[hibernate-dev] JVM metadata costs vs the use of inheritance across Logger interfaces

[Steve Ebersole]

I'm missing something.  IIUC the discovery was that, e.g., having something
like `BaseHibernateSearchLogger` in module-A and then something like
`RealLogger` in module-B that extends `BaseHibernateSearchLogger` causes
extra space to be taken up within the VM memory space.  Yet this is still
exactly what you do.  What am I missing?

Out of curiosity, is this specific to WildFly/Modules and how it handles
Class loading?  Or is this something in the JVM itself?

Another thing I'd like to point out that I believe helps with performance
in regards to loggers is what I have been doing in ORM which is to use just
a single instance of named loggers, which you can see in the loggers in the
`org.hibernate.internal.log` package :
https://github.com/hibernate/hibernate-orm/tree/master/hibernate-core/src/main/java/org/hibernate/internal/log

On Mon, Sep 25, 2017 at 6:32 AM Sanne Grinovero <sanne at hibernate.org> wrote:

> Our friend and colleague Andrew Dinn from the OpenJDK team is working
> on a series of blog posts to help people understand the impact of
> certain design choices on the cost of internal JVM metadata and native
> memory; this affects bootstrap costs of both the JVM and our
> libraries, overhead at runtime in terms of permanent memory waste,
> etc..
>
> So as these blogs will be out soon I'm not going to dive into more
> details or how I discovered this, but as a draft reviewer I had the
> privilege to already play with the technique and send a PR to
> Hibernate Search as result of verifying some theories.
>
> In a nutshell Andrew's work allowed me to spot that having a Logger
> interface in module A extended by another Logger in module B is
> causing it to generate a significant amount of duplication of Class
> definition metadata: the generated loggers are quite verbose in terms
> of such costs and don't benefit from inheritance as one would expect:
> the cost of B is (A+B), if you ignore benefits from symbol
> de-duplication. In this specific example I could measure more than
> 200K of wasted space. That's not small for a single jar! Incidentally
> in this case it also bloats the jar file size.
>
> A code patch might be more clear to explain than emails; this is what
> I recommend we do in all projects:
>  - https://github.com/hibernate/hibernate-search/pull/1546
>
> N.B. the verbosity of the generated code is related with runtime
> performance so I don't think we'll to remove the generated Logger
> metadata, and certainly don't plan to switch logger implementations as
> there are many more aspects to take into account - yet we might have
> identified an anti-pattern which is multiplying this cost N times
> without a good reason and it's quite easy and under our control to
> avoid that.
>
> Thanks,
> Sanne
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