bug#38345: 27.0.50; Permanent increase in memory consumption after opening images (or pdfs)

[Eli Zaretskii <eliz@gnu.org>]

> From: Ihor Radchenko <yantar92@gmail.com>
> Cc: juri@linkov.net, 38345@debbugs.gnu.org
> Date: Mon, 16 Dec 2019 14:18:58 +0800
> 
> In my earlier email, it described my trial to expunge the images from
> cache more aggressively:
> 
> >> also, I repeated my tests manually calling image-clear-cache every Nth
> >> image. Every invocation of image-clear-cache does reduce the memory
> >> consumption, but there is always some residual remaining unrealeased
> >> (see the attached images). The residual seems to scale with the number
> >> of images in the cache before clearing.
> 
> You suggested to look at the C code.
> 
> However, looking at the C code, I don't understand then why the
> memory consumption is increasing. I can only see that too many images in
> c->images array can cause extra memory consumption, which cannot cause
> the observed memory leak.

I guess that's because the freed memory is not at the end of the heap,
and this cannot be returned to the OS.

> Do you have any idea why even aggressive cache clearing causes memory
> consumption increase?

See above.  You are talking about "memory consumption", but actually
you trace the memory address space allocated to the Emacs process, and
that includes memory we freed, but the underlying C library couldn't
return back to the OS.