bug#63040: 30.0.50; Performance of buf_bytepos_to_charpos when a buffer has large number of markers

[Eli Zaretskii <eliz@gnu.org>]

[Resending with Stefan added.]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: 63040@debbugs.gnu.org
> Date: Mon, 24 Apr 2023 06:36:07 +0000
> 
> > Interesting.  Would it be possible to show the effect of different
> > values of the cut-off on the performance, so we could decide which
> > value to use?
> 
> I can do such test, but I do not think that playing with cut off is the
> best approach here.
> 
> The full code in question is below and there is already existing
> condition to cut the marker loop early based on the distance from
> best_above to the requested bytepos. So, another approach could be
> playing with BYTECHAR_DISTANCE_INCREMENT.

Yes, that would be an even better idea, IMO.

> Now, it is clearly not efficient enough for my large file.

Why do you say that?  Did you try something and the results were
unsatisfactory?  And what is not efficient enough -- the cutoff based
on the number of markers tested or based on the distance?

> Further, the later code creates markers to cache recent results and
> cutting too early may waste this cache.

And the technique that you tried doesn't waste the cache?

> Another idea could be moving the cache markers into a separate
> array, so that we can examine them without mixing with all other
> buffer markers.

Why would that separation be useful?

Thanks.