Re: Increase gc-cons-threshold during package-init?

[raman@google.com (T.V Raman)]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

By Now I meant "earlier that package-initialize was happening in my
startup file where the numbers were tweaked".

Agree to the rest of what you say -- the right thing might be to tweak
those during startup as a whole.>> With Emacs now calling package-initialize on its own,
>
> Not sure what you mean by "now": it's been the case since Emacs-24.1
> (recent changes on `master` have only moved the time at which it takes
> place).
>
>> that emacs startup was now  entailing 3 times as many gc calls during
>> startup as before.  One guess is that my init file was lexically
>> binding gc-cons-threshold to 64MB ---
>
> Lexically-binding it would have no effect on the GC, and since that var
> is declared as dynamically scoped, it was probably dynamically bound.
>
> Have you checked how much extra time is spent in the GC because of those
> extra calls?
>
>> the default gc-cons-threshold of 800K is likely from the past and
>> could be increased.
>
> This is a misunderstanding: the real gc-cons-threshold is computed
> dynamically based on the heap size (see gc-cons-percentage).
>
> Maybe those values need to be tweaked because tradeoffs have shifted
> somewhat, but it doesn't seem very likely.
>
> The problem with the above GC calls is much more fundamental and
> unrelated to the actual values we choose: during periods of
> initialization, programs typically allocate a lot of memory without
> generating much garbage.  So during those times running a GC tends to be
> a waste of effort (we scan the whole heap only to find a few crumbs here
> and there).
>
> People often look at situations like yours and conclude that
> gc-cons-threshold is too small.  Yes, it's too small for those cases,
> but bumping it once and forall makes things worse for the "usual" case,
> leading to increased heap size, fragmentation, and longer pauses during
> normal operation (tho this "usual case" is much more difficult to
> measure).
>
> There's no sadly way the GC can magically know that the last N megabytes
> of allocation generated very little garbage, tho.
>
> Maybe should bump those gc-knobs around the startup.el code (or maybe
> more generally we should bump those knobs while loading a file).
>
>> Even if we leave the default as is, it might make sense to temporarily
>> increase gc-cons-threshold within package-initialize
>
> Right.  Tho doing this will likely cause a GC to happen right after
> package-initialize, i.e. just before loading your ~/.emacs.
> Better would be to bump it around both, I guess.
>
>
>         Stefan
>
>

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