26.2; Strange slowness in some Info nodes

26.2; Strange slowness in some Info nodes

[ndame]

I don't know if it's a known bug or some system dependent issue
that's why I'm sending it to help.

I recently moved to emacs 26 and today I noticed a strange
scrolling slowdown in certain Info nodes.

It also occurs with emacs -q.

If I go to Elisp info and try to scroll in the main index with
page up/down then everything is fine and quick.

If I go to Lists and then to Building Lists and there try to
scroll down with page down, then the scrolling becomes slow, like
it takes one second for emacs to scroll the page after pressing
page down/up.

The problem seems to occur only in info nodes where there are
separate sections describing entities like functions in Building
Lists:

    -- Function: cons object1 object2
    This function is the most basic function for building new list
    structure.  It creates a new cons cell, making OBJECT1 the CAR,
    and OBJECT2 the CDR.  It then returns the new cons cell.  The
    arguments OBJECT1 and OBJECT2 may be any Lisp objects, but most
    often OBJECT2 is a list.

In nodes with free flow text there is no problem with scrolling, so
it's like info does something extra in entity describing nodes.

It's 26.2 on Windows taken from the official GNU site:

  GNU Emacs 26.2 (build 1, x86_64-w64-mingw32) of 2019-04-13

The problem also occurred with 26.1  I upgraded to 26.2. but it's
the same there.

Does anyone else experience this? Is this a known issue?
 

Re: 26.2; Strange slowness in some Info nodes

[Eli Zaretskii]

> Date: Fri, 2 Aug 2019 15:37:52 +0000 (GMT)
> From: ndame <emacsuser@freemail.hu>
> 
> I recently moved to emacs 26 and today I noticed a strange
> scrolling slowdown in certain Info nodes.
> 
> It also occurs with emacs -q.
> 
> If I go to Elisp info and try to scroll in the main index with
> page up/down then everything is fine and quick.
> 
> If I go to Lists and then to Building Lists and there try to
> scroll down with page down, then the scrolling becomes slow, like
> it takes one second for emacs to scroll the page after pressing
> page down/up.

Crystal ball says it's related to the fonts you use.  Does setting
inhibit-compacting-font-caches to a non-nil value help?

Re: 26.2; Strange slowness in some Info nodes

[ndame]

> Crystal ball says it's related to the fonts you use.  Does setting
> inhibit-compacting-font-caches to a non-nil value help?

Yes, thank you. With that set to t the scrolling slowness disappears.

The strange thing is I tested the problem with emacs -q, so it
didn't use any of my usual fonts, it picked up only the windows
system default fonts.

I checked the affected info nodes and I don't see any special
formatting or fonts there compared to other info pages which were
quick to scroll.

The only apparent thing is the symbol names. Those use the
Info-quoted face and curly quotes :

  Compare ‘make-list’ with ‘make-string’ ..

Maybe those cause the slowdown.
 

Re: 26.2; Strange slowness in some Info nodes

[Joe_Trivers]

> > Crystal ball says it's related to the fonts you use.  Does setting
> > inhibit-compacting-font-caches to a non-nil value help?
>
> Yes, thank you. With that set to t the scrolling slowness disappears.
>
> The strange thing is I tested the problem with emacs -q, so it
> didn't use any of my usual fonts, it picked up only the windows
> system default fonts.
>
> I checked the affected info nodes and I don't see any special
> formatting or fonts there compared to other info pages which were
> quick to scroll.
>
> The only apparent thing is the symbol names. Those use the
> Info-quoted face and curly quotes :
>
>   Compare ‘make-list’ with ‘make-string’ ..
>
> Maybe those cause the slowdown.

I get the same thing.
What's more, it seems to depend on the location in the file being displayed.
That info file is quite large, almost 4MB, and the "Building Lists" node is at line 6307.
Paging up and down after C-x n w (widen) still takes a lot of time, depending on the exact location in the file.
Moving to the middle of the file (M-g M-g 45000) and paging down is effectively instantaneous.
Repeatedly paging back up is also instantaneous, until I get to the page at line 44953, which takes half a second or more. The location in the file seems to be consistent.
That page contains a '⇒' character, as do essentially all the pages in the "Building Lists" section. Setting inhibit-compacting-font-caches to t fixes it for me, too. So I guess it's unrelated to file size. How about that.
I'm on GNU Emacs 26.1 (build 1, x86_64-w64-mingw32) of 2018-05-30.
/Joe
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Re: 26.2; Strange slowness in some Info nodes

[Eli Zaretskii]

> Date: Fri, 2 Aug 2019 19:24:14 +0000 (GMT)
> From: ndame <emacsuser@freemail.hu>
> 
> Yes, thank you. With that set to t the scrolling slowness disappears.
> 
> The strange thing is I tested the problem with emacs -q, so it
> didn't use any of my usual fonts, it picked up only the windows
> system default fonts.

If you have additional fonts installed, beyond those which came with
Windows out of the box, Emacs will examine them even in "emacs -Q".
When Emacs needs to find a font for some character, it enumerates all
the fonts installed on the system, and then examines them to find the
one that can actually display the character.  This happens even
without non-default customizations.

Re: 26.2; Strange slowness in some Info nodes

[ndame]

> If you have additional fonts installed, beyond those which came with
> Windows out of the box, Emacs will examine them even in "emacs -Q".
> When Emacs needs to find a font for some character, it enumerates all
> the fonts installed on the system, and then examines them to find the
> one that can actually display the character.  This happens even
> without non-default customizations.

Then that must be it, because recently I chose a new emacs base
font and installed several fonts to try them out. I currently
have 196 fonts installed.

Though I don't really get why repeated scrolling down is
slow. I'd think the first one is slow when emacs finds the font
for the character and then subsequent scroll downs should be
quick, because then the font for the character is known. 

It would be strange if the cached information were purged from
memory right after it was found and emacs had to find the font
again for the next scroll down.

Re: 26.2; Strange slowness in some Info nodes

[Eli Zaretskii]

> Date: Fri, 2 Aug 2019 20:23:06 +0000 (GMT)
> From: ndame <emacsuser@freemail.hu>
> 
> Though I don't really get why repeated scrolling down is
> slow. I'd think the first one is slow when emacs finds the font
> for the character and then subsequent scroll downs should be
> quick, because then the font for the character is known. 

It is known, but that knowledge can be forgotten.

> It would be strange if the cached information were purged from
> memory right after it was found and emacs had to find the font
> again for the next scroll down.

It is purged during GC.  At the GC level, there's no information
available that could tell Emacs not to purge a font (the "right after"
part is perhaps evident to a human, but not to a program).  The basic
problem with this situation is that some very large fonts cause very
frequent GC, which evicts those large fonts, which then cause a lot of
consing when they are again loaded, which again causes GC soon, etc.