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Re: Lost or corrupted `undo-tree' history
From: |
Eli Zaretskii |
Subject: |
Re: Lost or corrupted `undo-tree' history |
Date: |
Fri, 10 Jan 2020 11:31:05 +0200 |
> Cc: address@hidden
> From: Alexander Shukaev <address@hidden>
> Date: Fri, 10 Jan 2020 10:19:15 +0100
>
> > IMO, it would be a very bad mantra for a Lisp package operating on
> > this low level to disable GC, because that could cause the user's
> > system to run out of memory, and Emacs be killed by the likes of OOM
> > killer agents. Disabling GC is barely a good idea on the
> > user-customization level, certainly not in packages such as undo-tree.
>
> Maybe. I was under impression that for a "short-running" function this
> might be fine to prevent GC from running in-between some Emacs Lisp
> instructions.
You can never know whether a given function is "short-running" or not
in Emacs, not with the multitude of hooks and advices we have. You
_might_ be able to make that assumption safely on the C level, but
even there, it's non-trivial to prove to ourselves no Lisp could ever
run in between. For Lisp code, this is simply impossible to prove.
> Having said that, I agree that disabling GC sometimes may be dangerous
> practice. I remember how after reading [1], I tried that suggestion for
> minibuffer. After some time I noticed that I keep running out of
> memory. What was interesting is the actual test case. For example, I
> could run some search command which pushes matches into minibuffer, say
> up to 100 000 matches. As this is happening (and `gc-cons-threshold' is
> `most-positive-fixnum'), I can see memory consumption of Emacs growing
> rapidly say from 500MB up to 8GB at which point I cancel the search and
> exit the minibuffer command. As a result, `gc-cons-threshold' comes
> back to the default value and garbage collecting starts immediately.
> However, the memory consumption of Emacs does not fall back to 500MB,
> but rather goes down to only e.g. 6GB, which afterwards are never ever
> reclaimed.
I believe this is the expected behavior of memory allocation routines
on GNU/Linux. Freed memory is not necessarily returned to the system,
but kept in the process's address space as free memory to be used for
future allocations.
> If I repeat the test, the memory consumption would immediately
> continue to grow off 6GB further as if those 6GB are not reused and
> are somehow stuck holding something that cannot be reclaimed
> anymore. Hence, you can see that if I way same amount of time, the
> memory consumption would go to around 14GB. This is how one can
> quickly run out of memory as if there would be some memory leaks
> related to GC.
There are no memory leaks. Just don't set gc-cons-threshold too high,
that's all.
> Nevertheless, the second suggestion (for speeding up initialization
> code) from [1], IMO, is a good example of temporarily blowing
> `gc-cons-threshold' which I still use.
I disagree. Kids, don't try that at home! I posted several times on
reddit and elsewhere the procedure I propose to follow to raise the
threshold in a conservative and controlled way, to satisfy the
personal needs of any particular user, without risking any system-wide
degradation in memory pressure.