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From Dmitry Serebrennikov <dmitr...@earthlink.net>
Subject Re: Terminology etc
Date Tue, 24 May 2005 08:49:03 GMT
This is excellent! For one, I find Java-in-Java to be much clearer after 
this little set of definitions.

A quick proposal: perhaps "VM bootstrap" as used below should really be 
something like "VM initialization," "VM init," or even "VM startup", 
since the word "bootstrap" is very specific and to me at least, 
indicates something more akin to the "VM bootloader" + "VM boot image" 
(as used below).

I'm going to try to make a couple of observations. Please forgive the 
rambling :)  It just seems like things are starting to take shape...

* It looks like there is really not much in common between this 
architecture and one where "VM core" would be in C... For that one, "VM 
core" would be in C, as would be all of the components, such as GC, 
scheduler, etc. "VM init" doesn't sound like a big deal but would 
probably also be in C. "VM boot image" and the "VM bootloader" are 
simply not present in a C solution.

* "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.

* The meat of the VM seems to be in the "spokes" that connect to the "VM 
core-hub". It seems that this is where it would make the most sense to 
mix components written in C with those written in Java, to see which one 
can do a better job. If all spokes were in C, it would make little sense 
to have the hub be in Java... On the other hand if spokes are all Java, 
it makes little sense to have the hub be in C.

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?


-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
>
>


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