GNU bug report logs - #78773
[PATCH] Speedup url-retrieve-synchronously for low-latency connections

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Steven Allen <steven <at> stebalien.com>
To: bug-gnu-emacs <at> gnu.org
Cc: Lars Ingebrigtsen <larsi <at> gnus.org>, dick <dick.r.chiang <at> gmail.com>
Subject: [PATCH] Speedup url-retrieve-synchronously for low-latency connections
Date: Wed, 11 Jun 2025 21:08:45 -0700

[Message part 1 (text/plain, inline)]

Tags: patch


**Present context:**

I'm running into an issue in emacs-syncthing [1] where making a few
localhost network requests are taking a full second (blocking Emacs)
when they should be instantaneous. While digging into
`url-retrieve-synchronously', I found that waiting on the correct
network process instead of passing nil to `accept-process-output' made
these network requests instantaneous (although I have no idea why). See
the attached patch.

[1]: https://github.com/KeyWeeUsr/emacs-syncthing

To test this patch, you can:

1. Run a simple web server on localhost. I usually use

    python -m http.server --bind 127.0.0.1 8000

2. Evaluate:  (benchmark 100 '(url-retrieve-synchronously "http://127.0.0.1:8000"))

With this patch, this form is ~16x faster on my machine. I've also
tested this against a remote machine with a ~45ms latency and found a
1.25x speedup.

I've confirmed that this isn't busy-waiting by modifying this code to
print a message each time it loops: it loops the same number of times
with or without my patch.

I've confirmed that this speedup is strictly due to passing the target
process to `accept-process-output' by applying my patch then changing
JUST "proc" to "nil":

                    ;; ms, so split the difference.
                    (accept-process-output proc 0.05))

to

                    ;; ms, so split the difference.
                    (accept-process-output nil 0.05))

With this one change, this code goes back to being as slow as it was before.

**Historical context**

`url-retrieve-synchronously' was changed to wait on "nil" in:

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=49897,

Motivated by this bug report:

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=49861

However, the patch in question also changed the rest of
`url-retrieve-synchronously' so I'm hoping the issue lies elsewhere?

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, cairo version
 1.18.4) of 2025-06-11 built on Laptop
Repository revision: 87244ef1661f9e9d7c08f65047551e8a34cd92b2
Repository branch: makepkg
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: Arch Linux

Configured using:
 'configure
 'CPPFLAGS=-I/run/user/1000/build/emacs-git/src/mps-git/build/include '
 'LDFLAGS=-L/run/user/1000/build/emacs-git/src/mps-git/build/lib -Wl,-O1
 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now
 -Wl,-z,pack-relative-relocs -flto=auto' --prefix=/usr --sysconfdir=/etc
 --libexecdir=/usr/lib --localstatedir=/var --mandir=/usr/share/man
 --with-gameuser=:games --with-modules --without-m17n-flt
 --without-selinux --without-pop --without-gconf --disable-gc-mark-trace
 --with-mps=yes --enable-link-time-optimization
 --with-native-compilation=yes --with-xinput2 --with-x-toolkit=no
 --without-toolkit-scroll-bars --without-xaw3d --without-gsettings
 --with-cairo-xcb --without-xft --with-sound=no --with-tree-sitter
 --without-gpm --without-compress-install
 '--program-transform-name=s/\([ec]tags\)/\1.emacs/'
 'CFLAGS=-march=native -mtune=native -O3 -pipe -fno-plt -fexceptions
 -Wp,-D_FORTIFY_SOURCE=3 -Wformat -Werror=format-security
 -fstack-clash-protection -fcf-protection -fomit-frame-pointer
 -fno-math-errno -fno-trapping-math -fno-math-errno -fno-trapping-math
 -flto=auto''

[0001-Speedup-url-retrieve-synchronously-for-low-latency-c.patch (text/patch, attachment)]

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