Re: Scrolling xdisp.c. Room for optimisation in syntax.c/CC Mode.

[Alan Mackenzie]

On Sat, Oct 18, 2014 at 06:19:38PM +0000, Alan Mackenzie wrote:
> Hi, Emacs.

> I've measured the time Emacs takes to scroll a large C file, and how much
> of this is due to the inefficiency in backwards `scan-lists's when
> comments contain unbalanced string characters (with
> open-paren-in-column-0-is-defun-start nil).

[ ... ]

> On my set up, a Linux virtual terminal with a window 65 lines high, I get
> the following results:
>     no-odd-xdisp.c: 492 scrolls, total time = 42.2749125957489s
>            xdisp.c: 492 scrolls, total time = 69.40998315811157s.

> 69.4100 / 42.2749 = 1.642.  The original thus takes 64% longer (on
> average) for a backward scroll operation.

I forgot to mention that this was on Emacs 24.3, with full .emacs loaded,
and with an up-to-date stand-alone CC Mode.

> It would seem worthwhile to consider optimising the code to eliminate
> this 64%.  Two possibilities suggest themselves: (i) in syntax.c, by
> making use of the syntax-ppss cache (or similar); (ii) In CC Mode, by
> setting syntax-table text properties on unbalanced string quotes.

I've just repeated the measurement on the Emacs trunk, with emacs -Q.  I
now get these results:
    no-odd-xdisp.c: 500 scrolls, total time = 104.21487808227539s.
           xdisp.c: 500 scrolls, total time = 167.73530864715576s.

I don't know why there are more scroll operations needed.  But the fact
that scrolling is taking two and a half times longer in the trunk than in
24..3 is noteworthy, and possibly disturbing.  I'm think I compiled both
version with the same optimisation level, etc.
    
> -- 
> Alan Mackenzie (Nuremberg, Germany).