POV-Ray : Newsgroups : povray.unofficial.patches : jr's large csv and dictionary segfault in the povr fork. : jr's large csv and dictionary segfault in the povr fork. Server Time
27 Feb 2024 19:00:36 EST (-0500)
  jr's large csv and dictionary segfault in the povr fork.  
From: William F Pokorny
Date: 26 Mar 2023 02:46:56
Message: <641fea60$1@news.povray.org>
A status update.

---
First, let me put on the table the segfault which happens here should 
not be a segfault during a non-debug run! Someone had already added code 
comments in the cylinder and code code to this effect.

It's good fortune - of sorts - we get the segfaults in povr, but it's 
only happening because no POV-Ray source coder got around to changing 
that throw into a warning or possible error message. ;-)

---
I've had no luck finding the core problem.

The segfault comes and goes depending seemingly on a lot of things - 
which makes me wonder if it isn't sitting there in official POV-Ray 
releases too and we've just not tripped it.

The really odd fails seen, I've been unable to reproduce even once. At 
the moment I'm chalking those up to my fatigue and potential 
configuration and set up issues as I tried to run through just parts of 
the animation.

---
I have now a changed foreach.inc file where I've added checks for 
duplicate macro to be executed strings and duplicate i_ indexes which 
has been working for me without fail for a while!

I suspect it's because I've slowed an whole animation down to almost 
exactly the time it take p380 beta 2 to do it - and I've never seen a 
fail there either.

The code changed is now:

#macro fore_voidRun(a_)
   #for (i_, 0, dict_.ttl_ - 1)

     // After this in, not seen a fail myself.
     #ifndef (Last_i_)
         #declare Last_i_ = i_;
     #else
         #if (Last_i_=i_)
             #local DummyID = f_boom(9,8,7,6,5,4);
         #end
         #declare Last_i_ = i_;
     #end

     #if (i_)
       fore_cmdNext(dict_)
     #end
     #local cmd_ = fore_cmdStr();

     // Below code found a duplicate once - and only once.
     #ifndef (LastCmdStr)
         #declare LastCmdStr=cmd_
     #else
         #if (strcmp(LastCmdStr,cmd_)=0)
             #local DummyID = f_boom(1,2,3,4,5,6);
         #end
         #declare LastCmdStr=cmd_
     #end

     fore_exec(cmd_,"parse_fore_void.tmp")
     #if (fore_debug)
       #debug concat("called '",cmd_,"'.\n")
     #end
   #end
#end

So it feels like a race issue of some kind, but I cannot see how it can 
even come about... Each frame is indeed a new parser thread, but it 
should be nothing from the prior frame's thread carries forward. I 
thought for a while it was file I/O issue, but one fail on the macro 
string checking suggests it does have something to do with the 
dictionaries or the dictionary set up as you, jr, suspected.

---
On the animation itself. We are basically re-building the scene for each 
frame of the animation with early frames having very few objects and the 
last ones almost 9000. A curiosity I don't understand is the 
usaf__mkItems(4130,a_[4130],tmp_) package indexing goes well to values 
over 10000 on occasion. These often peak at those levels and then are 
not seen again - maybe these all come from the partial runs where the 
animation re-starts? Done so many of these things my head is mush on 
what's what.

Anyway, the who code set up, gets slower and slower as we get into the 
later frames. Some is because the parsing just gets big/long, but it's 
also because the dictionary gets slower. The dictionary is built on top 
of the parser symbol table mechanism and, while I've not dug to be 
certain, my bet is some of the chains hanging off the hash table entries 
are getting long and slow.

I don't know enough about what is changing in the animation to suggest 
it, but, if there are larger fixed portions on which we are building and 
building, it would almost certainly be faster to keep those fixed parts 
as larger include files we bring in all at once - as raw SDL.

Suppose the suggestion neither here or there with respect to the problem 
- which is certainly real.

I'll keep playing with it as I have time - and I feel like it - but 
given that even starting at say frame 3000 isn't saving me all that much 
time for each animation turn - this is a very painful bug to chase.

My plan at the moment is to kick off the full hour plus long animation 
(tiny image sizes) with the added delay in place - as I think to do it. 
At days end perhaps.

It's not failed in a long while for me, but it's slow even at tiny image 
sizes so I haven't really run that many full passes.

If it doesn't fail again for me over time - the delay / race condition 
idea perhaps holds enough water for me to start putting in random locks 
as I can maybe see places to insert them. The trouble has been that I 
perturb things and the the problem blinks in and out for I do not know 
what cause - if any really my doing. I cannot come up with a solid way 
to come at this problem! It's too flaky - and it takes a painful amount 
of time to try anything.

Yes, I'm whining now and should stop. :-)

Bill P.


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