emacs and large-address awareness under recent snapshots

Ken Brown kbrown@cornell.edu
Wed Aug 10 14:57:00 GMT 2011

On 8/9/2011 10:39 PM, Ryan Johnson wrote:
> On 09/08/2011 10:33 PM, Ken Brown wrote:
>> I submitted a bug report and may or may not get a useful response.
>> While waiting, I'd like to keep trying to figure out what the right
>> fix is.  Unless the dumping mechanism (unexec) is completely revamped,
>> we can't just ignore the static heap.  Some of it has already been
>> allocated by temacs and has to be taken into account by the memory
>> management scheme.  So when emacs starts up (as of 2011-07-21), the
>> heap is going to come in two pieces: the static heap in low memory and
>> the Cygwin-provided heap starting at 0x20000000 or 0x80000000.  I
>> can't think of any easy way of dealing with this, short of drastically
>> rewriting malloc.  Do you have any suggestions?
>> BTW, I don't necessarily have to use the malloc that comes with emacs.
>> I just verified that I can build emacs so that it uses Cygwin's
>> malloc.  I haven't done any testing yet to make sure there are no
>> glitches, but I think it will be OK.  Assuming this is the case, does
>> that simplify dealing with a heap that has two non-contiguous pieces?
> Given that the static heap is only 12MB, with most of that arguably
> occupied by stuff that isn't going away, what if we did "just ignore the
> static heap" (mostly)? Anything freed from that regionjust gets dropped
> on the floor and all new requests are served from the cygwin heap? I
> assume temacs stays away from the dynamic heap, since otherwise the dump
> would be corrupted.

I tried forcing malloc to reinitialize itself in emacs.c, and emacs 
crashed almost immediately.  A gdb backtrace showed that the memory got 
corrupted as soon as realloc got called on objects that were originally 
stored in the static heap.  After reinitialization, malloc had no 
knowledge of memory allocation in the static heap.


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