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bug#38644: 26.3; emacs uses 100% CPU with auto-revert-mode


From: Peter Ludemann
Subject: bug#38644: 26.3; emacs uses 100% CPU with auto-revert-mode
Date: Mon, 6 Jan 2020 16:34:19 -0800

Correction -- the "lock-up" I observed might have been due to paging (possibly related to emacs - I'll have to investigate later).
Toggling auto-revert-avoid-polling appeared to reduc emacs CPU usage, according to top(1) - it's a bit difficult to tell because there seems to be some lag in when the command takes effect and my compilation process switches a certain amount between CPU-intensive and IO-intensive. Anyway, CPU seemed to drop from ~60% to ~30% (please take this observation with a large grain of salt).
I'll try more things in the future and report if anything interesting shows up.
- peter

On Mon, 6 Jan 2020 at 16:01, Peter Ludemann <address@hidden> wrote:
If anything, auto-revert-avoid-polling makes responsiveness worse -- the window locked up a few times on me while doing ctrl-N while running a CPU-intensive (and possibly IO-intensive) compilation.
Although I might not have set the value correctly ... I did M-x customise-variable RET auto-revert-avoid-polling RET then "toggle" then "apply".
.

On Sun, 5 Jan 2020 at 11:57, Mattias Engdegård <address@hidden> wrote:
5 jan. 2020 kl. 20.31 skrev Peter Ludemann <address@hidden>:

> Which version of Emacs would you like me to try this with? And what result are you expecting/hoping to see? (e.g., might it reduce the current 30-80% CPU load for polling with emacs 28.0.50?) There are only a few open files directly under /tmp, so would this have any effect or does it propagate down to subdirectories?
>
> [Also, I'd need a few more details (not being an emacs-internals person) ... should I add this to my .emacs and restart, or execute in a scratch buffer, or ...?]

'auto-revert-avoid-polling' is a single global customisable variable, so you would set it using

 M-x customise-variable RET auto-revert-avoid-polling RET

, then turn it on and apply the change. I believe it was introduced in Emacs 27.

The idea is to save CPU by not having to look at files periodically to see if they have changed. I have no idea if you would see any improvement at all, but it shouldn't make anything worse.


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