Re: Shrinking the C core

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 9 Aug 2023 21:19:11 -0400
> From: "Eric S. Raymond" <esr@thyrsus.com>
> Cc: emacs-devel@gnu.org
> 
> Po Lu <luangruo@yahoo.com>:
> > "Eric S. Raymond" <esr@thyrsus.com> writes:
> > 
> > > When I first worked on Emacs code in the 1980s Lisp was already fast
> > > enough, and machine speeds have gone up by something like 10^3 since.
> > > I plain don't believe the "slower" part can be an issue on modern
> > > hardware, not even on tiny SBCs.
> > 
> > Can you promise the same, if your changes are not restricted to one or
> > two functions in fileio.c, but instead pervade throughout C source?
> 
> Yes, in fact, I can. Because if by some miracle we were able to
> instantly rewrite the entirety of Emacs in Python (which I'm not
> advocating, I chose it because it's the slowest of the major modern
> scripting languages) basic considerations of clocks per second would
> predict it to run a *dead minimum* of two orders of magnitude faster
> than the Emacs of, say, 1990.
> 
> And 1990 Emacs was already way fast enough for the human eye and
> brain, which can't even register interface lag of less than 0.17
> seconds (look up the story of Jef Raskin and how he exploited this
> psychophysical fact in the design of the Canon Cat sometime; it's very
> instructive). The human auditory system can perceive finer timeslices,
> down to about 0.02s in skilled musicians, but we're not using elisp
> for audio signal processing.

This kind of argument is inherently flawed: it's true that today's
machines are much faster than those in, say, 1990, but Emacs nowadays
demands much more horsepower from the CPU than it did back then.
What's more, Emacs is still a single-threaded Lisp machine, although
in the last 10 years CPU power develops more and more in the direction
of multiple cores and execution units, with single execution units
being basically as fast (or as slow) today as they were a decade ago.

And if these theoretical arguments don't convince you, then there are
facts: the Emacs display engine, for example, was completely rewritten
since the 1990s, and is significantly more expensive than the old one
(because it lifts several of the gravest limitations of the old
redisplay).  Similarly with some other core parts and internals.

We are trying to make Lisp programs faster all the time, precisely
because users do complain about annoying delays and slowness.  Various
optimizations in the byte-compiler and the whole native-compilation
feature are parts of this effort, and are another evidence that the
performance concerns are not illusory in Emacs.  And we are still not
there yet: people still do complain from time to time, and not always
because someone selected a sub-optimal algorithm where better ones
exist.

The slowdown caused by moving one primitive to Lisp might be
insignificant, but these slowdowns add up and eventually do show in
user-experience reports.  Rewriting code in Lisp also increases the GC
pressure, and GC cycles are known as one of the significant causes of
slow performance in quite a few cases.  We are currently tracking the
GC performance (see the emacs-gc-stats@gnu.org mailing list) for that
reason, in the hope that we can modify GC and/or its thresholds to
improve performance.

> If you take away nothing else from this conversation, at least get it
> through your head that "more Lisp might make Emacs too slow" is a
> deeply, *deeply* silly idea. It's 2023 and the only ways you can make
> a user-facing program slow enough for response lag to be noticeable
> are disk latency on spinning rust, network round-trips, or operations
> with a superlinear big-O in critical paths.  Mere interpretive overhead
> won't do it.

We found this conclusion to be false in practice, at least in Emacs
practice.

> > Finally, you haven't addressed the remainder of the reasons I itemized.
> 
> They were too obvious, describing problems that competent software
> engineers know how to prevent or hedge against, and you addressed me
> as though I were a n00b that just fell off a cabbage truck. My
> earliest contributions to Emacs were done so long ago that they
> predated the systematic Changelog convention; have you heard the
> expression "teaching your grandmother to suck eggs"?  My patience for
> that sort of thing is limited.

Please be more patient, and please consider what others here say to be
mostly in good-faith and based on non-trivial experience.  If
something in what others here say sounds like an offense to your
intelligence, it is most probably a misunderstanding: for most people
here English is not their first language, so don't expect them to
always be able to find the best words to express what they want to
say.