GNU bug report logs - #31852
Make memory-limit obsolete

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Emacs bug reports <bug-gnu-emacs <at> gnu.org>
Subject: Make memory-limit obsolete
Date: Fri, 15 Jun 2018 18:33:47 -0700

[Message part 1 (text/plain, inline)]

Tags: patch

The memory-limit function has an implementation that is a relic of 
long-ago days when the heap was allocated via sbrk. This assumption is 
often no longer true now that ASLR is popular, and once we get portable 
dumping work it'll be true even less often. Since memory-limit returns 
nonsense so often and since nobody cares, we can and should mark 
memory-limit as obsolete. On its way out we can move it to a Lisp 
implementation and cause it to be at least somewhat more plausible.

Proposed patches attached. The first patch moves memory-limit to Lisp, 
the second one marks it obsolete. I didn't know where to put the Lisp 
implementation so I put it into lisp/subr.el.

[0001-Rewrite-memory-limit-in-Lisp.patch (text/x-patch, attachment)]

[0002-Obsolete-memory-limit.patch (text/x-patch, attachment)]

Message #8 received at 31852 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 31852 <at> debbugs.gnu.org
Subject: Re: bug#31852: Make memory-limit obsolete
Date: Sat, 16 Jun 2018 09:31:23 +0300

> From: Paul Eggert <eggert <at> cs.ucla.edu>
> Date: Fri, 15 Jun 2018 18:33:47 -0700
> 
> The memory-limit function has an implementation that is a relic of 
> long-ago days when the heap was allocated via sbrk. This assumption is 
> often no longer true now that ASLR is popular, and once we get portable 
> dumping work it'll be true even less often. Since memory-limit returns 
> nonsense so often and since nobody cares, we can and should mark 
> memory-limit as obsolete. On its way out we can move it to a Lisp 
> implementation and cause it to be at least somewhat more plausible.
> 
> Proposed patches attached. The first patch moves memory-limit to Lisp, 
> the second one marks it obsolete. I didn't know where to put the Lisp 
> implementation so I put it into lisp/subr.el.

I don't have anything against moving this to Lisp (provided that all
the platforms we care about have a non-trivial implementation of
process-attributes), but I don't understand what we gain by declaring
this tiny function obsolete, especially since the alternative proposed
in the warning is exactly what's used in the proposed Lisp
implementation.  Wouldn't it be enough just to add to the function's
documentation a note that the estimation should be expected to be
inaccurate with modern memory-management technologies?

Thanks.

Message #11 received at 31852 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 31852 <at> debbugs.gnu.org
Subject: Re: bug#31852: Make memory-limit obsolete
Date: Sat, 16 Jun 2018 06:38:55 -0700

Eli Zaretskii wrote:
> I don't understand what we gain by declaring
> this tiny function obsolete

Nobody uses the function (clearly, since it doesn't work) so we gain simplicity, 
which is a win.

> Wouldn't it be enough just to add to the function's
> documentation a note that the estimation should be expected to be
> inaccurate with modern memory-management technologies?

Complicating the documentation would work against the goal of simplicity. If the 
goal is something other than simplicity, it'd be helpful to know what the goal 
is before proposing a different patch 2. The idea behind patch 2 is that 
simplicity trumps compatibility here, as nobody uses the function and nobody 
will use it even if patch 1 is applied, partly because of the inaccuracies that 
you mention.

Message #14 received at 31852 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 31852 <at> debbugs.gnu.org
Subject: Re: bug#31852: Make memory-limit obsolete
Date: Sat, 16 Jun 2018 18:02:03 +0300

> Cc: 31852 <at> debbugs.gnu.org
> From: Paul Eggert <eggert <at> cs.ucla.edu>
> Date: Sat, 16 Jun 2018 06:38:55 -0700
> 
> Eli Zaretskii wrote:
> > I don't understand what we gain by declaring
> > this tiny function obsolete
> 
> Nobody uses the function (clearly, since it doesn't work) so we gain simplicity, 
> which is a win.

If nobody uses the function, I see no gain at all, because obsoleting
a function tells those who use it not to.

> > Wouldn't it be enough just to add to the function's
> > documentation a note that the estimation should be expected to be
> > inaccurate with modern memory-management technologies?
> 
> Complicating the documentation would work against the goal of
> simplicity.

Then let's just move this to Lisp, and do nothing else.  That's even
simpler, IMO.

Message #19 received at 31852-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 31852-done <at> debbugs.gnu.org
Subject: Re: bug#31852: Make memory-limit obsolete
Date: Sat, 16 Jun 2018 08:34:52 -0700

Eli Zaretskii wrote:

> If nobody uses the function, I see no gain at all, because obsoleting
> a function tells those who use it not to.

If Emacs has useless and confusing features that consume documentation space and 
implementation resources, that is a net harm to current and future users. 
Omitting such features is therefore a net gain to users. The benefit to users by 
omitting useless and confusing features is worth the maintenance cost to us of 
obsoleting these features.

That being said, it appears I haven't convinced you to obsolete memory-limit, so 
I installed patch 1 but not patch 2 and am closing the bug report.

Message #22 received at 31852 <at> debbugs.gnu.org (full text, mbox):

From: "Daniel Colascione" <dancol <at> dancol.org>
To: 31852 <at> debbugs.gnu.org,
 eggert <at> cs.ucla.edu,
 eggert <at> cs.ucla.edu
Subject: Re: bug#31852: Make memory-limit obsolete
Date: Sat, 16 Jun 2018 08:51:47 -0700

> Eli Zaretskii wrote:
>
>> If nobody uses the function, I see no gain at all, because obsoleting
>> a function tells those who use it not to.
>
> If Emacs has useless and confusing features that consume documentation
> space and
> implementation resources, that is a net harm to current and future users.
> Omitting such features is therefore a net gain to users. The benefit to
> users by
> omitting useless and confusing features is worth the maintenance cost to
> us of
> obsoleting these features.
>
> That being said, it appears I haven't convinced you to obsolete
> memory-limit, so
> I installed patch 1 but not patch 2 and am closing the bug report.

I agree with obsoleting memory-limit. I don't like the vsize patch you
installed though: what does the amount of address space reserved actually
tell anyone? There's no relationship to actual resource consumption.

IMHO, memory-limit should return a combination of the Lisp heap size with
whatever malloc says is its own heap size.

Message #25 received at 31852 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Daniel Colascione <dancol <at> dancol.org>, 31852 <at> debbugs.gnu.org
Subject: Re: bug#31852: Make memory-limit obsolete
Date: Sat, 16 Jun 2018 09:27:30 -0700

Daniel Colascione wrote:
> I agree with obsoleting memory-limit. I don't like the vsize patch you
> installed though: what does the amount of address space reserved actually
> tell anyone? There's no relationship to actual resource consumption.

It's not that good a number, true; but it's better than the number we were 
returning, which was typically nonsense.

One of the benefits of moving memory-limit to Lisp was the removal of the bogus 
call to sbrk. Emacs should never call sbrk, and once the portable dumper code is 
merged, Emacs won't ever need to call sbrk again. Yay!

All things considered, I'd rather not go down the rabbit hole of trying to 
improve memory-limit further, because memory-limit was a mistake in the first 
place and nobody uses it (and rightly so) and we should be deprecating it rather 
than trying to improve it. That being said, if you'd like to make it better 
please feel free.

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