On May 24, 2005, at 4:49 AM, Dmitry Serebrennikov wrote:
* "OS interface" is perhaps one place where some code can be
shared. If C version can benefit from an OS abstraction layer for
portability, then it seems like this layer could be shared between
the C and the Java implementations.
I'm really hoping we can shelve the idea of having a C implementation
and a Java implementation. I think we should try to mix them into
one solution.
Steve, if the spokes were in Java but the hub in C, would we then
lose all of the aggressive inlining benefits that Java-in-Java
solution can provide?
I'll preface this and state that I have no idea what I'm talking
about. That said, this reminds me a little of what was done in the
Geronimo architecture that makes it differ from component
architectures that use central mechanisms like JMX to let modules/
components communicate. With Geronimo, the components are given
references to their dependencies, rather than having them work
through the central bus to talk between dependencies. This gives a
big speedup, as the overhead goes away. (Ok, there are 'smart'
proxies between the components, but that's a detail we can ignore here.
I've guessed (and steve confirmed) that the boundaries between the
modules in a VM aren't "flat" APIs, but something thicker, with a
good bit of interdependency. So I'd guess that while a central hub
would be used to let the modules resolve dependencies (The JIT needs
to talk to the GC and vice versa), it would be something on the order
of asking the 'hub' (won't use 'core') for something, and getting an
object back that could be as sophisticated as needed.
I'd sure like to see some of the existing thinking on this from the
JikesVM peeps....
As an aside to my aside, Ben and I were musing over ways to do things
like this, because we thought that in a multi-language-capable VM,
you'd probably define some minimum interface APIs so that parts in
different languages would have framework support for
intercommunication, but also provide a "discovery" mechanism to so
that like-minded components could talk in a more private, direct way.
For example, suppose we are able to define the boundary between GC
and JIT in a nice way, we'd want any object claiming to be a JIT to
support that standard API and be able to provide an to the hub to be
given to any other module an implementation of that interface. So
you could do any kind of JIT implementation or experimentation and
plug it in.
However, if I wrote both the JIT and GC, I would want my JIT and GC
to discover that and give each other a "private" API that took
advantage of each other's internals. Something like what we used to
do with COM and "QueryInterface", starting with something basic and
standard, and grubbing around in it to find something better.
Loosely speaking :
interface Discovery {
Object queryInterface(UUID interfaceID);
}
interface JIT extends Discovery {
// JIT API here
}
So in my GC implementation, an init() call that's involved when it's
being created :
void init(Discovery hubDiscovery) {
JIT commonJIT = (JIT) hubDiscovery.queryInterface
(UUID_FOR_STANDARD_JIT_API);
// now lets see if this JIT knows the secret handshake
CustomJIT goodJIT = (JIT) commonJIT.queryInterface
(UUID_FOR_JIT_I_ALSO_WROTE);
if (goodJIT != null) {
....
}
}
and in my JIT implementation :
Object queryInterface(UUID id) {
if (UUID_FOR_STANDARD_JIT_API.equals(id)) {
return this;
}
if (UUID_FOR_JIT_I_ALSO_WROTE.equals(id)) {
return myInternalCustomJIT;
return null;
}
Anyway, this reminded me of what Ben and I were talking about a few
days ago. Note that a) I'm just making this up, b) all the above is
trying to capture some loose ideas on the subject, and c)
<disclaimer> is all done pre-coffee in a strange hotel room after
waking up </disclaimer>
geir
-dmitry
Steve Blackburn wrote:
I thought it might be helpful to clarify some terminology and a few
technical issues. Corrections/improvements/clarifications
welcome ;-)
VM core
The precise composition of the VM core is open to discussion and
debate. However, I think a safe, broad definition of it is that
part of the VM which brings together the major components such as
JITs, classloaders, scheduler, and GC. It's the hub in the wheel
and is responsible for the main VM bootstrap (bootstrapping the
classloader, starting the scheduler, memory manager, compiler etc).
VM bootstrap
The bootstrap of the VM has a number of elements to it, including
gathering command line arguments, and starting the various
components (above).
In the context of a Java-in-Java VM, the above is all written in
Java.
VM boot image
The boot image is an image of a VM heap constructed ahead of time
and populated with Java objects including code objects corresponding
to the VM core and other elements of the VM necessary for the VM
bootstrap (all written in Java, compiled ahead of time, packaged
into Java objects and composed into a boot image). The boot image
construction phase requires a working host VM (ideally the VM is
self-hosting).
VM bootloader
In the case of Jikes RVM a dozen or so lines of assember and a few
lines of C are required to basically do the job of any boot loader
loader---mmap a boot image and throw the instruction pointer into
it. It will also marshal argv and make it available to the VM core.
This is technically interesting, but actually pretty trivial and has
little to do with the VM core (aside from ensuring the instruction
pointer lands a nice place within the boot image ;-)
OS interface
The VM must talk to the OS (for file IO, signal handling, etc).
There is not a whole lot to it, but a Java wrapper around OS
functionality is required if the VM is java-in-java. This wrapper
is pretty trivial and one half of it will (by necessity) be written
in C.
I hope this brief sketch provides folks with a slightly clearer view
of what a java-in-java VM looks like, and some (tentitive)
terminology
we can use to ensure we're not talking at cross purposes.
Cheers,
--Steve
--
Geir Magnusson Jr +1-203-665-6437
[EMAIL PROTECTED]
