bug#45200: Memory leaks: (garbage-collect) fails to reclaim memory

[Konstantin Kharlamov]

On Sun, 2021-01-24 at 22:00 +0300, Konstantin Kharlamov wrote:
> On Sun, 2021-01-24 at 13:51 -0500, Stefan Monnier wrote:
> > > # Steps to reproduce:
> > > 
> > > 1. Run `mkdir /tmp/.emacs.d`
> > > 2. Run emacs as `HOME=/tmp/ emacs`, and measure its PSS
> > > 3. Create a file /tmp/.emacs.d/early-init.el with content:
> > > 
> > >     ;; only run garbage collection on idle
> > >     (setq gc-cons-threshold most-positive-fixnum)
> > >     (run-with-idle-timer 2 t (lambda () (garbage-collect)))
> > > 
> > > 4. Run emacs as `HOME=/tmp/ emacs`, evaluate (garbage-collect), then
> > > measure
> > > its PSS
> > > 
> > > ## Expected
> > > 
> > > Size has no statistically-significant difference, because in both
> > > cases we garbage-collected memory.
> > 
> > I disagree with this expectation: it is perfectly normal for the amount
> > of memory allocated to the Emacs process to be left higher if you delay
> > the GC.  There are various reasons for that:
> > - fragmentation, of course.  Not much we can do about it short of using
> >   a moving collector.
> > - the desire to keep memory around rather than return it to the OS,
> >   under the assumption that we'll need it again soon.
> > 
> > And it's not considered as a memory leak as long as that memory has
> > indeed been needed in the past and that future allocations can still
> > make use of it.
> 
> I'm fine with Emacs possibly keeping a dozen of megabytes for personal use.
> The problem though is that the issue easily manifests itself in hundreds of
> megabytes, as can be seen with the testcase marked as "Message #11" in web-ui.

Case in point btw: after the previous weekend I found my Emacs instance that I 
did not touch during the weekend to bloat to as much as 6xxMB. I think that 
happened because the on-idle hook that I hooked garbage collector onto was not 
run for 2 days, whilst Emacs might have been doing something like communication 
to LSP server from time to time, so this is why it ended up taking so much 
memory. I think it's needless to say that I had to restart Emacs for the 
half-gigabyte of memory to get returned to my system.