Re: emacs and large-address awareness under recent snapshots

[Ken Brown <kbrown at cornell dot edu>]

On 8/7/2011 6:38 PM, Ken Brown wrote:
> I've built cygwin1.dll from the latest CVS and confirmed that the
> problem is fixed.  Unfortunately, I've just discovered a second problem,
> also starting with the 2011-07-21 snapshot, that only shows up when I
> try to start emacs under X (with emacs large address aware).  What
> happens here is that emacs keeps using more and more CPU (as shown by
> Windows Task Manager), but the emacs window never opens.  To reproduce,
> install emacs-X11 and then do the following:
>
> 1. $ peflags --bigaddr=1 /usr/bin/emacs-X11.exe
>
> 2. Start the X server.  (I use the Start Menu shortcut.)
>
> 3. Start emacs from an xterm window:
>
>      $ emacs -Q&
>
> As a slight variation on this, you can instead start emacs with the -nw
> option:
>
>      $ emacs -nw -Q
>
> This tells emacs to use the xterm window for display rather than opening
> its own window.  The result this time is that you can see the emacs
> display, but emacs is unresponsive while the CPU usage increases as before.

I attached gdb to the running process and got some more information.  It 
turns out that this has nothing to do with X.  It's just that starting 
emacs under X causes emacs to try to allocate memory, and this makes the 
problem show up very quickly.

It looks to me like emacs gets stuck in morecore_nolock() and/or 
_malloc_internal_nolock(), which are defined in src/gmalloc.c. 
Apparently, emacs has a peculiar way of managing memory on Cygwin, and 
this chokes on the changes to the heap start address as of 2011-07-21. 
I don't know enough programming to fix this.  If anyone wants to try, 
the relevant source files to look at are gmalloc.c, sheap.c, and 
unexcw.c.  The second and third are compiled only in the Cygwin build, 
and the first also has some Cygwin-specific stuff.

Maybe I should take this to the emacs-devel list at some point, but I'll 
wait a while to see if someone on this list can help.

Ken

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