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Parallel-for woes


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#1 SeraphLance   Members   -  Reputation: 1285

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Posted 04 September 2012 - 11:54 PM

So for quite some time I'd been writing a noise/fractal library, and today I decided to revisit it after a many-months break. The first thing I realized was that Perlin noise generation, being a pure function, ought to be embarrassingly parallel. Not having any experience whatsoever in parallelism, I get some weird results I'm having trouble fixing:

Posted Image

The artifacts in the left image are randomly placed each run, and with varying degrees of severity.

The code behind my for-loop is pretty straightforward:
		Color[] noisegen(Racket.Module basis)
		{
			Color[] ret = new Color[col.Length];
			int colx, coly;
			//sequential
			float d;
			for (int i = 0; i < col.Length; i++)
			{
				colx = i % img.Width;
				coly = i / img.Width;
				d = (float)(basis.Get((colx + xtrans) / (double)img.Width, (coly + ytrans) / (double)img.Width));
				ret[i] = new Color(d, d, d);
			}
			//end-sequential
			//Parallel.For(0, col.Length, (i) =>
			//{
			//		float d;
			//		colx = i % img.Width;
			//		coly = i / img.Width;
			//		d = (float)(basis.Get((colx + xtrans) / (double)img.Width, (coly + ytrans) / (double)img.Width));
			//		ret[i] = new Color(d, d, d);
			//});
			return ret;
		}

I'm not going to post the actual perlin noise function (noted here as basis.Get(...) ) for space economy reasons, but I'm pretty sure it's pure functional. It's pretty much a facelift of Perlin's "improved" 2002 implementation.

I really have no idea what's going on when I use my parallel implementation instead of my sequential one. Is it that the function is returning before some of the threads finish execution? Are parallel for loops not guaranteed to evaluate everything? I feel like I'm missing something very simple.

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#2 Telastyn   Crossbones+   -  Reputation: 3726

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Posted 05 September 2012 - 08:36 AM

It's hard to see the artifacts in the screenshot.

There doesn't look to be any issues here. My first guess would be some issue due to interaction with GDI/forms.

#3 6677   Members   -  Reputation: 1058

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Posted 05 September 2012 - 10:00 AM

I'd say approx 1/8th of the way across the screen from the right edge half way from the top theres a little white dot.

#4 SeraphLance   Members   -  Reputation: 1285

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Posted 05 September 2012 - 11:10 AM

I scaled down the image to avoid covering up the entire forum post. There's like 50 white pixels, mostly located in the top right. There's no real bias for where they are, it only happens to be that way in that particular image. It seems visible to me, even on this crappy laptop, but I'm the one that uploaded it after all. If you want a larger side-by-side, I can upload it tonight.

EDIT: I should add it becomes a lot more noticeable if you click to enlarge it.

Edited by SeraphLance, 05 September 2012 - 11:10 AM.


#5 Telastyn   Crossbones+   -  Reputation: 3726

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Posted 05 September 2012 - 11:25 AM

I saw some of that, but regardless there's nothing really in the code you posted that would influence that.

[edit: dmatter is more observant than I]

Edited by Telastyn, 05 September 2012 - 12:31 PM.


#6 dmatter   Crossbones+   -  Reputation: 2982

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Posted 05 September 2012 - 11:27 AM

Looks like you're sharing colx and coly between all the threads, creating a race condition. Try making those variables local within the lambda.

#7 SeraphLance   Members   -  Reputation: 1285

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Posted 05 September 2012 - 12:30 PM

Looks like you're sharing colx and coly between all the threads, creating a race condition. Try making those variables local within the lambda.

That's exactly the kind of thing I figured it'd be. I already fixed that with the d float, but hadn't noticed colx and coly. I'll fix those tonight and see if that solves the problem. Thanks.

EDIT: That fixed the problem. I'd figured it was related to some shared data, but I was having trouble finding it. Thanks again for the help!

Edited by SeraphLance, 05 September 2012 - 06:58 PM.


#8 6677   Members   -  Reputation: 1058

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Posted 06 September 2012 - 10:08 AM

There's like 50 white pixels, mostly located in the top right.

I see them now, seems to be a mix of my zoom level and screen res they almost blended in, zooming them to be completely fullscreened I see what you mean. Good that you've fixed it though




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