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el-job

Imagine you have a function you’d like to run on a long list of inputs. You could run (mapcar #'FN INPUTS), but that hangs Emacs until done.

This library lets you run the same function in many subprocesses (one per CPU core), each with their own split of the INPUTS list, then merge their outputs and pass it back to the current Emacs.

In the meantime, current Emacs does not hang at all.

Best of all, it completes faster than (mapcar #'FN INPUTS), owing to the use of all CPU cores!

For real-world usage, search for el-job in the source of org-node.el.

Design rationale

I want to shorten the round-trip as much as possible, between the start of an async task and having the results.

For example, say you have some lisp that collects completion candidates, and you want to run it asynchronously because the lisp you wrote isn’t always fast enough to avoid the user’s notice, but you’d still like it to return as soon as possible.

Processes stay alive

In the above example, a user might only delay a fraction of a second between opening the minibuffer and beginning to type, so there’s scant room for overhead like spinning up subprocesses that load a bunch of libraries before getting to work.

Thus, el-job keeps idle subprocesses for up to 30 seconds after a job finishes, awaiting more input.

An aesthetic drawback is cluttering your task manager with many processes named “emacs”.

Users who tend to run system commands such as pkill emacs may find that the command occasionally “does not work”, because it actually killed an el-job subprocess, instead of the Emacs they see on screen.

Emacs 30 fast-read-process-output

Some other libraries, like the popular async.el, are designed around a custom process filter.

Since Emacs 30, it’s a good idea to instead use the built-in process filter when performance is critical, and el-job does so. Quoting NEWS.30:

** The default process filter was rewritten in native code.
The round-trip through the Lisp function
'internal-default-process-filter' is skipped when the process filter is
the default one.  It is reimplemented in native code, reducing GC churn.
To undo this change, set 'fast-read-process-output' to nil.

News 2.1.0

  • DROP SUPPORT Emacs 28
    • It likely has not been working for a while anyway. Maybe works on the v0.3 branch, from 0.3.26+.

News 2.0.0

  • Jobs must now have :id (no more anonymous jobs).
  • Pruned many code paths.

News 1.1.0

  • Changed internals so that all builds of Emacs can be expected to perform similarly well.

News 1.0.0

  • No longer keeps processes alive forever. All jobs are kept alive for up to 30 seconds of disuse, then reaped.
  • Pruned many code paths.
  • Many arguments changed, and a few were removed. Consult the docstring of el-job-launch again.

Limitations

  1. The return value from the :funcall-per-input function must always be a list with a fixed length, where the elements are also lists.

    For example, org-node passes :funcall-per-input #'org-node-parser--scan-file to el-job, and if you look in org-node-parser.el for the defun of org-node-parser--scan-file, this is its final return value:

    (list (if missing-file (list missing-file)) ; List of 0 or 1 item
      (if file-mtime (list file-mtime))     ; List of 0 or 1 item
      found-nodes                           ; List of many items
      org-node-parser--paths-types          ; List of many items
      org-node-parser--found-links          ; List of many items
      (if problem (list problem))))         ; List of 0 or 1 item

    It may seem clunky to return lists of only one item, but you could consider it a minor expense in exchange for simpler library code.

  2. Some data types cannot be exchanged with the children: those whose printed form look like #<...>. For example, #<buffer notes.org>, #<obarray n=94311>, #<marker at 3102 in README.org>.

    IIUC, this sort of data only has meaning within the current process – so even if you could send it, it would not be usable by the recipient anyway.

    In days past, hash tables also looked like that when printed, but not since Emacs 25 or so: they now look like #s(hash-table data ...), which works fine to read back.