Re: Shrinking the C core

[Eric S. Raymond]

Eli Zaretskii <eliz@gnu.org>:
> 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.

Yeah, I've been thinking hard about that single-threadedness in the
last couple of weeks.  I have a design sketch in my head for a
re-partitioning of Emacs into a front-end/back-end pair communicating
via sockets, with the back end designed to handle multiple socket
sessions for collaborative editing. (No, this isn't my big secret idea,
it's something I think should be done *along with* my big secret idea.)

For this to work, a lot of what is now global state would need to be
captured into a structure associated with each socket session.  I notice
that it's difficult to find an obviously correct cut line between what
the session structure should own and what still needs to be shared state;
like, *some* keymaps definitely need to be session and buffers still need
to be shared, but what about the buffer's mode set and mode-specific kemaps?
Or marks?  Or overlays?

This is a difficult design problem because of some inherent features
of the Emacs Lisp language model.  I did not fail to notice that those
same features would make exploiting concurrency difficult even in the
present single-user-only implementation.  It is unclear what
could be done to fix this without significant language changes.

> 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.

Are you seriously to trying to tell me that the display engine rewrite ate
*three orders of magnitude* in machine-speed gains?  No sale.  I have
some idea of the amount of talent on the devteam and I plain do not
believe y'all are that incompetent.

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

I'm not persuaded, because your causal account doesn't pass my smell
test. I think you're misdiagnosing the performance problems through
being too close to them. It would take actual benchmark figures to
persuade me that Lisp interpretive overhead is the actual culprit.

Your project, your choices. But I have a combination of experience
with the code going back nearly to its origins with an outside view of
its present strate, and I think you're seeing your own assumptions
about performance lag reflected back at you more than the reality.

> Please be more patient,

That *was* patient. I didn't aim for his head until the *second*
time he poked me. :-)

I'll stop trying to make preparatory changes.  If I can allocate
enough bandwidth for the conversation, I may try on a couple of
hopefully thought-provoling design questions.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>