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Re: Concurrency, again

From: Stefan Monnier
Subject: Re: Concurrency, again
Date: Wed, 12 Oct 2016 15:43:12 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.2.50 (gnu/linux)

>> I urge you to have a look at the sources on the concurrency branch. There's
>> no threading, in the usual sense of that word: at any given time, only a
>> single thread is running. It's not at all what you seem to be imagining.
>> Think about idle timers that run without your having to do anything to start
>> them: that's what we have there, in a nutshell, plus a few niceties.
> More akin to green threading then?

Right.  It does introduce the possibility to have new interleavings, so
new bugs, but 99% of the bugs it uncovers should be reproducible (at
least in theory; tho maybe only in convoluted circumstances) in the
non-concurrent branch as well.

> It's interesting to note how many people agree with caution, or don't claim to
> need threading, vs. those calling for its inclusion.  I know many users who
> don't want Emacs locking up on them.  This is the problem that needs solving.
> Is threading in Emacs Lisp the answer? I still don't know.

I think there are many different needs/desires related to "concurrency".
A typical one (for me anyway) is:

- I want to use Gnus, but I want to be able to edit files while Gnus
  consults the IMAP server to see if there are new messages, or while
  Gnus is waiting for the damn NNTP server to answer.
  Currently I solve this problem by running Gnus in a separate Emacs
  session (i.e. I basically always have 2 Emacs processes).  I lived
  with it for the last 20 years, so it's not that bad, but
  it's inconvenient.

  The concurrency branch should be able to solve this problem.

  We could probably solve this problem also using a cps-translation like
  the one in generator.el, to take the synchronous code using
  accept-process-output, and turn it into code which runs
  asynchronously, from the process filters&sentinels.  It's likely to be
  somewhat brittle, tho.

- I want semantic-mode to scan my buffers in the background and to scan
  several buffers at the same time, to make use of my 8 cores.
  The concurrency branch doesn't really try to solve this problem.
  I'm not sure what's the best way to solve this problem, and I don't
  even know what would be a "simple" way to solve this problem.

-- Stefan

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