Subject: Re: bug#66117: 30.0.50; `find-buffer-visiting' is slow when opening large number of buffers

From: Ihor Radchenko <[email-masked]>
Date: Fri, 22 Sep 2023 13:30:41 +0000

Eli Zaretskii <[email-masked]> writes:

>> Will it be acceptable to implement the cache using variable watchers?
>
> I don't like us using variable watchers for such stuff.
>
> Why cannot we just cache the correspondence between files and buffers
> in some hash-table?

Because `buffer-file-name' can be modified from Lisp (via
`set-visited-file-name' or directly). Same for `buffer-file-truename'
and `buffer-file-number'.

Or should we just assume that these variables remain unchanged other
than by primitives?

>> The regression might happen when the number of buffers is small -
>> when hash tables become slower compared to simple list lookup.
>> But in such scenario, we will be talking about very small absolute
>> runtimes anyway, so it should probably not matter in practice.
>
> Alternatively, only create the hash-table when the number of buffers
> exceeds some threshold.

Indeed, this is a good idea.

-- 
Ihor Radchenko // yantar92,
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