slowness scrolling in *Help* windows

slowness scrolling in *Help* windows

[Perry E. Metzger]

Running a recent version of the trunk I've compiled on Mac OS X

(aka GNU Emacs 24.4.50.1 (x86_64-apple-darwin13.3.0, NS
appkit-1265.21 Version 10.9.4 (Build 13E28)) of 2014-09-04 on
jabberwock.cb.piermont.com)

I've been noticing that sometimes scrolling in *Help* gets very, very
slow sometimes. Typical case: C-h s (that is, view the syntax table),
and scroll through it, and it can take seconds for the screen to do
something after C-v

It is possible this is only happening if there are loads of "unusual"
Unicode characters in the buffer in question. I don't have any good
very long files of lots of Unicode characters to check if it is
happening on all buffers, or just *Help* buffers.

Is this a known problem? Should I file a bug report?

-- 
Perry E. Metzger		perry@piermont.com

Re: slowness scrolling in *Help* windows

[Andreas Schwab]

"Perry E. Metzger" <perry@piermont.com> writes:

> I've been noticing that sometimes scrolling in *Help* gets very, very
> slow sometimes. Typical case: C-h s (that is, view the syntax table),
> and scroll through it, and it can take seconds for the screen to do
> something after C-v

Emacs needs to look up the fonts for the characters in the buffer.  That
can take some time depending on the number of installed fonts.

> It is possible this is only happening if there are loads of "unusual"
> Unicode characters in the buffer in question. I don't have any good
> very long files of lots of Unicode characters to check if it is
> happening on all buffers, or just *Help* buffers.

It happens for any buffer that contains many different characters from
unusual scripts.  Try scrolling through the buffer generated by
mule-diag for an extreme example.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: slowness scrolling in *Help* windows

[Perry E. Metzger]

On Mon, 08 Sep 2014 20:05:11 +0200 Andreas Schwab
<schwab@linux-m68k.org> wrote:
> "Perry E. Metzger" <perry@piermont.com> writes:
> 
> > I've been noticing that sometimes scrolling in *Help* gets very,
> > very slow sometimes. Typical case: C-h s (that is, view the
> > syntax table), and scroll through it, and it can take seconds for
> > the screen to do something after C-v
> 
> Emacs needs to look up the fonts for the characters in the buffer.
> That can take some time depending on the number of installed fonts.

I can believe that this is part of the problem, but it seems like it
is anomalously long under the circumstances. It should not take
seconds on a modern processor to perform this action. Perhaps
something rather inefficient is going on, like a bad choice of a
search algorithm?

Perry
-- 
Perry E. Metzger		perry@piermont.com

RE: slowness scrolling in *Help* windows

[Drew Adams]

> I've been noticing that sometimes scrolling in *Help* gets very,
> very
> slow sometimes. Typical case: C-h s (that is, view the syntax
> table),
> and scroll through it, and it can take seconds for the screen to do
> something after C-v
> 
> It is possible this is only happening if there are loads of
> "unusual"
> Unicode characters in the buffer in question. I don't have any good
> very long files of lots of Unicode characters to check if it is
> happening on all buffers, or just *Help* buffers.
> 
> Is this a known problem? Should I file a bug report?

FWIW, I see this also.