Are ES Modules garbage collected? If so, do they re-execute on next import?
Herby Vojčík
herby at mailbox.sk
Thu Jul 16 15:24:08 UTC 2020
On 16. 7. 2020 15:23, Guy Bedford wrote:
> Node.js in the CommonJS loader and dynamic loaders like SystemJS have
> supported module unloading for many years by permitting eviction from
> the loader registry. Evicting the full parent tree was the traditional
> reloading workflow in SystemJS, but live binding pushes are also
> permitted in SystemJS as well for this.
>
> I agree there are issues and invariants of course to consider from a
> theoretical perspective, but those are decisions to be made in terms of
> what invariants are valued, and I don't feel they are necessarily
I understand there may be problems with static imports that are
statically analyzed, and there "only GC module loader as whole" makes
sense, but maybe, it would be possible to treat _dynamically loaded_
modules different way? Just thinking aloud.
> absolute constraints. These decisions should be made based on what is
> best for the JS users and engines. Not that I feel strongly this should
> be a requirement but that it should still be open to consideration.
>
> I'm not sure it was Allen's intention to ban any concept of reloading
> modules when defining the idempotency requirement for the host resolve
> function. Perhaps he could speak to that if he's around.
>
>
> On Tue, 14 Jul 2020 at 23:05, Mark S. Miller <erights at gmail.com
> <mailto:erights at gmail.com>> wrote:
>
> Only a module registry as a whole may be GCed. During the lifetime
> of any one module registry, it can only grow. No other solution is
> possible.
>
> Btw, I remember being surprised ages ago when the same issue came up
> for the Java ClassLoader. A classLoader holds on to all the classes
> it ever loaded. Each class holds onto its classLoader. Each instance
> holds on to its class. During the lifetime of a classLoader or any
> of its classes, the graph of that classLoader and its classes can
> only grow new classes. Not until the classLoader and all of its
> classes are unreachable at the same time can any of them be
> collected. This was equally unfortunate, surprising, and inescapable.
>
>
>
> On Tue, Jul 14, 2020 at 10:16 PM #!/JoePea <joe at trusktr.io
> <mailto:joe at trusktr.io>> wrote:
>
> How can we ensure that long-running applications (even if
> theoretical),
> that may load and unload an unlimited number of new modules over
> time
> (f.e. routes in a web page specified by 3rd parties as time
> progresses), not leak memory?
>
> Even if it is theoretical, I don't like the thought of something
> that
> only ever allocates memory that will never be freed.
>
> Is someone working on a solution for this?
>
>
> #!/JoePea
>
> On Wed, Jul 1, 2020 at 6:16 AM Mark S. Miller <erights at gmail.com
> <mailto:erights at gmail.com>> wrote:
> >
> > No, definitely not. The table from specifiers to module
> instances is indexed by specifiers. Specifiers are strings, so
> this table is not weak. It is not a "cache" in the sense that it
> is allowed to drop things. Rather it is a registry of module
> instances. Only a registry as a whole can be gc'ed, and which
> point that context is no longer around for instantiating or
> reinstantiating modules.
> >
> > As you suggest, if it could drop things because of GC that it
> would then need to regenerate, that would expose the
> non-determinism of gc. That would be a big deal. We carefully
> designed WeakMaps so that gc was non-observable. WeakMaps
> introduce no observable non-determinism. WeakRefs alone expose
> the non-determinism of gc, and are kept well quarantined from
> the rest of the language.
> >
> >
> > On Tue, Jun 30, 2020 at 5:42 PM #!/JoePea <joe at trusktr.io
> <mailto:joe at trusktr.io>> wrote:
> >>
> >> I am curious: can modules be garbage collected if the
> exports are not
> >> references by anything anymore? And if so, will the module be
> >> re-evaluated the next time it is imported?
> >>
> >> I haven't tried an experiment to answer this yet. I'll be
> back to post
> >> findings if someone doesn't post an official answer first.
> >>
> >> I'm thinking about code longevity. For example, if we make
> >> long-running web-based applications with many routes and
> features (for
> >> sake of example imagine a desktop environment, or a MMORPG
> game, with
> >> apps or components that are loaded within the same context).
> Over
> >> time, if imports are not collected, then it means we have a
> memory
> >> leak.
> >>
> >> Imagine, for example, an infinite-universe MMORPG where you
> can land
> >> on different planets where the code for features of a planet are
> >> provided by third parties as ES Modules. I know, this might
> not be a
> >> safe idea to import any code into an app, but just imagine
> it for sake
> >> of example (imagine we have a continuous integration system
> to test
> >> and verify code security, or something, before that code is
> allowed to
> >> be consumed in the app). Imagine you play this app for many
> many days,
> >> and visit many places, and you leave the app running the
> whole time
> >> (because farming for resources is disabled if the app is not
> running,
> >> or something).
> >>
> >> I would imagine that we want unused modules (when we leave a
> planet,
> >> for example) to be (destroyed) garbage collected so that we
> don't
> >> waste memory.
> >>
> >> #!/JoePea
> >> _______________________________________________
> >> es-discuss mailing list
> >> es-discuss at mozilla.org <mailto:es-discuss at mozilla.org>
> >> https://mail.mozilla.org/listinfo/es-discuss
> >
> >
> >
> > --
> > Cheers,
> > --MarkM
>
>
>
> --
> Cheers,
> --MarkM
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