bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Stephane Travostino]

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Heavy operations, such as scrolling back and forth in a buffer, are
noticeably laggier, for lack of better word, in the PGTK/Wayland version
than the X11, both tested on KDE in Wayland mode. 

Affects both 29.2 and the latest HEAD compiled a few days ago.

I am unsure whether it is a KDE or Emacs problem.

Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
scaling.

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.43, cairo version 1.18.0) of 2024-08-29 built on
toolbox.tranquility
Repository revision: b6f4ffcc106fdbc21dfea45c75fdc4f217d8f201
Repository branch: makepkg
Windowing system distributor 'The X.Org Foundation', version 11.0.12401002
System Description: Arch Linux

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bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Eli Zaretskii]

> Date: Mon, 02 Sep 2024 10:18:03 +0100
> From: "Stephane Travostino" <sph@combo.cc>
> 
> Heavy operations, such as scrolling back and forth in a buffer, are
> noticeably laggier, for lack of better word, in the PGTK/Wayland version
> than the X11, both tested on KDE in Wayland mode. 
> 
> Affects both 29.2 and the latest HEAD compiled a few days ago.
> 
> I am unsure whether it is a KDE or Emacs problem.
> 
> Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
> scaling.

AFAIU, this is a problem with GTK input methods.  From PROBLEMS:

  *** Emacs built with GTK lags in its response to keyboard input.
  This can happen when input methods are used.  It happens because Emacs
  behaves in an unconventional way with respect to GTK input methods: it
  registers to receive keyboard input as unprocessed key events with
  metadata (as opposed to receiving them as text strings).  Most GTK
  programs use the latter approach, so some modern input methods have
  bugs and misbehave when faced with the way Emacs does it.

  A workaround is to set GTK_IM_MODULE=none in the environment, or maybe
  find a different input method without these problems.

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Stephane Travostino]

On Mon, 2 Sep 2024, at 13:05, Eli Zaretskii wrote:
>> Date: Mon, 02 Sep 2024 10:18:03 +0100
>> From: "Stephane Travostino" <sph@combo.cc>
>> 
>> Heavy operations, such as scrolling back and forth in a buffer, are
>> noticeably laggier, for lack of better word, in the PGTK/Wayland version
>> than the X11, both tested on KDE in Wayland mode. 
>> 
>> Affects both 29.2 and the latest HEAD compiled a few days ago.
>> 
>> I am unsure whether it is a KDE or Emacs problem.
>> 
>> Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
>> scaling.
>
> AFAIU, this is a problem with GTK input methods.  From PROBLEMS:
>
>   *** Emacs built with GTK lags in its response to keyboard input.
>   This can happen when input methods are used.  It happens because Emacs
>   behaves in an unconventional way with respect to GTK input methods: it
>   registers to receive keyboard input as unprocessed key events with
>   metadata (as opposed to receiving them as text strings).  Most GTK
>   programs use the latter approach, so some modern input methods have
>   bugs and misbehave when faced with the way Emacs does it.
>
>   A workaround is to set GTK_IM_MODULE=none in the environment, or maybe
>   find a different input method without these problems.

Thank you, though without more scientific methods of measuring latency I can't tell if that helps or not. 

I noticed I had pixel precision scrolling mode on and that contributed a large part to that feeling of lag compared to other programs. If Firefox is able to smooth scroll at 60 Hz, I would say empirically Emacs PGTK would scroll at 15 Hz, making navigation in the buffer a choppy affair. 

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Stephane Travostino]

On Mon, 2 Sep 2024, at 13:12, Stephane Travostino wrote:
> On Mon, 2 Sep 2024, at 13:05, Eli Zaretskii wrote:
>>> Date: Mon, 02 Sep 2024 10:18:03 +0100
>>> From: "Stephane Travostino" <sph@combo.cc>
>>> 
>>> Heavy operations, such as scrolling back and forth in a buffer, are
>>> noticeably laggier, for lack of better word, in the PGTK/Wayland version
>>> than the X11, both tested on KDE in Wayland mode. 
>>> 
>>> Affects both 29.2 and the latest HEAD compiled a few days ago.
>>> 
>>> I am unsure whether it is a KDE or Emacs problem.
>>> 
>>> Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
>>> scaling.
>>
>> AFAIU, this is a problem with GTK input methods.  From PROBLEMS:
>>
>>   *** Emacs built with GTK lags in its response to keyboard input.
>>   This can happen when input methods are used.  It happens because Emacs
>>   behaves in an unconventional way with respect to GTK input methods: it
>>   registers to receive keyboard input as unprocessed key events with
>>   metadata (as opposed to receiving them as text strings).  Most GTK
>>   programs use the latter approach, so some modern input methods have
>>   bugs and misbehave when faced with the way Emacs does it.
>>
>>   A workaround is to set GTK_IM_MODULE=none in the environment, or maybe
>>   find a different input method without these problems.
>
> Thank you, though without more scientific methods of measuring latency 
> I can't tell if that helps or not. 
>
> I noticed I had pixel precision scrolling mode on and that contributed 
> a large part to that feeling of lag compared to other programs. If 
> Firefox is able to smooth scroll at 60 Hz, I would say empirically 
> Emacs PGTK would scroll at 15 Hz, making navigation in the buffer a 
> choppy affair.

Update: GTK_IM_MODULE=none does not make it any less laggier. It is mostly felt in typing and editing source code, and switching to the X11 build makes it immensely snappier and doesn't feel like I'm working through a remote connection.

FYI there are other reports online of people noticing major latency in HiDPI mode with the PGTK version, especially when the frame is fullscreen (so there's more pixels to update):  

https://old.reddit.com/r/emacs/comments/ucv0at/awful_performance_with_pgtk_on_wayland/

https://old.reddit.com/r/emacs/comments/1acdieh/pgtk_emacs_high_input_lag_at_large_frame_sizes_on/

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Eli Zaretskii]

> Date: Tue, 03 Sep 2024 12:27:09 +0100
> From: "Stephane Travostino" <sph@combo.cc>
> Cc: 72960@debbugs.gnu.org
> 
> On Mon, 2 Sep 2024, at 13:12, Stephane Travostino wrote:
> > On Mon, 2 Sep 2024, at 13:05, Eli Zaretskii wrote:
> >>> Date: Mon, 02 Sep 2024 10:18:03 +0100
> >>> From: "Stephane Travostino" <sph@combo.cc>
> >>> 
> >>> Heavy operations, such as scrolling back and forth in a buffer, are
> >>> noticeably laggier, for lack of better word, in the PGTK/Wayland version
> >>> than the X11, both tested on KDE in Wayland mode. 
> >>> 
> >>> Affects both 29.2 and the latest HEAD compiled a few days ago.
> >>> 
> >>> I am unsure whether it is a KDE or Emacs problem.
> >>> 
> >>> Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
> >>> scaling.
> >>
> >> AFAIU, this is a problem with GTK input methods.  From PROBLEMS:
> >>
> >>   *** Emacs built with GTK lags in its response to keyboard input.
> >>   This can happen when input methods are used.  It happens because Emacs
> >>   behaves in an unconventional way with respect to GTK input methods: it
> >>   registers to receive keyboard input as unprocessed key events with
> >>   metadata (as opposed to receiving them as text strings).  Most GTK
> >>   programs use the latter approach, so some modern input methods have
> >>   bugs and misbehave when faced with the way Emacs does it.
> >>
> >>   A workaround is to set GTK_IM_MODULE=none in the environment, or maybe
> >>   find a different input method without these problems.
> >
> > Thank you, though without more scientific methods of measuring latency 
> > I can't tell if that helps or not. 
> >
> > I noticed I had pixel precision scrolling mode on and that contributed 
> > a large part to that feeling of lag compared to other programs. If 
> > Firefox is able to smooth scroll at 60 Hz, I would say empirically 
> > Emacs PGTK would scroll at 15 Hz, making navigation in the buffer a 
> > choppy affair.
> 
> Update: GTK_IM_MODULE=none does not make it any less laggier. It is mostly felt in typing and editing source code, and switching to the X11 build makes it immensely snappier and doesn't feel like I'm working through a remote connection.

Please try profiling the lagging cases with "M-x profiler", and post
the profile here.

Po Lu, any other ideas or suggestions?

> FYI there are other reports online of people noticing major latency in HiDPI mode with the PGTK version, especially when the frame is fullscreen (so there's more pixels to update):  
> 
> https://old.reddit.com/r/emacs/comments/ucv0at/awful_performance_with_pgtk_on_wayland/
> 
> https://old.reddit.com/r/emacs/comments/1acdieh/pgtk_emacs_high_input_lag_at_large_frame_sizes_on/

I don't doubt what you report is real.

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Stephane Travostino]

On Tue, 3 Sep 2024, at 13:52, Eli Zaretskii wrote:
>> Date: Tue, 03 Sep 2024 12:27:09 +0100
>> From: "Stephane Travostino" <sph@combo.cc>
>> Cc: 72960@debbugs.gnu.org
>> 
>> On Mon, 2 Sep 2024, at 13:12, Stephane Travostino wrote:
>> > On Mon, 2 Sep 2024, at 13:05, Eli Zaretskii wrote:
>> >>> Date: Mon, 02 Sep 2024 10:18:03 +0100
>> >>> From: "Stephane Travostino" <sph@combo.cc>
>> >>> 
>> >>> Heavy operations, such as scrolling back and forth in a buffer, are
>> >>> noticeably laggier, for lack of better word, in the PGTK/Wayland version
>> >>> than the X11, both tested on KDE in Wayland mode. 
>> >>> 
>> >>> Affects both 29.2 and the latest HEAD compiled a few days ago.
>> >>> 
>> >>> I am unsure whether it is a KDE or Emacs problem.
>> >>> 
>> >>> Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
>> >>> scaling.
>> >>
>> >> AFAIU, this is a problem with GTK input methods.  From PROBLEMS:
>> >>
>> >>   *** Emacs built with GTK lags in its response to keyboard input.
>> >>   This can happen when input methods are used.  It happens because Emacs
>> >>   behaves in an unconventional way with respect to GTK input methods: it
>> >>   registers to receive keyboard input as unprocessed key events with
>> >>   metadata (as opposed to receiving them as text strings).  Most GTK
>> >>   programs use the latter approach, so some modern input methods have
>> >>   bugs and misbehave when faced with the way Emacs does it.
>> >>
>> >>   A workaround is to set GTK_IM_MODULE=none in the environment, or maybe
>> >>   find a different input method without these problems.
>> >
>> > Thank you, though without more scientific methods of measuring latency 
>> > I can't tell if that helps or not. 
>> >
>> > I noticed I had pixel precision scrolling mode on and that contributed 
>> > a large part to that feeling of lag compared to other programs. If 
>> > Firefox is able to smooth scroll at 60 Hz, I would say empirically 
>> > Emacs PGTK would scroll at 15 Hz, making navigation in the buffer a 
>> > choppy affair.
>> 
>> Update: GTK_IM_MODULE=none does not make it any less laggier. It is mostly felt in typing and editing source code, and switching to the X11 build makes it immensely snappier and doesn't feel like I'm working through a remote connection.
>
> Please try profiling the lagging cases with "M-x profiler", and post
> the profile here.

I don't know how to make a consistent test case. I have tried here to profile opening Emacs (same commit with and without PGTK) on the same 547-line Elixir file, and holding the Down key until it reaches the bottom and then back to the top of the buffer. I have (setopt scroll-conservatively 101) so after the first page the contents are continuously redrawn for every new line. 

The PGTK version feels like it's skipping frames while it's relatively smooth on X11:

X11:        
        8795  86% + redisplay_internal (C function)
        1141  11% + command-execute
          54   0% + direnv--maybe-update-environment
          49   0% + gcmh-register-idle-gc
          42   0% + winner-save-old-configurations
          20   0% + timer-event-handler
          18   0% + ...
          18   0% + jit-lock--antiblink-post-command


PGTK:
        9387  91% + redisplay_internal (C function)
         698   6% + command-execute
          19   0% + ...
          19   0% + timer-event-handler
          12   0% + direnv--maybe-update-environment
          11   0% + winner-save-old-configurations
 
I have run this a few times and in Wayland `redisplay_internal` takes always a few percent more time than on X11, though I am not sure these numbers can prove anything as they are quite close.

Is there some kind of consistent UI benchmark I can run? The frame skipping reminds me of missed vsync deadlines one might experience in games.

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Eli Zaretskii]

> Date: Tue, 03 Sep 2024 16:36:44 +0100
> From: "Stephane Travostino" <sph@combo.cc>
> Cc: 72960@debbugs.gnu.org
> 
> > Please try profiling the lagging cases with "M-x profiler", and post
> > the profile here.
> 
> I don't know how to make a consistent test case. I have tried here to profile opening Emacs (same commit with and without PGTK) on the same 547-line Elixir file, and holding the Down key until it reaches the bottom and then back to the top of the buffer. I have (setopt scroll-conservatively 101) so after the first page the contents are continuously redrawn for every new line. 
> 
> The PGTK version feels like it's skipping frames while it's relatively smooth on X11:
> 
> X11:        
>         8795  86% + redisplay_internal (C function)
>         1141  11% + command-execute
>           54   0% + direnv--maybe-update-environment
>           49   0% + gcmh-register-idle-gc
>           42   0% + winner-save-old-configurations
>           20   0% + timer-event-handler
>           18   0% + ...
>           18   0% + jit-lock--antiblink-post-command
> 
> 
> PGTK:
>         9387  91% + redisplay_internal (C function)
>          698   6% + command-execute
>           19   0% + ...
>           19   0% + timer-event-handler
>           12   0% + direnv--maybe-update-environment
>           11   0% + winner-save-old-configurations
>  
> I have run this a few times and in Wayland `redisplay_internal` takes always a few percent more time than on X11, though I am not sure these numbers can prove anything as they are quite close.

Thanks.  Maybe Po Lu will have some ideas.

> Is there some kind of consistent UI benchmark I can run? The frame skipping reminds me of missed vsync deadlines one might experience in games.

Try this:

  (defun scroll-up-benchmark ()
    (interactive)
    (let ((oldgc gcs-done)
	  (oldtime (float-time)))
      (condition-case nil (while t (scroll-up) (redisplay))
	(error (message "GCs: %d Elapsed time: %f seconds"
			(- gcs-done oldgc) (- (float-time) oldtime))))))

Evaluate this function, then visit a large file, like src/xdisp.c from
the Emacs sources, and invoke "M-x scroll-up-benchmark RET".  It will
show the time it took at the end.  Record the results and compare with
the other configuration.

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> Po Lu, any other ideas or suggestions?
>
>> FYI there are other reports online of people noticing major latency in HiDPI mode with the PGTK version, especially when the frame is fullscreen (so there's more pixels to update):  
>> 
>> https://old.reddit.com/r/emacs/comments/ucv0at/awful_performance_with_pgtk_on_wayland/
>> 
>> https://old.reddit.com/r/emacs/comments/1acdieh/pgtk_emacs_high_input_lag_at_large_frame_sizes_on/
>
> I don't doubt what you report is real.

I proposed a theory in a number of other tickets concerning this lag,
specifically that the GTK 3 toolkit cannot take advantage of hardware
accelerated graphics on Wayland, which produces perceptible delays on
large displays.

bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> Thanks.  Maybe Po Lu will have some ideas.

I mentioned one.  I think a C profiler (e.g. gprof) would provide more
insightful data.