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Re: Accelerating a one-line program doing matrix multiplication [message #72663 is a reply to message #72662] Wed, 29 September 2010 03:22 Go to previous messageGo to previous message
Axel Martínez is currently offline  Axel Martínez
Messages: 22
Registered: June 2010
Junior Member
On Sep 29, 8:57 am, chris <rog...@googlemail.com> wrote:
> On 28 Sep., 15:10, nata <bernat.puigdomen...@gmail.com> wrote:
>
>
>
>> Concatenation is a very slow action in IDL and, if you are copying
>> memory, the time of computation increases...
>> If v0, v1, v2 and v3 are each of them 3-element vectors then you will
>> not see the difference. TEMPORARY function is great when you are
>> copying large arrays. I think you can not improve your code because
>> the problem is the matrix multiplication and you can not change that.
>> Try putting timers to see what's the time to compute each instruction.
>
>> tt=SYSTIME(/SEC)
>> aux=[[v1],[v2],[v3]]
>> PRINT, SYSTIME(/SEc)-tt
>
>> tt=SYSTIME(/SEC)
>> aux=aux # vc
>> PRINT, SYSTIME(/SEC)-tt
>
>> etc.
>
>> Cheers,
>> nata
>
>> On Sep 28, 2:17 am, Axel M <axe...@gmail.com> wrote:
>
>>> On 27 Sep., 15:31, nata <bernat.puigdomen...@gmail.com> wrote:
>
>>>> You can use the TEMPORARY function if you can set the input to
>>>> undefined...
>>>> When you do [[v1],[v2],[v3]] you are duplicating data. v1, v2 and v3
>>>> are copied and you are not conserving memory.
>
>>>> You could try:
>>>> RETURN, [[TEMPORARY(v1)],[TEMPORARY(v2)],[TEMPORARY(v3)]] # vc +
>>>> REBIN(v0, SIZE(vc, /DIMENSIONS))
>
>>>> Cheers,
>>>> nata
>
>>> Thanks nata.
>
>>> v0, v1, v2 and v3 are each of them 3-element vectors. I can add that
>>> but, as I understand it, it will only save the place of 12 floating
>>> values in memory (48 bytes?).
>
>>> But I am happy that you did not see any other obvious thing. I started
>>> feeling depressed seeing that I am not being able to improve this
>>> single line of code... maybe it is ok, and the whole thing is just
>>> slow...? ahh.
>
> Yes, and you can substitute some of your calls:
>
> aux #= vc (makes no copy of aux as far as i know)
> invert -> la_invert (much much speedier)
> sometimes (replicate({temp:input},newsize)).(0) is faster then rebin
> exchange ## with matrix_multiply
>
> Cheers
>
> CR

Thanks a lot for all suggestions!

I have tried the different ideas. Unfortunately no improvements. Not
even the /SAMPLE in the rebin call helped, which I thought could make
a difference.

So I guess the matrix multiplication is just too computationally
intensive to get fast results.

What I also thought is that the method is fast enough when only a few
points are given, but very time-consuming when "vc" contains all
coordinates of the images, and they are sorted like [[0,0,0], [0,0,1],
[0, 0, 2], [0, 1, 0], ... [2, 2, 1], [2, 2, 2]]. Maybe there is some
way to use this property of vc to accelerate it. Like multiplying the
v1, v2, v3 by INDGEN and then somehow adding them smartly... I will
give it a try.

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