Building/distributing more than one VM in a single JDK distribution

[Glavo]

Hi,

As far as I know, BellSoft LibericaJDK provides both server vm and client
vm for 64-bit platforms.

> D:\Java\Windows\BellSoft\jdk-23.0.2-full\bin\java.exe -client --version
>
> openjdk 23.0.2 2025-01-21
>
> OpenJDK Runtime Environment (build 23.0.2+9)
>
> OpenJDK 64-Bit Client VM (build 23.0.2+9, mixed mode, sharing)
>
>
I thought maybe some users were using them.
We noticed that for our GUI application, the client VM consumed
significantly less memory than the server VM,
so we also considered whether we should use client vm as much as possible,
until we found that other JDK distributions often do not provide client vm.

Glavo

On Tue, Mar 11, 2025 at 6:53 PM Magnus Ihse Bursie <
magnus.ihse.bursie at oracle.com> wrote:

> Since time immemorial, the JDK has had the ability to build more than
> one variant of Hotspot, and let the user select which one to use at
> runtime. The canonical example was how 32-bit Windows included both
> "client" and "server", and defaulted to "client".
>
> This flexibility comes at a cost, as it creates a lot of complexity in
> different parts of the JDK -- including, but not limited to, the build
> system, jlink, and the launcher.
>
> Hotspot has support for these variants: server, client, minimal, core
> and zero. Of these, "core" is apparently an old remnant that I even
> forgot where it was used (embedded, I think?), and "client" has only
> been used for 32-bit systems, which are now on their way out. Neither of
> these are tested regularly in the Oracle CI, or on GHA.
>
> That leaves only server, minimal and zero as the remaining relevant
> variants. These are kept up to date with testing, at least so that they
> are able to build.
>
> However, we do not build *both* server and minimal, or *both* server and
> zero, neither on GHA or on the Oracle CI. Instead, we *replace* server
> with either zero or minimal.
>
> The point of zero is to have a JVM that can run on hardware that server
> does not support (or, in the special case of the iOS mobile port -- a
> platform where JITting code is not allowed). It makes no sense to me to
> distribute a JDK that includes both the server and the zero variants of
> Hotspot. If server works, then zero is suboptimal in performance, and
> not needed. If server does not work, then it need not be included.
>
> The point of minimal is to create a JVM with the smallest possible
> footprint, by excluding functionality. If you distribute the a JDK with
> both the server and minimal JVM included, you fail doubly: the footprint
> will be larger, and there will never be any use of minimal, since it is
> just more limited than server.
>
> Hence, I see no need anymore to keep the ability to build more than one
> variant of Hotspot at the same time. I propose we drop this
> functionality, which will allow for us to clean up and remove a lot of
> complexity in several areas of the codebase. (Just to be clear: I do not
> propose removing zero or minimal, I'm just saying that you need to build
> *only* the zero or minimal JVM if that is what you want.)
>
> Feedback, thoughts?
>
> /Magnus
>
>
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