Subject: bug#60691: 29.0.60; Slow tree-sitter font-lock in ruby-ts-mode

From: Dmitry Gutov <[email-masked]>
Date: Thu, 19 Jan 2023 20:28:41 +0200

Hi Yuan,

On 18/01/2023 08:50, Yuan Fu wrote:
>>>> Should treesit--font-lock-fast-mode be locally bound inside that
>>>> function, so that it's reset between chunks? Or maybe the condition
>>>> for its enabling should be tweaked? E.g. I don't think there are any
>>>> particularly large or deep nodes in ruby.rb's parse tree. It's a
>>>> very shallow file.
>>
>> Yeah that is a not-very-clever hack. I’ve got an idea: I can add a C
>> function that checks the maximum depth of a parse tree and the maximum
>> node span, and turn on the fast-mode if the depth is too large or a node
>> is too wide. And we do that check once before doing any fontification.
>>
>> I’ll report back once I add it.
> 
> I wrote that function. But I didn’t end up using it. Instead I added a
> "grace count", so that the query time has to be longer than the
> threshold 5 times before we switch on the fast mode instead of 1.
> 
> My main worry is that simply looking at the parse tree would not catch
> all the case where there will be expensive queries.

That might be true, but a criterion that doesn't specify conditions 
exactly can give no guarantee against false positives.

> Could you try the latest commit and see if the fast mode still switches
> on when it shouldn’t?

At first it seemed to help, but then I switched the major mode a couple 
more times, and ran the benchmark twice more, and the "fast mode" 
switched on again.

Which seems to make sense: there is no resetting the counter, right?

So if previously it happened once somehow during a certain scenario, now 
I have to repeat the same scenario 4 times, and the condition is met.