emacs and large-address awareness under recent snapshots

Ken Brown kbrown@cornell.edu
Wed Aug 10 02:33:00 GMT 2011

On 8/9/2011 2:21 PM, Ken Brown wrote:
> On 8/9/2011 11:21 AM, Corinna Vinschen wrote:
>> However, whatever you do, it will not really work.  Keep in mind that
>> the large address awareness only makes sense (and has any effect!) on
>> systems which provide a large address area.
>> To me the bottom line here is, that emacs is doing the wrong thing.
>> There are a couple of assumptions how a system maintains memory, which
>> are just not valid on all systems.  The malloc initialization and the
>> assignment of the heapbase (the first call to sbrk(0)) should happen
>> in emacs every time it starts.
> That makes sense to me.  I thought that was what I was accomplishing
> (for Cygwin) by setting __malloc_initialized to 0 before dumping.  I'm
> not sure why it didn't work.  In any case, the fix shouldn't be Cygwin
> specific.  It's probably time to report this as an emacs bug.

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?


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