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RE: Performance problems (CPU 100%) with NULs in files


From: Ludwig, Mark
Subject: RE: Performance problems (CPU 100%) with NULs in files
Date: Sat, 24 Sep 2011 00:20:24 +0000

> From: Eli Zaretskii
> Subject: Re: Performance problems (CPU 100%) with NULs in files
> 
> > From: "Ludwig, Mark" <address@hidden>
> > Thread-Topic: Performance problems (CPU 100%) with NULs in files
> > Date: Wed, 21 Sep 2011 21:08:42 +0000
> >
> > What happens is that as I scroll through the file, when the NULs are
> visible, Emacs gets into some intensive processing for a long time
> (minutes, sometimes!).  It eventually unwinds and repaints the display,
> but any movement of point sends it into this loop again.  I have found
> that M-< or M-> will quickly reposition away from the problem (assuming
> the beginning and/or end of the file do not contain NULs).  Most other
> movement operations send it into the loop.
> 
> Does it help to visit such files without code conversions, i.e.
> 
>   M-x find-file-literally RET FILENAME RET
> 
> ?
> 
> If not, please file a bug report and attach to it an example file that
> causes this slowdown.

Thanks for the advice, but I have investigated and decided this is probably too 
unusual to expect any "fix" for it.

What I have found is that the "problem" is due to a "line" of text being 
extremely long.  In the test file I have, it is ~800,000 characters (bytes).  
(It came to me with NULs, but I can replace those with any other printable 
character and get the same result.)

What I find is that some movement actions are rather slow -- take 4-7 seconds 
-- while others are extremely quick.  Specifically, the "forward" movement 
actions (C-e, M-f) are slow, while the "backward" movement actions (C-a, M-b) 
are instantaneous.  Reposition (C-l) is also slow, as are the line-oriented 
commands (C-p, C-n).

Thinking through the magnitude of the oddity, I don't think it would be 
reasonable to expect Emacs to handle this any better than it does.  It's just 
gratifying that C-g works, so I can interrupt it when I stumble into some junk, 
and now that I know which actions are fast and slow, I can work around it.

OTOH, if you guys really think this is worth asking any developer to fix, I'll 
file a bug report.  I don't need to send any data, because it's easy to 
reproduce this behavior starting with an empty buffer.

Thanks!

Mark




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