Re: [gl4java-usergroup] GC Bug

[Kenneth B. R. <kbr...@al...>]

> While using the GLU tesselator with 700+ points, I encountered very 
> peculiar behavior. Above a certain amount of points, the end result of 
> the tesselation would be completely messed up. After reading the 
> comments in the Nvidia/DirectBuffer demo about the GC bug, I took a look 
> at the native code that handles a gluTessVertex call and found that no 
> copy is made of the data that is passed to this function. The OpenGL 
> manual states that a call to gluTessVertex only keeps a pointer to the 
> data array and does not make a copy itself. I assume that what's 
> happening in my program is that the garbage collector is moving around 
> (and probably even freeing) some of the coordinate arrays, because they 
> are neither pinned or copied. Is this assumption correct?

It's hard to tell what the GLU tesselator is doing under the hood
from looking at its man pages. If it is holding a pointer to the
passed GLdouble* then you could be correct. An easier way to
determine whether this is the problem is to run with -XX:+PrintGC
(HotSpot only) or -verbose:gc (any JVM, I think). If you see GC
messages at the times when the vetices are corrupted then this is
the problem. Otherwise it could be a bug in your application or 
possibly in GL4Java.

> If it is correct will this be fixed any time soon? I know that
> the problem can be resolved by using direct buffers, but i
> would like to have pre 1.4 compatibility.

There is no good solution for this kind of problem on earlier
JDKs. Alternatives include performing a data copy via JNI (which
requires an associated free somewhere, which is sometimes hard to
figure out when to do, or having a static data buffer, which
requires synchronization) or performing pinning, which is
complicated or impossible depending on the GC algorithm.