POV-Ray : Newsgroups : povray.beta-test : v3.8+ crackle instability (facets?) with >1 uses per thread. : v3.8+ crackle instability (facets?) with >1 uses per thread. Server Time
18 Jul 2024 06:37:50 EDT (-0400)
  v3.8+ crackle instability (facets?) with >1 uses per thread.  
From: William F Pokorny
Date: 8 Jun 2024 15:12:53
Message: <6664ad35@news.povray.org>
I've been playing with 'facets' and 'crackle' of late. I've turned up a 
bug (or two) (*).

Documenting now - partly so I can think through what I'm seeing as I write.

The crackle pattern and facets perturbation maintain thread local 
storage so information can be cached in a thread safe way.

The issue, I think, is that that storage is set up to work with one 
crackle and/or facets use per thread and no more.

Once we run >1 of either in the same thread they share the thread local 
storage. This >1 usage per thread happens, for example, when we layer 
textures both based upon crackle.

See the two attached scene files which result in images like those 
attached when things go wrong. (Using v3.7 beta 2 and the not yet 
released yuqk R15 for the renders)

1) Things don't always go wrong. The problem is flaky. Scene renders 
with artifacts often later run cleanly and visa versa. The two scene 
files attached are good at having problems.

2) The v38 scene uses 'repeat <>' and the yuqk one 'ip_strength <>' - 
which twiddles with the strength of the noise used to push the point per 
cube around inside the cube for the pseudo random-ish point set. My 
guess at the moment is these features turn over the thread crackle cache 
more often.

3) I've only seen the buggy results when the text output reports less 
than 100% cache hits. For example:

   Crackle Cache Queries:          960000
   Crackle Cache Hits:             888124 ( 93 percent)

4) The problem seems slightly worse - with more of a render block 
signature - when multiple threads are used. This doesn't really line up
with my best guess as to the issue! At the moment I think this probably 
a secondary bug where maybe the cache is supposed to be cleared at 
render block end, but it isn't, or similar. Might be a secondary thread 
safety issue too.

5) I've not looked at the v3.7 code as yet for these issue(s).

I know. The yuqk fork's tearing result looks kinda cool - wish it 
reflected intent... :-)

Bill P.

(*) - There are a couple other minor bugs too in crackle with offset and 
<=0 metric settings patched / fixed in the yuqk fork.

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Download 'crackle2_v38.pov.txt' (2 KB) Download 'crackle2_v38_00.jpg' (224 KB) Download 'crackle2_yuqk.pov.txt' (2 KB) Download 'crackle2_yuqk2.jpg' (315 KB)

Preview of image 'crackle2_v38_00.jpg'

Preview of image 'crackle2_yuqk2.jpg'


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