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Re: Preview: portable dumper

From: Eli Zaretskii
Subject: Re: Preview: portable dumper
Date: Tue, 29 Nov 2016 20:40:50 +0200

> From: John Wiegley <address@hidden>
> Cc: address@hidden,  address@hidden,  address@hidden
> Date: Mon, 28 Nov 2016 14:18:58 -0800
> > Accepting this patch means we believe in this direction, consider it a
> > perspective one, enough so that we are willing to invest a non-trivial
> > amount of energy into making it happen, fix the fallout, debug the results,
> > etc. We are talking about at least one major release, maybe more, and
> > definitely several minor releases. IOW, we are talking several years. I
> > don't think we should gamble with such high stakes, not unless we sure we
> > want to go there, because this is the best alternative of [...]
> If I didn't know you were talking about pdumper vs. unexec here, I'd swear
> this above paragraph was expressing my own concerns about merging Tromey's
> concurrency branch.

Superficially, perhaps.  But as soon as you look at the details, there
are two important differences.

First, the only piece of code in the concurrency patch that is not
entirely trivial is a 60-line function (30% of whose line count is
comments and ifdef's).  That's all there is to understand there.  The
rest is extremely simple, almost trivial: functions that directly map
into the corresponding system threads APIs, or 2-liners for creation,
checking, and otherwise simple management of the few new object types.
By contrast, the proposed patch, even in its initial version, weighs
in at 3200 lines in its core part, and almost none of it is trivial or

What's more, it comes with the need to make important and non-trivial
decisions wrt which objects and data structures need to be dumped, and
how, something that requires intimate knowledge of the internals and
their use patterns, both at dump time and in interactive sessions.
These decisions will need to be revisited when the related objects are
added or modified, which is the part that averts me the most, because
it means extending the Lisp interpreter, which is already not easy,
will have become even harder.

The other important difference is that threads are an opt-in feature:
if you don't start any threads, Emacs with threads still works as it
did before, and you can just ignore anything related to threads.  IOW,
threads are a feature we offer to Lisp programs that want to use it.
By contrast, dumping code is a central and mandatory part of building
Emacs and of its startup process.  It cannot be ignored or made

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