libgcj and incremental gc

Wed Mar 23 19:05:00 GMT 2005

Boehm, Hans writes:
 > Thanks for trying this. This was on X86? I wonder if the trampolines
 > just lose execute permission?
I don't think so. What seems to happen is that when I jump to a
trampoline, it has turned into a string. It could be, of course, that
some insane piece of code has written a string to the wrong place in
memory, but I reckon that it's a good guess the trampoline has been
reclaimed.
 > I would not in general turn this on by default. But it might be a
 > useful option. The vague plan I had was to add some mechanism for
 > turning this on only if both:
 > 
 > - the user asks, and
 > - the page size is equal to HBLKSIZE
 > 
 > It may be that the GC_ENABLE_INCREMENTAL environment variable should
 > acquire this meaning. It currently doesn't check the second
 > condition.
 > 
 > The second condition is necessary to ensure that pointer-free objects
 > are never write protected.
Yes, I can understand that one.
 > I think the hard part in making this work is that we would have to
 > ensure that system calls only write to pointer-free objects. I
 > think that's hard only because I'm not sure there's an easy way to
 > find and check all of them.
I can't think of any case in Java where a system call writing to a
Java object that contains pointers would not be a bug.
My interest here is to get some proper numbers about performance
issues.
Andrew.