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Re: master afc0bfd380: Speed up loaddefs-generate on slow disks

From: Stefan Monnier
Subject: Re: master afc0bfd380: Speed up loaddefs-generate on slow disks
Date: Thu, 02 Jun 2022 21:18:57 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

Eli Zaretskii [2022-06-02 19:16:00] wrote:
>> > I'm really surprised this is faster, even on slow disks.
>> > Does anyone have a idea of why that is?
> Isn't that clear?  file-newer-than-file-p calls 'stat' for both files,
> and one of them is a loop invariant AFAIU.  Taking its time outside
> the loop speeds up things (but perhaps not on GNU/Linux).

Under GNU/Linux, I'd expect `stat` to be significantly faster than
`file-attributes` (and the extra `stat` on the loop-invariant file
should be especially fast since it should be as close as it gets w.r.t
the caches (both OS-level and CPU-level)).

>> I didn't actually benchmark it -- either way is fast on my laptop.  But
>> the old autoloads code did it with time-less-p, and Eli said that it was
>> faster, so...  Perhaps Eli can report back whether it made a difference
>> or not.
> It did -- for the better.  Thanks.

This suggests that the kind of change I suggested would make it even
a bit faster, tho it should reduce the run time by less than 50%
under Windows (because the time to do the `stat`s is apparently already
>50% of the total).


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