Performance degradation from long lines

[mithraeum]

A recent r/emacs thread[1] discussed the long standing[2][3]

issue of Emacs performing horribly on very long lines.

In summary, it seems that it might be possible to significantly

improve performance by altering Emacs' display engine to use simpler

techniques at cost of increased maintenance for the extra code that

will be needed, and the loss of some display capability.

I encourage reading the original thread for details.

To me it seems that the aforementioned loss of display capabilities

is not relevant to terminal Emacs, as it doesn't have those capabilities

in the first place.  So none of the objections apply except those

regarding extra maintenance and the effort to make this work.

In the case of gui Emacs, the reduction of display capability is real,

but it seems prefereable to freezing Emacs or slowing it down to a crawl.

It also seems better than, or at least a viable alternative to, opening

files with M-x find-file-literally, which loses font-lock and leaves

the buffer stuck in Fundamental mode.

In the r/emacs thread I was encouraged to post to emacs-dev to get

the input of more developers, so here I am.

I had three other ideas, which I did not yet have a chance to mention

in the thread.

First is that Emacs could have a user-configurable timeout after which

it could warn the user that it's taking an unexpectedly long time to

open a file (or find the end of a line) and ask whether to abort,

continue trying, or drop down to a mode with reduced display

capabilities (which could be either the equivalent of

find-file-literally or the above simplified display techniques, if

they're implemented).

The second idea is to use to ignore anything that would slow Emacs

down while doing a simple count of the line length, and if the line

length so far is over a certain threshold to ask the same questions as

above about aborting, continuing, or dropping down to a fail-safe

mode.

The third is to use some of Emacs' new async abilities to try to open

the file.  It might still take forever to open, but at least it shouldn't

freeze all of Emacs in the meantime.

Any thoughts?

[1] - https://old.reddit.com/r/emacs/comments/9qtpak/what_would_it_take_to_make_emacs_perform_well_on/

[2] - https://debbugs.gnu.org/cgi/bugreport.cgi?bug=13675

[3] - https://github.com/codygman/figure-out-emacs-long-lines-issue/blob/master/figuring-out-emacs-display-issues.org