Preload attribute

[Dan Heidinga]

On Thu, Jun 8, 2023 at 4:51 PM John Rose <john.r.rose at oracle.com> wrote:

> On 8 Jun 2023, at 9:52, Dan Heidinga wrote:
>
> On Thu, Jun 8, 2023 at 12:44 PM John Rose <john.r.rose at oracle.com> wrote:
>
> On 8 Jun 2023, at 9:01, Dan Heidinga wrote:
>
> If we decouple the list of preloadable classes from the classfile, how
> would non-jdk classes be handled?> What if instead of ditching the
>
> attribute, or treating it like an
>
> optimization, we firmed up the contract and treated it as a guarantee…
>
> If we go down this route, let’s consider putting the control information
> into a module file (only) for starters. (Maybe class file later if
> needed.) There would be fewer states to document and test, since (by
> definition) class files could not get out of sync.
>
> A module would document, in one mplace, which types it would “prefer” to
> preload in order to optimize its APIs (internal or external).
>
> This might lead to more class loading than intended. The current approach
> has each classfile register the list of classes it wants preloaded to get
> the best linkage which means we only have to load those classes if we link
> the original class. There's a natural trigger for the preload and a
> limited set of classes to load.
>
> There’s a spectrum of tradeoffs here: We could put preload attributes on
> every method and field, to get the maximum amount of fine-grained lazy
> (pre-)loading, or put them in a global file per JVM instance. The more
> fine-grained, the harder it will be to write compliance testing, I think.
>

Agreed.  There's a sweet spot between expressiveness and overheads
(testing, metadata, etc).  Classfiles have historically been the place
where the JVM tracks this kind of information as that fits well with
separate compilation and avoids the "external metadata" problems of ie:
GraalVM's extra-linguistic configuration files.

When compiling the current class, javac already requires directly
referenced classes to be findable and thus has the info required to write a
preload attribute.  Does javac necessarily have the same info when
compiling the module-info classfile?  Maybe when finding the non-exported
packages for the module javac (or jlink? or jmod?) could also find the
value classes that need preloading?

Moving it into a separate pass like this doesn't feel like quite the right
fit though as it excludes the classpath and complicates the other tools
processing of the modules.

> Moving to a single per-module list loses the natural trigger and may
> pre-load more classes than the application will use. If Module A has
> classes {A, B, C} and each one preloads 5 separate classes, with a
> per-module list that's forcing the loading of 15 additional classes (plus
> supers, etc). With a per-class list, we only preload the classes on a
> per-use basis. More of a pay for what you use model.
>
> Is there a natural trigger or way to limit the preloads to what I might
> use
> with the per-module file?
>
> That’s a very good question. I think what Preload *really is* is a list
> of “names that may require special handling before using in APIs”. They
> don’t need to be loaded when the preload attribute is parsed; they are
> simply put in a “watch list” to trigger additional loading *when
> necessary*. (This is already true.) So I think if we move the preload
> list to (say) the module level (if not a global file), then the JVM will
> have its watch list. (And, in fewer chunks than if we put all the stuff all
> the time redundantly in all class files that might need them: That requires
> frequent repetition.) The JVM can use its watch list as it does today, with
> watch lists populated separately for each class file.
>
I initially thought a global list would lead to issues if two different
classloaders defined classes of the same name but since this is a "go and
look" signal, early loading based on name should be fine even in that case
as each loader that mentions the name would be asked to be asked to load
their version of the named class.  So I think a per-JVM list would be OK
from that perspective (though I still don't like it).


> To emphasize: A watch list does not require loading. It means, “if you see
> this name at a point where you could use extra class info, then I encourage
> you to load sooner rather than later”. The only reason it is “a thing” at
> all is that the default behavior (of loading either as late as possible, or
> as part of a CDS-like thingy) should be changed only on an explicit signal.
>
While true for what the JVM needs, this is hard behaviour to explain to
users and challenging for compliance test writers (or maybe not if we
continue to treat preload as an optimization).  Is this where we want to
spend our complexity budget?  Part of why I'm circling back to treating
preload as a per-classfile attribute that forms a requirement on the VM
rather than as an optimization is that the model becomes clearer for users,
developers and testers.


> And, hey, maybe CDS is all the primitive we need here: Just run -Xdump
> with all of your class path loaded. Et voila, no Preload at all.
>
 Users may find this behaviour surprising - I ran with a CDS archive and my
JVM loaded classes earlier than it would have otherwise?

--Dan
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