Re: Question about memory usage

[Michał Kondraciuk]

On 04/03/2018 08:28 AM, Eli Zaretskii wrote:
> > From: Michał Kondraciuk <k.michal@zoho.com>
> > Date: Mon, 2 Apr 2018 13:57:26 +0200
> >
> > Basically, when I run the sexp below in emacs -Q, Emacs keeps allocating
> > a lot of memory. In 10 minutes, it goes from 18 MB to over 200 MB.
> >
> > (while t
> >     (with-temp-buffer
> >       (setq buffer-undo-list nil)
> >       (insert "a")))
> >
> >
> > Calling garbage-collect afterwards or even inside the body of the loop
> > doesn't help (except the loop obviously runs slower, so after 10
> > minutes, Emacs uses ~100 MB of memory).
>
> What do you mean by "afterwards"?  The while-loop never ends, so
> there's no "afterwards" AFAIU.  Am I missing something?

You're right, I meant after I stop the loop with C-g.

> To answer your question: yes, I think this is expected, given that you
> set buffer-undo-list to nil (what is the purpose of that, btw?). 

No purpose, but some external packages display information in a way 
similar to this sexp, where undo information is also recorded (needlessly):

(with-current-buffer (get-buffer-create "info")
   (let ((inhibit-read-only t))
     (erase-buffer)
     (insert "info"))
   (setq buffer-read-only t)
   (display-buffer (current-buffer)))

I tested it in a loop (but also killed the "info" buffer in each 
iteration) and Emacs also keeps allocating memory, as with 
(with-temp-buffer (setq buffer-undo-list nil) ...).
Obviously normally this isn't a problem because you won't run this code 
in a loop, but if you leave Emacs open for a long time and run this sexp 
as part of some command many times, memory can accumulate (I think, it's 
why I asked if Emacs will finally free this unused memory).

> When Emacs ends up requesting more memory from the OS, it usually
> doesn't release that memory when it is no longer needed, but keeps it
> in the process's address space and reuses it if/when it needs more
> memory.
But shouldn't Emacs reuse the memory from previous loop iteration 
instead of allocating it? Also, if the sexp is modified like this, Emacs 
memory usage is at the same level:


(while t
  (with-temp-buffer
    (insert "a")
    (setq buffer-undo-list (list (cons (point-min) (point-max))))))