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Re: FOR loops and efficiency [message #66609 is a reply to message #66568]

Sat, 23 May 2009 13:24

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

On May 22, 2:24 pm, Christopher Thom <ct...@oddjob.uchicago.edu>
wrote:
> Quoth Craig Markwardt:
>
>> A FOR loop will only be slow(er) when the time spent executing the
>> loop overhead is much more than the time spent doing the computations
>> in one loop iteration. A simple test would be to execute a dummy
>> loop:
>> NMAX = 100000L
>> FOR I = 0L, NMAX do begin & dummy = 1
>> Keep raising the value of NMAX until the execute time of the loop is
>> perceptible. Don't bother trying to optimize loops smaller than this.
>
>> In your case, you are only doing ten iterations, and each iteration
>> does a lot of work, so you won't gain by removing the loop.
>
> I've heard this description about FOR loops a lot, but one general
> question I've never been able to answer is, "how do i know when my loops
> are doing enough work?". How do I know when my loop overhead is a large
> fraction of the time spent on an iteration?
>
> I guess the real underlying question here is recognising when to optimise,
> and when to simply move on to more important things. Does anyone have any
> rules of thumb to help guide this recognition?

I still stand by my rule of thumb. The problem with FOR loops is the
amount of time spent doing loop overhead stuff. If you run your loop
but *take all the calculations out*, and the total execution time is
not perceptible, then you probably won't gain by optimizing/
vectorizing.

Craig

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[Message index]

		FOR loops and efficiency By: Rachel on Thu, 21 May 2009 09:04
		Re: FOR loops and efficiency By: JDS on Tue, 26 May 2009 14:51
		Re: FOR loops and efficiency By: Craig Markwardt on Wed, 27 May 2009 22:38
		Re: FOR loops and efficiency By: Craig Markwardt on Sat, 23 May 2009 13:24
		Re: FOR loops and efficiency By: JDS on Fri, 29 May 2009 13:39
		Re: For loop By: Hassan on Tue, 27 October 2009 01:16
		Re: For loop By: Michael Galloy on Mon, 26 October 2009 09:47
		Re: For loop By: Hassan on Mon, 26 October 2009 08:45
		Re: For loop By: penteado on Mon, 26 October 2009 08:01

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