Re: Strange slowness when killing words interactively

[Taylor Venable]

On Tue, Apr 26, 2011 at 21:30, Taylor Venable <taylor@metasyntax.net> wrote:
> Hi, I'm having a strange problem with C-<backspace>
> (backward-kill-word) and looking for some help in trying to debug it.
> When I use this keystroke, the CPU usage spikes. The same goes for
> C-<delete> (forward-kill-word) or any other key that I bind to either
> of these functions. Other functions, such as backward-kill-sexp and
> backward-kill-sentence, are similarly problematic. As a result, when I
> hold the key down, my high repeat rate makes Emacs unresponsive for a
> second until whatever is slowing down catches up. It happens with the
> GTK GUI, but not with the text user interface. None of my other
> machines have this behaviour with the same version of Emacs (that
> includes one machine which is running the same distribution [Arch
> Linux] with the same packages). No other programs on this system are
> affected by performance problems when deleting words of text from a
> block. The problem occurs with and without using -q. There does not
> seem to be the same problem if I run the kill function itself with
> C-u; for example, C-u 1000 backward-kill-word is instant. What's the
> best way to profile this and figure out where my CPU cycles are being
> eaten? Thanks for any help, my version information is below. I built
> from fully updated bzr trunk tonight.
>
> M-x emacs-version
> GNU Emacs 24.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.22.1) of
> 2011-04-26
>
> gcc --version
> gcc (GCC) 4.6.0 20110415 (prerelease)
>
> uname -a
> Linux system76 2.6.38-ARCH #1 SMP PREEMPT Sun Apr 17 15:18:58 CEST
> 2011 x86_64 Intel(R) Core(TM) i7 CPU 870 @ 2.93GHz GenuineIntel
> GNU/Linux

(Adding emacs-devel since I've started looking at the source code.)

I've found the location where the slowness creeps into kill-word and
friends. Looking at kill-region in simple.el, the part that is very
slow for my system is adding to the kill ring. If I comment those
lines out (as shown in http://paste.lisp.org/+2LWP) then the sluggish
response disappears. It's odd to me that I don't see this behaviour
when I start Emacs with -nw as I would (perhaps naively) think that
slowness in kill-region would be independent of what user interface is
active.

-- 
Taylor C. Venable
http://metasyntax.net/