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digitalmars.D.bugs - [Issue 7095] New: GC doesn't return or reuse unused memory [D2.056]

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7095

           Summary: GC doesn't return or reuse unused memory [D2.056]
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Mac OS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: druntime
        AssignedTo: nobody puremagic.com
        ReportedBy: soul8o8 gmail.com


--- Comment #0 from Heywood Floyd <soul8o8 gmail.com> 2011-12-11 08:59:32 PST
---
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
$ _

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next sibling parent d-bugmail puremagic.com writes:
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David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |dsimcha yahoo.com
         Resolution|                            |INVALID


--- Comment #1 from David Simcha <dsimcha yahoo.com> 2011-12-11 09:47:56 PST ---
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.

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--- Comment #2 from Heywood Floyd <soul8o8 gmail.com> 2011-12-11 10:01:25 PST
---
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?)

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--- Comment #3 from David Simcha <dsimcha yahoo.com> 2011-12-11 10:12:27 PST ---
(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.

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deadalnix <deadalnix gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |deadalnix gmail.com


--- Comment #4 from deadalnix <deadalnix gmail.com> 2011-12-11 10:27:18 PST ---
(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 ?

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--- Comment #5 from David Simcha <dsimcha yahoo.com> 2011-12-11 10:38:41 PST ---
(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.

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