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bug#5099: 23.1; Performance problems in Emacs 23?

From: Stefan Monnier
Subject: bug#5099: 23.1; Performance problems in Emacs 23?
Date: Wed, 02 Dec 2009 09:58:55 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1.50 (gnu/linux)

> However, having tried Emacs 23 for an hour, I have noticed very much
> the same phenomenon. My main mail file is roughly 100MB large.
> It takes Emacs 22 only about 5 seconds to save it, and the auto-save
> pause is probably half that long.  (I use a reasonably fast Solid
> State Drive.) The CPU utilisation is an A-shaped curve, implying that
> there is nothing much for the CPU to do. In contrast, Emacs 23 takes
> about more than 20 seconds to save the same mail file and the CPU
> utilisation is a pretty intensive burn for most of that time (20% on
> one processor and 60% on a second processor).  The auto-save pauses
> are about 10-15 seconds long.

> Now, VM mail files are raw-text.  Saving them should be an I/O-bound
> process.  It is not clear why Emacs 23 should be burning CPU cycles to
> save raw-text files and taking 3-5 times as long as Emacs 22.

That's indeed surprising and I can't think of anything that could cause
such a major slowdown.  Maybe if the VM mail buffers (containing
"raw-text") are left in multibyte mode, we do something inefficient
somehow (but then again, it'd be surprising for those buffers to be in
multibyte mode anyway).


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