Garbage collector: is 800kb a good default?

[Dmitrii Korobeinikov <dim1212k@gmail.com>]

Hi all,

So, I was debugging scrolling, it felt evidently clunky for large
files for me. Font lock is the elephant in the room, but I decided to
profile my custom half-screen scroll functions anyway and was quite
surprised to find that GC was gobbling 38% of the CPU time. That
didn't have anything to do with my functions, the default full page
scroll has it the same.

That lead to some experimenation with gc-cons-threshold. Higher values
of this variable resulted in much less frequent freezes/pauses while
scrolling [1]. Setting garbage-collection-messages to t and
gs-cons-threshold to 80MB, I figured I can scroll a voluminous sheet
of 7k lines before gc has to do its job. With the default 0.8 it's
every 2-3 pagescrolls. That's a big difference.

Of course, raising the threshold significantly higher on its own is
not a very good idea. But if paired with an idle timer like suggested
here [2], then it all starts looking like a decent combination:

(setq gc-cons-threshold mem)
(run-with-idle-timer 2 t 'garbage-collect)

It would be good to find a threshold which the user barely has a
chance of reaching unless running a long uninterrupted task. And, by
the way, in this latter case, it is better to have a higher threshold
too due to the increase in speed, like with startup [3]. Setting the
threshold just a bit higher than the current default wouldn't help
much but may actually be worse as the pauses would be longer, but
about as frequent. Too high a value would create very long freezes for
uninterrupted tasks. But there should be a good middle ground.

So, as far as I can see, defaulting to a higher threshold with an idle
timer could yield better user experience with practically no
significant tradeoffs. Is there something I might be missing here?

Best,
DK

[1] This is evident in vanilla emacs if you pay attention, but it's
much more so for me w/ my the bloated config. I haven't yet found the
culprit.
[2] https://emacs.stackexchange.com/questions/34342/is-there-any-downside-to-setting-gc-cons-threshold-very-high-and-collecting-ga
[3]
| threshold (MB) | startup time (s) |
|----------------+------------------|
|            0.8 |             9.14 |
|              2 |             8.77 |
|              5 |             7.54 |
|             20 |             6.88 |
|             50 |             6.65 |
|             80 |             6.73 |
|            100 |             6.17 |