Subject: bug#71644: 30.0.50; Severe slowdown in larger files with markers beginning in emacs 29+

From: Stefan Monnier <[email-masked]>
Date: Sat, 22 Jun 2024 11:52:08 -0400

> (BTW, we should probably merge this bug and bug#63040 where I first
> shared this patch - just to demonstrate the problem and discuss possible
> solutions)

Ah, thanks, I'll take a look at that bug.

>     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.

🙂

>> Using markers as a cheap cache of conversions was a cute hack but we
>> really need to replace it.
>>
>> Some options that come to mind:
>>
>> - Keep the tradition of abusing an existing data structure, and stash
>>   the bytepos info inside the overlay tree or the text properties.
>>   This way the conversion is bounded by O(log BUFFERSIZE).
>
> For overlay tree, it might be even better to stash all the markers in
> Emacs into itree structure.

Yes, I don't really distinguish the itree structure and the overlay
tree, but indeed we could have a separate itree for the markers.

> For now, every operation involving
> adjusting/searching markers scales linearly - not ideal.

Adjusting only happens upon insertion, and while it's definitely not
ideal, it's surprising how little it bites in practice.
And if we move to an array-with-gap it'll bite even much less.

As for searching markers, AFAIK the only time we do that is for ....
.... the bytes<->chars conversion 🙂

[ Admittedly, we also do a search when we delete a marker, but that'd be
  easy to optimize away in the common case if we cared about it.  ]

IIRC, XEmacs doesn't use a linked-list of markers but an array-with-gap
of markers instead.  Not sure if they keep it sorted, but if they do, such
an array-with-gap can even be binary-searched, so it's quite efficient.

>> - Use a dedicated data-structure.  E.g. a pair of array-with-a-gap
>>   (one indexed by BYTEPOS/STEP the other indexed by CHARPOS/STEP, where
>>   STEP would be a large enough constant to make those arrays cheap yet
>>   small enough that the remaining scan is cheap).
>>   This way the conversion is O(STEP), i.e. "constant-time".
> I think that it will be less efficient compared to using a tree-like
> structure (if we can manage to use it). Will it be easier?

Given we've survived with a *really* poor data-structure until now,
I suspect that we don't need to worry about choosing the most efficient
option. 🙂

>> BTW, among my various local hacks, I've been using the hack below, which
>> aims to randomize the order in our markers-list, so as to minimize the
>> risk that we have to wade through lots of markers all clumped around the
>> same position.  I don't think it does a good job of it, but maybe we can
>> improve the execution of this idea, tho it still doesn't help if there's
>> no GC involved.
> I am not sure if I believe that this approach can yield practical gains.

Neither am I.  Another idea I had in the same vicinity (and hence
arguably just as unconvincing) was that in buf_*pos_to_*pos, when we
exit the

    for (tail = BUF_MARKERS (b); tail; tail = tail->next)

loop, we could move the markers we just skipped to the end of the list,
based on the idea that we've just found them to be useless at the head.
Doing it efficiently requires keeping a pointer to the end of the list, tho.

> AFAIU, the problem with the slowdown we are discussing here is markers
> that are all around the same position. It's rather too many markers in
> general, spaced not far from each other.

Sounds about right.  But if we could keep them nicely randomized (which
was my original goal with my quick hack), then the total number of
markers wouldn't matter that much because the first N markers in the
list would still be (probabilistically) nicely spread over the
whole buffer.

>> BTW, if/when we use some other data-structure to convert bytes<->chars,
>> then we could presumably get rid of our markers-list and stash markers
>> inside our overlay tree (basically represent them as degenerate overlays
>> with beg==end and no properties).
> I am wondering why it is impossible to stash markers inside overlay tree
> without doing anything special about bytes<->chars conversion (other
> than changing the linear loop with itree query).

I think the answer is that it's not not possible.

        Stefan