurifour light-years away from Earth. Obviously, we do get things off our disk at reasonable speeds (which really implies that cache memory is phenomenally fast!), but the analogy does emphasize how very different the levels of the hierarchy are in speed. The bottom line is that the more you have of the faster memory, the faster your processor can get the information that you want and the faster your overall processing will be.

You'll see advertisements occasionally mentioning the system bus. The system bus is how signals are sent around your computerfrom video or network to hard disk, from RAM to the printer. A faster system bus clearly implies a faster overall system, but a faster system bus might not influence (for example) the speed of a Java program. First, even the fastest bus is much slower than the processor400 million pulses per second versus 4 billion pulses per second. Second, the system bus doesn't usually influence the access to cache or memory, and that's where the majority of the speed is won or lost anyway.

There are things that you can do to make your hard disk as fast as possible for your computation. The speed of the disk isn't that significant for processing timeeven the fastest disks are still far slower than the slowest RAM. Leaving enough free space on your disk for swapping is important. When your computer doesn't have enough RAM for what you're asking it to do, it stores some of the data that it isn't currently using from RAM on to your hard disk. Moving data to and from your hard disk is a slow process (relatively speaking, compared to RAM access). Having a fast disk with enough free space that the computer doesn't have to search around for swap space helps with processing speed.

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How about the network? In terms of speed, the network doesn't really help you. The network is magnitudes slower than your hard disk. There are differences in network speeds that do influence your overall experience, but not necessarily the speed of processing on your computer. Wired Ethernet connections tend to be faster than wireless Ethernet connections. Modem connections are slower.

15.3.3. Display

How about the display? Does the speed of your display really impact the speed of your computer? No, not really. Computers are really, really fast. The computer can repaint everything on even really large displays faster than you can perceive.

The only place that one might imagine someone arguing that the display speed matters is with really high-end computer gaming. Some computer gamers claim that they can perceive a difference between 50 frames per second and 60 frames per second updates of the screen. If your display was really large and everything had to be repainted with every update, then maybe a faster processor would make a difference you could perceive. But most modern computers today update so quickly, you just couldn't tell a difference.