bug#59265: 27.1; emacs apparently redraws the entire X window for every update

bug#59265: 27.1; emacs apparently redraws the entire X window for every update

[Francesco Potortì]

This bug report is vague, because I don't know the details yet, but I should be able to produce them later.

When using a remote window using Xpra (which is more or less equivalent to Screen for X, see xpra.org) the amount of information sent by Emacs is huge, apparently redrawing the entire window at every keystroke.

I discovered this while reporting an Xpra bug and Xpra's author commented on this, see https://github.com/Xpra-org/xpra/issues/3660

While I work with him to solving the bug, I will try to get more info on the matter, assuming this is not a known issue.  If the problem is real, that makes Emacs resource hungry when run on remote X servers.



In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2021-11-27, modified by Debian built on x86-csail-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Debian GNU/Linux bookworm/sid

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bug#59265: 27.1; emacs apparently redraws the entire X window for every update

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

Francesco Potortì <pot@gnu.org> writes:

> This bug report is vague, because I don't know the details yet, but I
> should be able to produce them later.
>
> When using a remote window using Xpra (which is more or less
> equivalent to Screen for X, see xpra.org) the amount of information
> sent by Emacs is huge, apparently redrawing the entire window at every
> keystroke.
>
> I discovered this while reporting an Xpra bug and Xpra's author
> commented on this, see https://github.com/Xpra-org/xpra/issues/3660
>
> While I work with him to solving the bug, I will try to get more info
> on the matter, assuming this is not a known issue.  If the problem is
> real, that makes Emacs resource hungry when run on remote X servers.

Emacs generally does not redraw the entire X window upon an update.

Judging by the debug info provided in the Xpra bug report, it is relying
on the Damage extension to report window damage.  The Damage extension
always reports damage to the entire window upon a DBE buffer swap, which
is almost inconsequential when connected to an actual remote X server (I
do that every day), but not when a program like Xpra tries to ferry
pixmap contents over the wire.

Relying on Composite and Damage to send application contents over the
wire is fundamentally the wrong approach.  I suspect Xpra has to be
written as an X server driver (as its name applies), and that after
doing so, its DBE implementation has to be fixed to only call the
CopyArea GC op on areas of the back buffer that really changed.

So this isn't really a bug in Emacs, sorry.  You can work around the
problem by turning off double buffering, but that means you will also
need to put up with flicker.

bug#59265: 27.1; emacs apparently redraws the entire X window for every update

[Eli Zaretskii]

> From: Francesco Potortì <pot@gnu.org>
> Date: Mon, 14 Nov 2022 17:39:20 +0100
> 
> This bug report is vague, because I don't know the details yet, but I should be able to produce them later.
> 
> When using a remote window using Xpra (which is more or less equivalent to Screen for X, see xpra.org) the amount of information sent by Emacs is huge, apparently redrawing the entire window at every keystroke.

Emacs redraws only the minimum that is required to update its windows
on the glass.  It actually compares the current window contents with
the desired contents, and redraws only where they differ.  This should
result in very small updates, and the only situation for which I can
imagine Emacs updating the entire frame is when the whole frame is
different, or it was obscured by another window, or something similar.

If all you do is type one character, basically only that character's
line will be redrawn, in some case even less.  As an extreme example,
if the line's contents was

   xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

and you type "y" in the middle, you should see one "x" redrawn as "y",
and one "x" drawn at the end of the line, and that's all.

Of course, there could also be updates of other parts of the Emacs
display, such as the mode line.  For example, if you have
column-number-mode enabled, typing a character will also update the
mode line.  But again, it should only redraw a small part of the mode
line near the column number.  Likewise, if what you type affects the
tyool bar or the menu bar, these will be redrawn.

One thing I can suggest to try is disable double-buffering.  I have no
idea what it can do with Xpra (since I don't really know how does Xpra
do its job, whatever that is).

bug#59265: 27.1; emacs apparently redraws the entire X window for every update

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

Eli Zaretskii <eliz@gnu.org> writes:

> One thing I can suggest to try is disable double-buffering.  I have no
> idea what it can do with Xpra (since I don't really know how does Xpra
> do its job, whatever that is).

Yes, I explained what the problem is (and it isn't in Emacs) earlier:

Judging by the debug info provided in the Xpra bug report, it is relying
on the Damage extension to report window damage.  The Damage extension
always reports damage to the entire window upon a DBE buffer swap, which
is almost inconsequential when connected to an actual remote X server (I
do that every day), but not when a program like Xpra tries to ferry
pixmap contents over the wire.

Relying on Composite and Damage to send application contents over the
wire is fundamentally the wrong approach.  I suspect Xpra has to be
written as an X server driver (as its name applies), and that after
doing so, its DBE implementation has to be fixed to only call the
CopyArea GC op on areas of the back buffer that really changed.

So this isn't really a bug in Emacs, sorry.  You can work around the
problem by turning off double buffering, but that means you will also
need to put up with flicker.

bug#59265: 27.1; emacs apparently redraws the entire X window for every update

[Francesco Potortì]

Po, Eli,

thanks you for the explanation.  Hope I will manage to channel these to Xpra's developers.

bug#59265: 27.1; emacs apparently redraws the entire X window for every update

[Stefan Kangas]

tags 59265 + notabug
close 59265
thanks

Po Lu <luangruo@yahoo.com> writes:

> So this isn't really a bug in Emacs, sorry.  You can work around the
> problem by turning off double buffering, but that means you will also
> need to put up with flicker.

I'm therefore closing this bug report.