Dual vs. Single Processors

Xanthippe asks: "I'm currently looking to build a new system and hunting for a within-reasonable-budget ideal. A friend suggested going with dual-P500 processors instead of an Athlon650. I wouldn't mind going with either, but I'd like to know how they compare. I know zilch about processors and their mysterious voodoo-god-like inner workings, but I can read a benchmark as well as the next geek-wannabe. Has anyone tried pitting a dual system against one of the 600+ AMDs? Are there any major advantages/disavatages in going with a dual system?"

Comments

It depends on the apps

[Tet]

The first thing to remember is that each CPU can only ever do one thing at a time. An operating system like Unix will simulate being able to do multiple things simultaneously by allocating each process a small amount of CPU time, and then sending it to sleep, while another process is given access to the CPU. If you have multiple CPUs, then there is more total CPU time to allocate to each of your available processes, and so, with an SMP aware OS, the machine runs faster overall.

Individual processes, however, can only make use of multiple CPUs if they are specifically written to do so (e.g., by using a threads library). Otherwise, they will simply run at the same speed as they would do on a single CPU. This gives rise to some interesting situations. At my last company, we were doing some fairly hefty image manipulation. The main conversion program ran quicker on the departmental laptop than it did on a 64 CPU Sun Starfire. This is because the program could only make use of 1 CPU, and the Starfire's ran at 200MHz, compared to the laptop's 250MHz. However, as soon as you try running lots of those processes concurrently, the SMP machine comes into its own.

So, in answer to your question, if you're running computationally intensive single-threaded applications, and absolute performance is a high priority, then a faster single processor will be better. Otherwise, you're probably better off with two slightly slower CPUs.

Do You Need 500 mHz Let Alone Dual 500mHz?

[InitZero]

Probably not. I'd be willing to bet that most of the time (SETI, Dnet, etc. aside), you're current PC is sitting there spinning its wheels.

Except for a relative few, computers have far more CPU cycles than they need. Even my NT servers sitting on 450mHz processors are idle more often than not.

So, unless you have a clearly defined reason and performance statistics showing you that you need dual processors, you probably don't. Even if you do have performance statistics saying that you need more power, think twice about going dual.

Is your bottleneck the CPU? Can your application really use more than one processor?

For a single-user system, chances are one CPU is more than enough.

And, though I know this has been covered before, never buy a dual CPU board unless you are ready to put both CPUs on it when purchased. I've never met anyone who has actually put a second CPU in the box later. By the time you have the money or need to add a second processor, you can get a single processor that out-performs the dual along with a motherboard with all sorts of new bells and whistles for about what the the old processor cost you new.

InitZero