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# Optimizing Emacs Lisp Code
Dmitry Gutov
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[[!table header="no" class="speaker-details" data="""
Name pronunciation: | d-MEET-ri GOO-tov
Pronouns: | he/his
Homepage: | <https://github.com/dgutov/>
Preferred contact info | <dgutov@yandex.ru>
"""]]
- Before optimizing, benchmark first.
- Different benchmarking approaches.
- Live evaluation, step-debugging, measuring from a debugger breakpoint.
- How to determine if a function is expensive. How to pick one from
competing alternatives (cl-lib, seq, dash, lean core).
- Print-benchmarking.
- Byte-compiled code can give a very different picture, changing where
the bottleneck is. How to quickly load a byte-compiled version.
- Steps taken to speed up the Xref package recently.
# Discussion
IRC nick: dgutov
Pad:
- Q1: Why are overlays slow compared to text-properties? I (believe
to) understand that it is (in part only?) due to "get element n in
vector vs list". If so, then why don't we change that? There could
be a text-property called "overlays", so that lookup would also be
like in a vector. What benefits does the datastructure currently
used for overlays have that make that undesirable? Would a mixed
approach make sense; i.e. allow non-modifiyng lookups to use the
"cached" overlays that are stored in the "overlay" text-property
and make text-inserting and overlay-moving actions store in the
currently used datastructure as well as in the indirect
text-property=>overlay cache?
- A: "There is a pending patch to represent the set of a
buffer's overlays as an AAtree or somesuch.."
- Sounds promising :)
- For more details, check out these threads:
- <https://lists.gnu.org/archive/html/emacs-devel/2014-09/msg00616.html>
- <https://lists.gnu.org/archive/html/emacs-devel/2016-11/msg00475.html>
- <https://lists.gnu.org/archive/html/emacs-devel/2018-03/msg00565.html>
- <https://lists.gnu.org/archive/html/emacs-devel/2019-12/msg00115.html>
- Q2: As a non-programmer, would these sorts of optimizations be
helpful to do on a personal init.el file?
- A: Probably not
- Though too much mode-line customisation may slow things down.
- Q3: What's a good approach for benchmarking destructive
operations? If you delete elements from a list in-place, all
subsequent runs will be artificially fast.
- A: There is an example of a comparison between operations from
different libraries in the example file provided by the talk.
Particularly check the benchmarks for delete and remove
operations (destructive and non-destructive, respectively).
- Q4:Cl-lib constructors, getters, and setters usually expand into
multiple levels of let-bindings. AFAIU, every let-binding is an
extra memory allocation. Do you recommend avoiding cl-defstruct in
favour of "pure" lists/vectors?
- A: basically no. if defstruct allows you to organise better, go
ahead.
- Q5: Is it possible to optimize some emacs packages by making use of
code compiled from other languages (like C or Common Lisp) ((i.e. in
the same way python is able to import C code))?
- A: yes emacs modules allow you to run C or Rust, transitioning
between emacs proper and module (passing the data) might slow
things down? Because of copying that's necessary to avoid
issues with gc.
- Q6:You mentioned that overlays are much slower compared to text
properties. What about text properties vs. buffer-local variables to
store position cache?
- A: I haven't measured it but offhand I'm going to guess that
buffer-local variables will be faster.
- Also depends on the structure you're going to use for the
cache - is it a single cons, or a list, or a tree, etc.
BBB:
- AVL tree
- defstruct accessors should expand with compiler macros to aref calls, which are very fast
- They have extra if though
- oh you mean for testing whether the value is such a struct?
- yes there is that test, but I wouldn't expect that to make it 3x slower, AFAIK
IRC:
- If somebody wants to do a remote session with me: I do have processes such as updating column view dynamic blocks that take maybe 40 minutes. So far, I avoid executing those functions when I'm around the computer myself. However, there may be room for improvement and I really can't tell wether it is in my personal setup or not because it's not always that easy to re-create a use-case with plain Emacs cnofig
- Thanks for doing this talk. FYI you might find the this bench-multi-lexical macro useful: https://alphapapa.github.io/emacs-package-dev-handbook/#outline-container-Optimization
- dgutov: I can't seem to find the exact macro you are referring to. But if it covers a use case benchmark-progn does not, consider contributing it to benchmark.el in the core.
- Sorry, try this link directly to that macro: https://github.com/alphapapa/emacs-package-dev-handbook#bench-multi-lexical The purpose of the macro is to compare different forms and show how they perform relative to each other
- dgutov: Ah yeah, that looks pretty cool. Less sure about the org format, but it must be nice for presentations.
- The Org format is good for documentation too. But it just uses the output of benchmark-run, so it could easily be left in Lisp form. :)
- dgutov: These things are really handy to have available in 'emacs -Q', though. When you're working on shaving some extra handful of percents.
- Yes, a few lines of code could be added to run the compiled code in a separate Emacs process.
- https://github.com/alphapapa/emacs-package-dev-handbook compares some common ways to do common things in Elisp so you can see which is generally faster, e.g. https://github.com/alphapapa/emacs-package-dev-handbook#inserting-strings
- PSA: buffer-local-value is generally much faster than with-current-buffer if all you need to do is get the value of a variable in a buffer
- For more info about the performance of overlays vs text properties data structure, there's an Emacs TODO about it. C-h C-t and search for "Move overlays to intervals.c".
- cl-defstruct getters/setters have compiler macros that expand into simple aref calls on vectors, they are very efficient
Links:
- you might find the this bench-multi-lexical macro useful:
<https://alphapapa.github.io/emacs-package-dev-handbook/#outline-container-Optimization>
or
<https://github.com/alphapapa/emacs-package-dev-handbook#bench-multi-lexical>
- <https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/emacs-lisp/elp.el>
- <https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/emacs-lisp/benchmark.el>
- "Use hash tables kids!"
- PSA: buffer-local-value is generally much faster than
with-current-buffer if all you need to do is get the value of a
variable in a buffer
- EIEIO's object construction is slow because it goes through
`make-instance` which is a generic function and it itself calls
various other generic functions, so there's a lot of cl-generic
dispatch overhead; and then there's the fact that the (keyword)
arguments are laboriously parsed at run-time so it itself is slow as
well.
- There is a pending patch to represent the set of a buffer's
overlays as an AAtree or somesuch.
- <https://media.emacsconf.org/2021/emacsconf-2021-faster--optimizing-emacs-lisp-code--dmitry-gutov.el>
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