CODEWITHSUNDEEP
c
suvirai
suvirai

Reputation: 89

Why time keeps on varying?

I executed one program and when I calculated the time elapsed I found that time is not constant. It's varying under some range. I wanted to know why is it so?

Upvotes: 2

Views: 166

Answers (2)

paxdiablo
paxdiablo

Reputation: 881653

You often find this sort of behaviour when you measure elapsed times. That's because elapsed times depends on all of the other things that your computer may be doing.

See for example:

pax> time sleep 1
real  0m1.012s
user  0m0.004s
sys   0m0.000s

pax> time sleep 1
real  0m1.002s
user  0m0.004s
sys   0m0.000s

pax> time sleep 1
real  0m1.007s
user  0m0.004s
sys   0m0.000s

In all those cases, the elapsed real time varies but the actual use of the processor, user+sys is remarkably consistent.

For timing, you should use the most accurate measurement you can (such as user+sys) to remove external influences. You should also use statistical techniques to get a more accurate picture.

For example, I tend to do twelve runs, throw away the outliers (fastest and slowest), then average the remaining ten.

Upvotes: 2

NullUserException
NullUserException

Reputation: 85468

Because there is a number of processes running concurrently with your application.

Even if you managed to isolate your application completely, there are no guarantees that the same code will run at the same speed every time. That's why you should really on averages over multiple runs if you are testing performance (assuming that's what you are doing here).

If you are measuring efficiency, there are more objective/formal ways of defining it:
See: Big-O notation

Upvotes: 6

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