Feature Request: cache syntax highlighting to improve 'relativenumber' scroll performance

[bluz71]

Hello Vim team,

Spawning a new feature request from the existing Ruby high CPU usage #282 issue. Note, the vim-ruby #243 issue is fundamentally the same issue as well.

Basically the issue boils down to syntax highlighting needing to be calculated for every visible line when scrolling up or down with relativenumber enabled. The more lines visible and the slower the host CPU the worse the performance hit especially for languages with complex syntax highlighters such as Ruby.

In issue #282 Bram said this (Dec 2015):

When moving the cursor around, with 'relativenumber' or 'cursorcolumn'
set, cause a full redraw, because every line changes. I'm wondering if
it is possible to cache the result of syntax highlighting. It would
require storing the "attr" value, before it's mixed with other
attributes, such as from 'cursorcolumn'. So long as we don't scroll the
screen update would be much faster. These days memory is plenty, thus
that isn't a problem.

As soon as you page forward it's just as slow, of course, thus making
the syntax highlighting faster is still desired.

I suspect, if implemented, that this suggestion indeed would greatly help scroll performance for those of us who enable relativenumber. A new setting possibly? cachesyntax?

I leave it to the Vim masters to either: do this, or not do this, or schedule later, or close if too hard.

Thank you.

[mgraham]

Ok, I don't know the internals of cursorline or vim's syntax highlighting, but it seems to me that it should be possible to completely separate cursorline highlighting from syntax highlighting?

Usually, the cursorline just changes the background colour for the current line. So...

• when drawing the line the cursor is on, set the background colour to the highlight colour.
• when drawing any other line set the background colour to the regular background colour.

I don't understand why the entire screen's syntax highlighting needs to be recalculated because the background colour of one line gets toggled.

[mgraham]

For instance, the screen's syntax highlighting does not seem to need to be recalculated every time a visual text selection changes.

[neoadventist]

Awesome! I've been wondering why my screen has been flickering every time I go to the next line. I would love to know how to fix this! @mgraham @bluz71

[bluz71]

@neoadventist,

The enhancement needs to happen inside Vim & Neovim, so for now there is not much you can do except disable relativenumber.

[her]

If you comb through the various postings about this issue across the internet these suggestions come up frequently as a fix:

set ttyfast
set lazyredraw
set regexpengine=1

However these I suspect are actually just fixing people's issues with 3rd party plugins.

The real fix for this is:

set norelativenumber

I think this feature is simply broken when applied to ruby filetypes due to what @bluz71 and @brammool already mentioned.

Hopefully this helps others as it took me some time to find the culprit. Cheers.

[bluz71]

@her,

I agree with:

set ttyfast
set regexpengine=1

However, enabling lazyredraw has lead to weird redraw and slow character deletion issues in my experience. I experimented with it quite a bit and in the end concluded that it is best left disabled.

I also recommend this setting:

set synmaxcol=200

Only syntax highlight the first 200 chars of a line. Very long lines are a killer with relativenumber.

The Ruby syntax highlighter contains hundreds of regexps, multiple by the number of visible lines can equal very bad scroll performance with relativenumber. Caching syntax highlighting is my number one most wanted Vim enhancement. Good to see the first post of this issue now with 62 likes (and slowly growing).

[folofjc]

I have this issue and do not do pgp or ruby syntax highlighting. It also just started happening recently, so might have come from an update.

[bluz71]

I noticed that #282 has been closed due to patches 8.1.0372 - 8.1.0374

Commit bd9a53c (patch 8.1.0374) implements a relativenumber optimisation, only redraw the number column, not all lines.

I have updated and tested, and indeed when scrolling within a page, with relativenumber enabled, navigating the cursor seems much smoother.

Note, scrolling off the top or bottom of the page, via long-pressing j or k may still be slow sometimes, but in that case every line on the page is changing AND syntax highlighting will be applied to lines coming into view. That was never the crux of this issue which was only about scrolling within a single static screen.

Thanks to the Vim maintainers, very much appreciate the enhancement.

Resolving.

[her]

Oh that’s awesome @bluz71, thanks for the update!