[riot-devel] HotSpot JVM on RIOT-OS?

Joakim Gebart joakim.gebart at eistec.se
Tue Aug 11 14:48:56 CEST 2015

See my response inline below.

On Tue, Aug 11, 2015 at 11:58 AM, Zac Harvey <zharvey at pobox.com> wrote:
> Thanks for that Lejos link, Kaspar, I will definitely dig into it later
> today.
> You mentioned that RIOT *targets* devices that require a small footprint,
> but you didn't state that RIOT only supports devices with small RAM.
> What's the max size/RAM that RIOT can support, or that RIOT is addressable
> for?  What would be entailed with refactoring it to handle larger apps?

So far, RIOT does not support 64 bit systems (native included, it is
compiled as a 32 bit binary even on amd64 afaik), and it will take
some effort to add that support, because there may be hidden
assumptions in the code that some data type is 32 bit long.

The biggest problem, however, with porting to a larger system than a
microcontroller is that these larger systems have MMU (memory
management unit)-hardware which performs address space translations
between virtual addresses and physical addresses, and ensures that
each process has its own address space to run inside. This MMU must be
configured when booting in order to access all of the memory. My area
of expertise is constrained systems, such as the current RIOT
platforms, so I may have some factual errors in the above explanation,
please correct me if I am wrong about anything above.

> I ask this because to me, and for many others I'm sure, the main value in an
> RTOS (as opposed to Linux) is its deadline guarantees. When I run Java on
> Linux, the whole JVM process might come to a hault for a few seconds while
> Linux flushes some stream. Or perhaps some system-level utility runs for a
> few minutes and bogs down everything on the server (including my Java app).
> So to me, the real appeal of RIOT is:
> 1. Its a legitimate real-time OS when hard deadlines, task execution
> guarantees, etc.; and

Can you really get any real-time guarantees on a Java VM? There are
background processes happening inside the Java VM which may cause
delays, for example memory garbage collection (this applies to Python
too and probably others, but I don't know any other interpreted
languages well enough to comment on them)

> 2. It's an OS where I can just deploy my app and I know that *nothing else*
> besides the OS is running
> So to me, if RIOT-OS can technically handle JVM apps (2GB - 4GB in size),
> then why not try to get a JVM like HotSpot to run on it? And if for some
> reason it can't handle apps that size (e.g. perhaps the largest integer it
> can handle is only 65,536, etc.), then I'm wondering what level of
> refactoring would be required to enable it to handle beefier/server-like
> apps.
> Traditionally, the counter-argument here is: "Well then just write your apps
> in C."  However, in 2015, all hobbyists, commercial and open source
> organizations have an enormous amount of time & effort ($$$) invested in
> existing software systems, which may be Java-based, Python-based,
> Ruby-based, Node-based, etc.
> I can't speak for anyone else, but I would argue that the ability to run a
> JVM app on a real-time OS is going to be of *enormous* utility, especially
> in this modern "Internet of Things" era we're careening towards.  It might
> also spawn up a whole subculture of getting other systems (again, Python,
> Ruby, etc.) running in an embedded context.  I'm just sayin'...!
> Thoughts? Criticisms? Hate mail?

By all means, go for it! Even if it may not give the same real-time
guarantees as running a process hand-coded in C on the bare metal
platform it is still a useful experiment and concept.

For anyone attempting this: I don't believe this is an easy task. You
will probably have to implement quite a large part of POSIX ioctl,
some improvements to the supervisor call handler (at leas if running
on ARM, to handle more than just task scheduling). You may also need
to write an adaptation layer between gnrc (the network stack) and
whatever network API is expected by the JVM. Filesystem storage may
not be strictly necessary, but probably mandatory for almost any apps
you want to run on this system.

Best regards,

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