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bug#33275: 27.0.50; Image cache pruning

From: Lars Ingebrigtsen
Subject: bug#33275: 27.0.50; Image cache pruning
Date: Fri, 09 Nov 2018 13:26:50 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Eli Zaretskii <eliz@gnu.org> writes:

> After thinking about this and looking at the code, I don't think I
> understand your proposal(s).  Here are the issues that I saw with what
> I think you proposed:
>  . The hash table used by the image cache is not a Lisp hash-table, so
>    weakness doesn't apply.  We cache pointers to C structs, so using a
>    Lisp hash-tables would need "some work".

Yeah, it'd have to be rewritten substantially to use Lisp hash tables.

>  . The code you show above is called from redisplay, so it will not be
>    executed as long as a Lisp program (such as the one you show at the
>    beginning of this discussion) runs, and you will still have the OOM
>    killer get you before that code get a chance to clear the cache.

That's true, but calling the cache pruning function from the function
that enters images into the cache would be trivial, I think.

>  . Running such code from GC could be tricky, because freeing images
>    needs to remove references of those images from the glyph matrices,
>    and that cannot be safely done from arbitrary places.

Ah, I see.  Yes, that's a major complication...

> I think if we want to prevent the OOM killer from killing Emacs due to
> many cached images, we should inspect inside cache_image the current
> VM size of the process (like system_process_attributes does), compare
> it to the memory limits, and display a warning and/or signal an error
> when we get dangerously close to the limit.

I think that sounds like a good idea, yes.  Having such a check is good
for Emacs sanity, but we should also have a way for programmers to avoid
ending up in that pathological situation to begin with.

So either a DONT-CACHE parameter to `image-size' or just a docstring
thing.  :-)

(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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