emacs and large-address awareness under recent snapshots

Corinna Vinschen corinna-cygwin@cygwin.com
Sun Aug 7 11:34:00 GMT 2011

On Aug  5 19:16, Ken Brown wrote:
> Starting with the 2011-07-21 snapshot, emacs doesn't work well with
> the large-address-awareness flag set (under 64-bit Win7).  As soon
> as emacs is started, a *Warning* buffer is created with the
> following message:
>   Emergency (alloc): Warning: past 95% of memory limit
> To reproduce, install emacs and do the following:
> $ peflags --bigaddr=1 /usr/bin/emacs-nox.exe
> $ emacs-nox.exe -Q

Yes, I can reproduce the message, but I have not the faintest idea
why emacs thinks so.  If you look into the process map, you'll
see the following:

  $ ps | grep emacs
     280    2852     280       2796    0 11001 13:02:21 /usr/bin/emacs-nox
  $ less /proc/280/maps
  80000000-8064E000 rw-p 00000000 0000:0000 0                   [heap]
  8064E000-98000000 ===p 0064E000 0000:0000 0                   [heap]

Starting with the 2011-07-21 the heap starts at 0x80000000 if the
application (and the system) is large address aware.  Even if you
dont see the "[heap]" decoration(*), the heap is at that address.
What you can see is this:

- The heap is located at 0x80000000 and has a size of 384 Megs (the
  default start size), up to address 0x98000000.

- Only the first 0x64e000 (== 6610944) bytes are allocated so far, so
  there are still about 254 Megs left on the heap.

I did set breakpoints to all functions returning malloc information,
but emacs doesn't call one of them.  Is there a chance that emacs
does some invalid 32 bit pointer arithmetic and just gets confused?


(*) I just checked in a patch to Cygwin which fixes printing the
    "[heap]" text in the right column.  In your case there's a good
    chance that it's missing.

Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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