Don't Miss the (Front-Side) Bus
SHA Newsletter: Vol 2. No. 8
I recently worked on a Pentium 4 laptop that was running very slowly and
sporadically crashing. This didn't make much sense, because the system
featured a 1.60 GHz Pentium 4 processor and 512MB of RAM. These are good
numbers to have; by the numbers, it should have been a pretty fast little
machine. I was curious, to say the least, about why it ran so slowly.
After some investigation, I tested the memory and found one of the memory sticks to be bad. I removed the bad memory stick, and the mysterious crashes vanished. That's also when I discovered the subtle cause of the poky performance, "PC-133" labels on the memory sticks. That told me that the memory was considerably slower than the processor itself. To understand the whole picture, you need to know about the front-side bus (FSB).
The front-side bus is the electrical connection between the processor and the computer's main memory, or RAM, If you have a Pentium, Xeon, or Centrino label on your PC, then you have an Intel-based computer, and it has a front-side bus (some other machines don't use a front side bus). The speed of the bus is given in units of MHz (millions of cycles per second), because the CPU and memory exchange data in cycles. The PC-133 label meant that the memory stick was intended for a 133 MHz front-side bus.
This sounds wonderful, until you consider that Pentium 4 processor chip. At 1.6GHz, it can ask for data at well beyond a billion-and-a-half times per second (1.6 GHz is 1,600 MHz) -- twelve times faster than the memory sticks. When the processor needs to fetch data (which happens continually), it will demand it 12 times faster than the memory sticks can respond. As a result, that zippy Pentium processor is mostly in idle, waiting for data from memory.
If traffic lights worked like this, when you drove up to a red light, it would be like waiting for 11 red-to-green cycles before you could drive through the intersection. You might have a very powerful car, but you just couldn't race across town.
Ways To Get Better Performance
CPUs are always going to be faster than those memory sticks that snap into the computer. That's a fact of life. So computer designers have some neat ways to compensate for the difference. Here are some of them.
On-Chip Cache Memory: Modern processors include a nice chunk of RAM memory (a "cache") right inside the chip itself, running at full speed. If your program (or a section of it) fits in cache, there's no need to move data on the front-side bus. Unfortunately, cache can't be the only solution. Eventually your programs have to move data over the FSB, and pay the performance penalty. Nevertheless, cache helps a great deal. The more cache you have, the faster your computer can run. To save money, manufacturers can use processors that have smaller chunks of cache memory.
Double Data Rate Memory: DDR or DDR2 identifies specially designed memory sticks that can deliver data two or more times per clock cycle. The front-side bus might still be rather slow, like 133 MHz, but the memory responds faster to compensate. If the spec sheet talks about DDR or DDR2 RAM, this is what it means. Sometimes, a computer uses "dual-channel" memory as well, which divides each memory stick into halves. The CPU can take data from one half while it's waiting for the other half to deliver some more. This can also help pick up the pace.
Faster Front Side Buses: Speeds of 400, 533, 800MHz and faster are possible. The faster the FSB runs, the smaller the performance hit when your program gets data from main memory. If your FSB is very fast, your memory sticks will have to be faster too, in order to keep up. All this makes the computer more expensive.
In the case of our poky laptop, the cause was clear. It was an older machine, one of the early Pentium 4 PC's. It used relatively slow memory and FSB. The result was really slow performance on a computer that advertised a really fast CPU, because the computer moved a lot of information over the slow FSB.
When You Go Shopping
Look for overall balance. An ultra-fast CPU will always go faster than the speed of the FSB, but a multiple of more than 4 or 5 will cause a big performance hit. A speedy CPU chip won't be able to sustain its top speed unless the mainboard, the memory, and all the supporting chips are able to keep up.
If you see big differences in price between computers that seem to have equivalent specifications, ask why. Pairing a fast processor with a slow mainboard and memory can lower the cost, but also the performance, of your new computer. Unfortunately, the specifications for on-chip cache, processor speed and FSB speed aren't always easy to find and understand.
Remember to start with your needs and budget. Even the slowest of modern computers are plenty fast for most jobs. Getting a great deal on a computer that's fast enough is a wise choice.
Please note: Any trademarks and trade names of others mentioned in this message are the property of their owners, and not Stoney Hill Associates, LLC. We respect the intellectual property of others. The information provided is believed to be reliable, but we cannot guarantee that the procedures and information given here will work correctly for your specific situation.
If you would like help with a computer or software problem you face, contact us. Send an email to request@stoneyhillassociates.com.
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