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digitalmars.D.bugs - [Issue 7251] New: GC not working

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7251

           Summary: GC not working
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Severity: regression
          Priority: P2
         Component: druntime
        AssignedTo: nobody puremagic.com
        ReportedBy: wfunction hotmail.com


--- Comment #0 from wfunction hotmail.com 2012-01-08 23:40:33 PST ---
On DMD 2.057, the code:


import core.memory, std.stdio;
extern(Windows) int GlobalMemoryStatusEx(ref MEMORYSTATUSEX lpBuffer);
struct MEMORYSTATUSEX
{
    uint Length, MemoryLoad;
    ulong TotalPhys, AvailPhys, TotalPageFile, AvailPageFile;
    ulong TotalVirtual, AvailVirtual, AvailExtendedVirtual;
}
void testA(size_t count)
{
    size_t[] a;
    foreach (i; 0 .. count)
        a ~= i;
}
void main()
{
    MEMORYSTATUSEX ms;
    ms.Length = ms.sizeof;
    foreach (i; 0 .. 32)
    {
        testA(16 << 20);
        GlobalMemoryStatusEx(ms);
        stderr.writefln("AvailPhys: %s MiB", ms.AvailPhys >>> 20);
    }
}



Gives output:

AvailPhys: 3711 MiB
AvailPhys: 3365 MiB
AvailPhys: 3061 MiB
AvailPhys: 2747 MiB
AvailPhys: 2458 MiB
core.exception.OutOfMemoryError

Obviously, the GC is non-functional...

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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> 2012-01-09 14:47:00 PST ---
This looks like false pointers.  testA allocates 48 MB for its largest array on
each run.  We really need precise heap scanning.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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wfunction hotmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |


--- Comment #2 from wfunction hotmail.com 2012-02-29 20:08:19 PST ---
Just wondering, why was this marked as "Resolved, Invalid" with no comment?

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
                 CC|                            |clugdbug yahoo.com.au
         Resolution|                            |INVALID


--- Comment #3 from Don <clugdbug yahoo.com.au> 2012-03-01 01:22:39 PST ---
(In reply to comment #2)

 Just wondering, why was this marked as "Resolved, Invalid" with no comment?


Huh? Comment #1 was added when it was closed. Reclosing.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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wfunction hotmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |REMIND


--- Comment #4 from wfunction hotmail.com 2012-03-01 02:01:33 PST ---
(In reply to comment #3)

 (In reply to comment #2)

 Just wondering, why was this marked as "Resolved, Invalid" with no comment?


 
 Huh? Comment #1 was added when it was closed. Reclosing.


Wait what?

"This looks like false pointers [...] We really need precise heap scanning."

Doesn't that mean this needs to be fixed? Or is one of us misunderstanding the
meaning of the above?

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--- Comment #5 from Don <clugdbug yahoo.com.au> 2012-03-01 03:39:55 PST ---
(In reply to comment #4)

 (In reply to comment #3)

 (In reply to comment #2)

 Just wondering, why was this marked as "Resolved, Invalid" with no comment?


 
 Huh? Comment #1 was added when it was closed. Reclosing.


 
 Wait what?
 
 "This looks like false pointers [...] We really need precise heap scanning."
 
 Doesn't that mean this needs to be fixed? Or is one of us misunderstanding the
 meaning of the above?


No, it's inevitable that a mark-and-sweep GC on a 32 bit system won't work if
large numbers of false pointers are present. And this code:

   foreach (i; 0 .. count)
        a ~= i;

is flooding the system with false pointers.

You marked this as regression, is there any previous compiler where this
worked?
(reopen if it there is).

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--- Comment #6 from wfunction hotmail.com 2012-03-01 14:53:44 PST ---
(In reply to comment #5)

 No, it's inevitable that a mark-and-sweep GC on a 32 bit system won't work if
 large numbers of false pointers are present. And this code:
 
    foreach (i; 0 .. count)
         a ~= i;
 
 is flooding the system with false pointers.


I think you might have misunderstood the problem, like I originally did.

I am **********  NOT  *********** filling the system with false pointers.
See David Simcha's comment in response to mine:
http://stackoverflow.com/a/8796226/541686

It's inherently defective -- I'm merely _allocating_ a lot of memory, that's
all. The rest is a problem with the GC's algorithm.
(Unless you expect that no one will allocate one 20-MB chunk of RAM?)


 
 You marked this as regression, is there any previous compiler where this
 worked?
 (reopen if it there is).


I marked it as "remind", since I'm sorta reminding about this bug... if that's
not the implied meaning then my bad, but this still needs to be fixed.

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Rob Jacques <sandford jhu.edu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P2                          |P4
                 CC|                            |sandford jhu.edu
           Platform|x86                         |All
         OS/Version|Windows                     |All
           Severity|regression                  |minor


--- Comment #7 from Rob Jacques <sandford jhu.edu> 2012-03-04 09:53:01 PST ---
(In reply to comment #6)

 (In reply to comment #5)

 No, it's inevitable that a mark-and-sweep GC on a 32 bit system won't work if
 large numbers of false pointers are present. And this code:
 
    foreach (i; 0 .. count)
         a ~= i;
 
 is flooding the system with false pointers.


 
 I think you might have misunderstood the problem, like I originally did.
 
 I am **********  NOT  *********** filling the system with false pointers.
 See David Simcha's comment in response to mine:
 http://stackoverflow.com/a/8796226/541686
 
 It's inherently defective -- I'm merely _allocating_ a lot of memory, that's
 all. The rest is a problem with the GC's algorithm.
 (Unless you expect that no one will allocate one 20-MB chunk of RAM?)


The static data segment and the stack generate _always_ contain false pointers.
So even if you're not generating them yourself much, they do exist and things
will get stuck. 

Also, direct use of ~= is discouraged; appender is recommended instead and
there  patch for appender that solves this issue.

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prev sibling parent d-bugmail puremagic.com writes:
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Leandro Lucarella <leandro.lucarella sociomantic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |leandro.lucarella sociomant
                   |                            |ic.com
         Resolution|REMIND                      |DUPLICATE


--- Comment #8 from Leandro Lucarella <leandro.lucarella sociomantic.com>
2013-05-30 04:54:25 PDT ---
I think is better to set this as a duplicate of #3463, as when that one gets
fixed, this one should be fixed too.

*** This issue has been marked as a duplicate of issue 3463 ***

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May 30 2013