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x_display_ok() is slow over using SSH X11 forwarding

From: Michael Stapelberg
Subject: x_display_ok() is slow over using SSH X11 forwarding
Date: Wed, 2 Sep 2020 15:48:40 +0200


Like undoubtedly many of you, I started Working From Home a while ago and will continue to do so for some time.

I am using Emacs over SSH’s X11 forwarding, which—for various reasons—is the best setup for me, and yes, I have already tried numerous others (TRAMP, remote desktop, nx, xpra, …).

The one issue I have with my setup is that Emacs takes a noticeably long time to open a new frame (X11 window).

I have discovered that Emacs opens 4 (!) X11 connections when a new Emacs process opens a frame: https://twitter.com/zekjur/status/1293892602274746370

I then measured how long opening an X11 connection takes, and found that when using SSH X11 forwarding, opening an X11 connection is really slow (≈120ms) compared to locally (≈0.5ms): https://twitter.com/zekjur/status/1296763221936939010

It turns out that 2 of the 4 connections are made by x_display_ok(), which can easily be short-circuited to verify: https://twitter.com/zekjur/status/1298002575267110919. Setting NO_AT_BRIDGE=1 removes the third connection.

Now, entirely short-circuiting that function is a big hammer, and probably we don’t want to break existing users of x_display_ok(), but I do wonder what the best way to remove these unnecessary connection attempts is?

From a high-level perspective, the current behavior is unnecessary because:

1. If the X11 $DISPLAY variable is not okay, I’ll get an error message anyway.

2. Just because $DISPLAY worked at the time of check doesn’t mean it’ll work at time of use.

3. Even if checking is desired, there is no need to check twice during startup, and we should retain the connection used for checking so that we don’t need to connect over and over.

Any thoughts?


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