The garbage collector refuses to collect garbage. MacOS X 10.7.2, DMD v2.056 import core.memory; void main(){ long[] arr; foreach(i;0..100){ arr = new long[10_000_000]; GC.collect(); GC.minimize(); } } $ rdmd gcrun.d core.exception.OutOfMemoryError $ _
D's GC isn't fully precise yet. In other words, it can't always tell the difference between a pointer and a non-pointer. When you allocate an 80-megabyte array, the probability that some non-pointer will look like it points into the array if interpreted as a pointer is pretty high. This is a fundamental limitation of the current garbage collector design, rather than a bug. As a workaround, it's best to use C's malloc for very large arrays.
I understand. Thanks for the reply! (With risk of polluting the bug tracker here, but perhaps to clarify the workaround: Could GC.malloc/free be used as well? Also, I assume if a large array of structs that in turn contain references to other heap objects, those would not get scanned had the array been C-alloced risking active objects to get collected?)
(In reply to comment #2) > I understand. Thanks for the reply! > > (With risk of polluting the bug tracker here, but perhaps to clarify the > workaround: Could GC.malloc/free be used as well? You could use GC.free(). It's somewhat dangerous, though, because you risk corrupting the GC if you free something incorrectly. > Also, I assume if a large > array of structs that in turn contain references to other heap objects, those > would not get scanned had the array been C-alloced risking active objects to > get collected?) Use GC.addRange() to get around this.
(In reply to comment #1) > D's GC isn't fully precise yet. In other words, it can't always tell the > difference between a pointer and a non-pointer. When you allocate an > 80-megabyte array, the probability that some non-pointer will look like it > points into the array if interpreted as a pointer is pretty high. > > This is a fundamental limitation of the current garbage collector design, > rather than a bug. As a workaround, it's best to use C's malloc for very large > arrays. Isn't GC has a flag that says if something may contains pointer or not ? Why this flag is set when allocation an array of long ?
(In reply to comment #4) > Isn't GC has a flag that says if something may contains pointer or not ? > > Why this flag is set when allocation an array of long ? It's not. If a bit pattern in `arr` would point into GC memory if interpreted as a pointer, then the pattern is ignored. The GC has some level of precision in that it knows whether a heap-allocated block contains any pointers or not. However, the issue is that pointers **from** somewhere else point **at** arr, keeping it alive incorrectly. There are two sources for false pointers: 1. The stack, where the GC has no knowledge whatsoever of what's a pointer and what isn't. I think this also applies to the static data segment. 2. Heap allocated data structures that contain both pointer and non-pointer data, e.g.: struct Foo { void* ptr; size_t num; } If you allocate a Foo on the heap, the whole block will be marked as potentially containing pointers. The GC won't know what parts of it aren't pointers. `num` could have a bit pattern that would be a valid pointer and will keep some objects alive unnecessarily.