If you get the error message ‘Virtual memory exceeded’, save
your modified buffers with C-x s (save-some-buffers).
This method of saving them has the smallest need for additional
memory. Emacs keeps a reserve of memory which it makes available when
this error happens; that should be enough to enable C-x s to
complete its work. When the reserve has been used, ‘!MEM FULL!’
appears at the beginning of the mode line, indicating there is no more
reserve.
Once you have saved your modified buffers, you can exit this Emacs session and start another, or you can use M-x kill-some-buffers to free space in the current Emacs job. If this frees up sufficient space, Emacs will refill its memory reserve, and ‘!MEM FULL!’ will disappear from the mode line. That means you can safely go on editing in the same Emacs session.
Do not use M-x buffer-menu to save or kill buffers when you run out of memory, because the Buffer Menu needs a fair amount of memory itself, and the reserve supply may not be enough.
On GNU/Linux systems, Emacs does not normally get notified about
out-of-memory situations; instead, the OS can kill the Emacs process
when it runs out of memory. This feature is known as the
out-of-memory killer, or OOM killer. When this
behavior is in effect, Emacs is unable to detect the out-of-memory
situation in time, and won’t be able to let you save your buffer as
described above. However, it is possible to turn off this behavior of
the OS, and thus allow Emacs a chance to handle the out-of-memory
situation in a more useful manner, before it is killed. To do that,
become the super user, edit the file /etc/sysctl.conf to
contain the lines shown below, and then invoke the command
sysctl -p from the shell prompt:
vm.overcommit_memory=2 vm.overcommit_ratio=0
Please note that the above setting affects all the processes on the system, and in general the behavior of the system under memory pressure, not just the Emacs process alone.