lsp-ui-flycheck is (sometimes) slow

[sebastiansturm]

I'm using lsp-ui-flycheck with cquery, which works great most of the time. Sometimes though (and I don't know why this issue seems to pop up so inconsistently) editing slows down to a crawl, with Emacs's profiler telling me that most of the CPU time is spent within flycheck. For now, I have locally changed lsp-ui-flycheck-enable to mark diagnostics as stale using a global variable and have flycheck constantly check that variable with an idle timeout.

Does anyone else experience this issue or is something wrong with my configuration?

If this is a general performance problem, it might make sense to have lsp-ui-flycheck--start check if the lsp backend has sent any new diagnostics since flycheck had last been run and use flycheck's own timeout mechanism instead of manually invoking it from within lsp-after-diagnostics-hook?

[MaskRay]

I also suffer from the slowness in lsp-ui-flycheck. For cquery, every textDocument/didChange requests results in a textDocument/publishDiagnostics response. There should be some way to cancel unfinished rendering. This is just one place where it can be improved, but the root cause may be otherwhere.

BTW, how do you mark diagnostics as stale?

[sebastiansturm]

nothing sophisticated, I just replaced lsp-ui-flycheck-enable with the following:

(defvar lsp-ui-flycheck--stale-diagnostics nil)

(defun lsp-ui-flycheck-enable (_)
  "Enable flycheck integration for the current buffer."
  (setq-local flycheck-check-syntax-automatically nil)
  (setq-local flycheck-checker 'lsp-ui)
  (lsp-ui-flycheck-add-mode major-mode)
  (add-to-list 'flycheck-checkers 'lsp-ui)
  (run-with-idle-timer 0.2 t
                       (lambda () (when (and lsp-ui-flycheck--stale-diagnostics flycheck-mode)
                                    (flycheck-buffer)
                                    (setq lsp-ui-flycheck--stale-diagnostics nil))))
  (add-hook 'lsp-after-diagnostics-hook (lambda ()
                                          (setq lsp-ui-flycheck--stale-diagnostics t)
                                          ;; (when flycheck-mode
                                          ;;   (flycheck-buffer))
                                          )))

BTW, is the automatic textDocument/publishDiagnostics mandated by the Language Server protocol or could cquery also be changed to only send diagnostics when requested by the editor?

[topisani]

An idle timer on sending the didChange events helps a lot for this and other things. I think this is the correct way to do it. It should probably be fixed up a bit and merged into core lsp-mode though:

(defcustom lsp-on-change-idle-timer-timeout 0.2
  "Timeout to send `textDocument/didChange' notification"
  :group 'lsp-mode
  :type 'number)

(defvar lsp-on-change-idle-timer-id nil)

(defun my-lsp-on-change-timer-advice (orig-func &rest args)
  "Add an idle timer to lsp-on-change"
  (-some->> lsp-on-change-idle-timer-id (cancel-timer))
  (setq lsp-on-change-idle-timer-id
        (run-with-idle-timer lsp-on-change-idle-timer-timeout nil
                             (lambda ()
                               (funcall orig-func (point-min) (point-max) (- (point-max) (point-min)))
                               (setq lsp-on-change-idle-timer-id nil)))))

(advice-add 'lsp-on-change :around 'my-lsp-on-change-timer-advice)

This works very well for me however, along with using font-lock for cquery semantic highlighting instead of overlays, and disabling git-gutter+ on long files, since it uses overlays as well.

Some issues with it:

• it seems completions sometimes rely on this being run, so we need a way to force it to happen now.
• it sends a didChange event for the entire buffer, since i didnt implement the correct calculations for that yet

However, this fixes the flycheck issue without touching flycheck. It might however still be a good idea to use flycheck's own timers for that instead of lsp-after-diagnostics

Edit:

using the feature/noverlay emacs branch removes pretty much all issues with overlays. My buffer with ~10000 overlays from cquery semantic highlighting is fast as ever

[sebastiansturm]

maybe we can define a convenience function or macro that automatically converts "immediate" functions into ones triggered by idle timers. We could then use that to directly replace lsp-on-change (as well as the symbol-highlight callback and possibly other functions) by their timed counterparts. The following seems to work for a toy function, but I haven't yet tried it with the functions defined by lsp-mode. Also, I'm not sure if those timers should be buffer-local, but I guess UI-oriented modes are probably fine with a single global timer per function

(defmacro make-coalescing (fn default-timeout) ()
          (let* ((fn-name (intern (symbol-name fn)))
                 (timer-sym (intern (format "coalescing-%s-timer-id" fn-name)))
                 (timeout-var-sym (intern (format "coalescing-%s-timeout" fn-name)))
                 (cls-fn (intern (format "coalescing-%s" fn-name)))
                 (timer-docstring (format "Timeout to invoke %s" fn-name))
                 (cls-fn-docstring (format "Execute %s when idle" fn-name)))
            `(progn
               (defcustom ,timeout-var-sym ,default-timeout ,timer-docstring :type 'number)
               (defvar ,timer-sym nil)
               (defun ,cls-fn (&rest args) ,cls-fn-docstring
                      (-some->> ,timer-sym (cancel-timer))
                      (setq ,timer-sym
                            (run-with-idle-timer ,timeout-var-sym nil
                                                 ;; don't know if we have to
                                                 ;; pass around args in this
                                                 ;; clumsy manner
                                                 (lambda (args)
                                                   (apply #',fn args)
                                                   (setq ,timer-sym nil)) args))))))

;; defines a function named coalescing-some-function that (I hope) accepts the
;; same arguments as some-function and executes after 0.2 seconds of idle time
;; (user-configurable via coalescing-some-function-timeout)
;; Also, something like this might already exist in Emacs's standard library?
(make-coalescing some-function 0.2)

[topisani]

hmm, dont really think that's necessary - we should just expand the above patch a bit (mainly to not refresh the entire buffer each time, we need to collect the required args from all the didChange events we have skipped), and then get it added to lsp-mode. I think this is the only function where we will really need this

[sebastiansturm]

this issue also seems to affect lsp-ui-doc--make-request, at least on my machine. Can someone confirm this?

[MaskRay]

Do you mean lsp-ui-doc--make-request is also slow?

[sebastiansturm]

I don't think it's slow, I assume it is just called far too often. It figured heavily in the profiler report, so I made it use a timer instead and the problem seemed to go away (doc display still works). Didn't do systematic benchmarks though

[Fuco1]

There's definitely some slowness in the lsp-ui-mode. And by some I mean when I type there is a 1-2 second delay between letters. I don't know which feature is doing it (how can I check?) but after looking at the profiler report there was a lot of time spent in lsp-on-change. I'm using the php lang server.

After disabling lsp-ui-mode everything is super responsive again.

[yyoncho]

@Fuco1 can you post the perf report?

[codesuki]

To give this issue here some new life... I'd really like to use this plugin but enabling lsp-ui-mode in a go buffer makes my emacs slow to a crawl. Just going up down with C-p C-n is almost unbearably slow.

edit: I noticed that running emacs in a small window does not result in this very dramatic slowdown. My normal use is in fullscreen where it's impossible to use.

[zhenwenc]

If you still experiencing slowness, try switch to flycheck-posframe.

btw, this is the report I have profiled just a few (< 5) minutes, with rust:

[garyo]

I'm also seeing real slowness (1-2 sec delay to echo chars) with lsp-ui. My profile run starts with lsp-ui-doc--make-request which is also called on every textDocument/didChange event on every keystroke. I tried @topisani 's patch, which does speed things up, but I get errors ("void function orig-func") using that, and it also sends the whole document (as @topisani noted). Anyone working on this?

[sebastiansturm]

I haven't had performance issues with flycheck(-ui) in a very long time now though I sometimes sabotage header files in template-heavy C++ projects so it shouldn't be for a lack of errors. I'd try setting lsp-log-io to t, opening the workspace log on the side and seeing what (if any) lsp traffic is generated by the actions you perceive as slow.

Maybe post the results of your profiling run, and also the output of lsp-describe-session (make sure to open the Capabilities bullet point, that should list among other things the currently used sync method).

Also, severe performance problems can be caused by large numbers of overlays; though for flycheck to suffer from this, the files in question would have to contain a rather extreme number of errors. If you think this could be it, try evaluating (length (overlays-in (point-min) (point-max))). Beyond a few thousand or so, lag can get noticeable (this will be fixed by a new implementation of overlays which will probably find its way into Emacs master some time after the 27 release, cf. https://lists.gnu.org/archive/html/emacs-devel/2019-09/msg00544.html)

[garyo]

I can't run lsp-describe-session: I always get an error "Invalid image file name 'xpm'". This is with a recent nightly emacs 27 on Windows. \<goes to check on why> OK, I figured that out -- there's a var I have to set to turn off images in tree widgets. Sigh. See below. Yes, there's lots of traffic on every keystroke; in this file, there's a few thousand possible completions sent each time lsp asks, maybe around 300kB of JSON. Horrifying, yes, but VS Code can keep up...

There's only 4 overlays in my test file, so I don't think that's it.

[-] c:/tmp/
 `-[-] vls:4044
    |-[-] Buffers
    |  |-[+] test.vue
    |  `-[+] KeyframeGraph.vue
    `-[-] Capabilities
       |-[X] colorProvider: t
       |-[X] codeActionProvider: t
       |-[X] referencesProvider: t
       |-[X] definitionProvider: t
       |-[X] documentSymbolProvider: t
       |-[-] documentLinkProvider:
       |  `-[X] resolveProvider: nil
       |-[X] documentHighlightProvider: t
       |-[X] hoverProvider: t
       |-[X] documentFormattingProvider: nil
       |-[-] signatureHelpProvider:
       |  `-[X] triggerCharacters: [(]
       |-[-] completionProvider:
       |  |-[X] triggerCharacters: [. : < " ’ / @ *]
       |  `-[X] resolveProvider: t
       `-[X] textDocumentSync: 1

[sebastiansturm]

so lsp is using full sync here (textDocumentSync: 1); guess vls doesn't support anything else. While that's not optimal, I'd expect debouncing to still keep editing latency acceptable. Maybe check what lsp-debounce-full-sync-notifications and lsp-debounce-full-sync-notifications-interval are set to. If that doesn't provide a useful clue, what does your profile report look like?

[jcs090218]

In my case, it was flycheck-pos-tip. It's quite slow to show pos-tip for some reason. 😕