[maemo-developers] [maemo-developers] Speed test with vfp (floating point) on N800

[From: Visti Andresen]

On Sun, 11 Feb 2007 21:55:52 +0100
Klaus Rotter <klaus at rotters.de> wrote:

> Hi folks,
> 
> I was very interesseted in the floating point copro in the N800 cpu and 
> did some tests with a progamm which calculates the mandelbrod set and 
> outputs it via SDL. I put this program online on 
> bomberman.garage.maemo.org. No installer, you have to run the benchmarks 
> on xterm or via ssh (so did I).

It might interest you to know that I have an integer implementation of the mandelbrot set.
It's not quite finished yet but, so its only available though subversion from the N770Demos project

> 
> // Results of Mandelbrot set on N800 800x480 pixels, 100 interations
> // real -1 to 2
> // imag	-1.3 to 1.2
> 
> // dbl - uses doubles
> // flt - uses floats
> // vfp - compiled with "-mfpu=vfp -mfloat-abi=softfp"
> 
> // All binarys are compiled with -O2
> 
> // Results with full PixelDraw and SDL_Update every pixel
> ./mandel_armel_dbl.bin - Time: 178.284 seconds
> ./mandel_armel_dbl_vfp.bin - Time: 151.816 seconds
> ./mandel_armel_flt.bin - Time: 169.486 seconds
> ./mandel_armel_flt_vfp.bin - Time: 152.148 seconds
> 
> // Results with full PixelDraw and _NO_ SDL_Update.
> ./mandel_armel_dbl.bin - Time: 26.377 seconds
> ./mandel_armel_dbl_vfp.bin - Time: 1.808 seconds
> ./mandel_armel_flt.bin - Time: 19.813 seconds
> ./mandel_armel_flt_vfp.bin - Time: 1.709 seconds
> 
> // Results without any Drawing
> ./mandel_armel_dbl.bin - Time: 26.525 seconds
> ./mandel_armel_dbl_vfp.bin - Time: 1.672 seconds
> ./mandel_armel_flt.bin - Time: 19.647 seconds
> ./mandel_armel_flt_vfp.bin - Time: 1.601 seconds
> 
> // Results with full PixelDraw and SDL_Update only every column
> ./mandel_armel_dbl.bin - Time: 27.512 seconds
> ./mandel_armel_dbl_vfp.bin - Time: 2.447 seconds
> ./mandel_armel_flt.bin - Time: 20.689 seconds
> ./mandel_armel_flt_vfp.bin - Time: 2.451 seconds
> 
> What could that mean? First, SDL_Update is _very_ expensive. In the 
> first try, I call SDL_Update just for every Pixel 
> (SDL_Update(screen,x,y,1,1) and it slows down execution about six times, 
> the vfp about 80 times. So I think there must be a kind of sync inside 
> the SDL_Update function. If I update only every column, the speed loss 
> is only about 80%.

When you do an update or flip the surface data is transferred to video memory (a slow and painful operation)
This is also true even if SDL tells you that you got a HW surface. 
Unfortunately there are no sync, to the vertical blank...
At least so have I been told. 

> 
> Vfp float can give a speed increase about factor 10 and more. And there 
> is not much difference between vfp float and double.

Humm the N800 even has a floating point unit... and its quite fast.
I would say roughly as fast as an 770 using a long for the math.
Only timed by counting aloud, and with a different setup -1, 1 and -1, 1 with 256 iterations.
(The current subversion code uses long long math for that extra resolution, and it costs quite some speed :)

> 
> -Klaus
> 
> -- 
> Klaus Rotter * klaus <at> rotters <dot> de * www.rotters.de
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