Question

I have a single thread console application.

I am confused with the concept of CPU usage. Should a good single thread application use ~100% of cpu usage (since it is available) or it should not use lots of cpu usage (since it can cause the computer to slow down)?

I have done some research but haven’t found an answer to my confusion. I am a student and still learning so any feedback will be appreciated. Thanks.

Answer 1

It depends on what the program needs the CPU for. If it has to do a lot of work, it’s common to use all of one core for some period of time. If it spends most of its time waiting for input, it will naturally tend to use the CPU less frequently. I say “less frequently” instead of “less” because:

Single threaded programs are, at any given time, either running, or they’re not, so they are always using either 100% or 0% of one CPU core. Programs that appear to be only using 50% or 30% or whatever are actually just balancing periods of computational work with periods of waiting for input. Devices like hard drives are very slow compared to the CPU, so a program that’s reading a lot of data from disk will use less CPU resources than one that crunches lots of numbers.

It’s normal for a program to use 100% of the CPU sometimes, often even for a long time, but it’s not polite to use it if you don’t need it (i.e. busylooping). Such behavior crowds out other programs that could be using the CPU.

The same goes with the hard drive. People forget that the hard drive is a finite resource too, mostly because the task manager doesn’t have a hard drive usage by percentage. It’s difficult to gauge hard drive usage as a percentage of the total since disk accesses don’t have a fixed speed, unlike the processor. However, it takes much longer to move 1GB of data on disk than it does to use the CPU to move 1GB of data in memory, and the performance impacts of HDD hogging are as bad or worse than those of CPU hogging (they tend to slow your system to a crawl without looking like any CPU usage is going on. You have probably seen it before)

Chances are that any small academic programs you write at first will use all of one core for a short period of time, and then wait. Simple stuff like prompting for a number at the command prompt is the waiting part, and doing whatever operation ad academia on it afterwards is the active part.