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Re: Today's IDL Lesson [message #62009 is a reply to message #61926]

Fri, 15 August 2008 02:05

Allan Whiteford
Messages: 117
Registered: June 2006

Senior Member

David Fanning wrote:
> Kenneth P. Bowman writes:
>
>
>> Shouldn't that be "increases linearly with the image size and
>> number of invocations of WHERE"?
>
>
> I don't think so. A 1500x1500 array took about 3.2 sec.
> a 2500x2500 array took about 35 sec. Just eyeballing it,
> it doesn't look linear to me. :-)
>
> Cheers,
>
> David

This scared me!

So, I wrote the following:

pro profile_where

max=26
time=fltarr(max)
sizes=(1+2*(findgen(max) mod 3))*10l^float(indgen(max)/3)

for i=0l,max-1 do begin
array=findgen(sizes[i])
time[i]=call_external('libidl.so','clock')
idx=where(array lt 3)
time[i]=call_external('libidl.so','clock') -time[i]
end

plot,sizes,time/1e6,xtitle='Number of elements',ytitle='Time in s'
oplot,sizes,time/1e6,psym=2

; or...

plot,alog10(sizes),alog10(time/1e6),xrange=[5,10]
oplot,alog10(sizes),alog10(time/1e6),xrange=[5,10],psym=2

end

and I get something which looks basically linear. The maximum array size
were chosen so that I wasn't hitting swap space and the where()
criterion so that each iteration returned the same number of results
(hence allocated the same amount of memory).

I have no doubt that in real world applications we get non-linear
scaling due to hitting swap or various other reasons but I think the
above shows that the basic usage of where() has linear scaling.

I hope this is as much a relief to others as it is to me.

Thanks,

Allan

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[Message index]

		Today's IDL Lesson By: David Fanning on Thu, 14 August 2008 12:05
		Re: Today's IDL Lesson By: David Fanning on Fri, 15 August 2008 05:11
		Re: Today's IDL Lesson By: Allan Whiteford on Fri, 15 August 2008 02:05
		Re: Today's IDL Lesson By: David Fanning on Thu, 14 August 2008 19:12
		Re: Today's IDL Lesson By: Kenneth P. Bowman on Thu, 14 August 2008 12:41
		Re: Today's IDL Lesson By: MarioIncandenza on Wed, 20 August 2008 10:01

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