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Emacs LSP performance booster

Build and test status Release status

Improve the performance of lsp-mode or eglot using a wrapper executable.

Background & Prior work

(Huge thanks to @yyoncho for both maintaining lsp-mode and giving me the inspiration of this project).

According to yyoncho, the are several performance issues related to lsp-mode (mostly the same for eglot):

  1. Json parsing in Emacs is slow
  2. The server may block on sending data to emacs when the buffer is full, because Emacs is consuming the data too slowly
  3. Similarly, Emacs may block while attempting to send data to the server (hence blocking the Emacs UI), because the server may be busy

@yyoncho tried to solve these issues by implementing a native async non-blocking jsonrpc fork of emacs. The result is very good regarding performance. However, it requires modifications in the Emacs source code and it seems unlikely that those changes could be merged upstream. Also, it could prove difficult to maintain, since it also requires a separate code path in lsp-mode (I myself encountered some issues).

How this project works

This project provides a wrapper-executable around lsp server programs, to work around the above-mentioned issues:

  • It converts json messages from the server at high speed directly into elisp bytecode (in text representation) for Emacs to read.
    • e.g. {"objs":[{"a":1},{"a":2}]} would be converted to #[0 "\301\302\300\303D\300\304D\"D\207" [:a :objs vector 1 2] 13]
    • This improves the message parsing performance in Emacs by ~4x for large json objects; see benchmark result here
    • Although Emacs still needs to parse the text representation and interpret it into elisp objects, the performance gain mainly comes from the following:
      • Parsing (reading) elisp object is apparently better optimized and simpler in Emacs
      • By using bytecode to construct objects, we can eliminate duplicated objects (e.g. the "a" json key in above example)
  • It separates reading and writing into different threads and keeps pending messages in internal buffers within each thread, to avoid blocking on IO. This solves issues (2) and (3) mentioned above.

Overall, this lsp server wrapper strategy achieves similar result as the native async non-blocking jsonrpc approach without requiring modifications in Emacs source code.

Important

At present only local lsp server programs which communicate by standard input/output can be wrapped, not servers communicating over network ports (local or remote).

How to use

Generally, what you need to do is:

  1. Wrap your lsp server command with this emacs-lsp-booster executable. For example, if the original lsp server command is pyright-langserver --stdio, configure lsp-mode or eglot to run emacs-lsp-booster [flags --] pyright-langserver --stdio instead.
  2. Advise or update the json parsing function in lsp-mode or eglot to parse any bytecode input seen, prior to parsing it as json.

See more detailed configuration steps below.

Obtain or build emacs-lsp-booster

For Linux or Windows users, you may download the prebuilt binary from release. (The macOS binary in the release page lacks proper code signing for now.) A flake for nix users is available here.

Alternatively, you may build the target locally:

  1. Setup Rust toolchain
  2. Run cargo build --release
  3. Find the built binary in target/release/emacs-lsp-booster

Then, put the emacs-lsp-booster binary in your $PATH (e.g. ~/.local/bin).

Configure lsp-mode

Note

Make sure NOT to use the native-jsonrpc custom version of Emacs

  1. Use plist for deserialization for lsp-mode
  2. Add the following code to your init.el:
(defun lsp-booster--advice-json-parse (old-fn &rest args)
  "Try to parse bytecode instead of json."
  (or
   (when (equal (following-char) ?#)
     (let ((bytecode (read (current-buffer))))
       (when (byte-code-function-p bytecode)
         (funcall bytecode))))
   (apply old-fn args)))
(advice-add (if (progn (require 'json)
                       (fboundp 'json-parse-buffer))
                'json-parse-buffer
              'json-read)
            :around
            #'lsp-booster--advice-json-parse)

(defun lsp-booster--advice-final-command (old-fn cmd &optional test?)
  "Prepend emacs-lsp-booster command to lsp CMD."
  (let ((orig-result (funcall old-fn cmd test?)))
    (if (and (not test?)                             ;; for check lsp-server-present?
             (not (file-remote-p default-directory)) ;; see lsp-resolve-final-command, it would add extra shell wrapper
             lsp-use-plists
             (not (functionp 'json-rpc-connection))  ;; native json-rpc
             (executable-find "emacs-lsp-booster"))
        (progn
          (when-let ((command-from-exec-path (executable-find (car orig-result))))  ;; resolve command from exec-path (in case not found in $PATH)
            (setcar orig-result command-from-exec-path))
          (message "Using emacs-lsp-booster for %s!" orig-result)
          (cons "emacs-lsp-booster" orig-result))
      orig-result)))
(advice-add 'lsp-resolve-final-command :around #'lsp-booster--advice-final-command)

Done! Now try to use lsp-mode as usual.

Configure eglot

Please see https://github.com/jdtsmith/eglot-booster for information on configuring eglot. Huge thanks to @jdtsmith

How to verify it's working

  1. Check that emacs-lsp-booster process is running
  2. Check the stderr buffer (e.g. for lsp-mode, *pyright::stderr* buffer; for eglot, the EGLOT (...) stderr* buffer, note the leading space); it should contain emacs_lsp_booster related log.

Advanced usage

Run emacs-lsp-booster --help for more options.