Hi Rafal,
My guess is that there are a dozen people on this list right now who,
given a month, could build a simple VM from scratch, on their own.
However, building something like ORP or Jikes RVM is an enormous task,
well beyond the scope of any individual.
In other words, I don't think the challenge we collectively face is that
of identifying the right abstractions for a simple VM---this is
relatively simple. My feeling is that the real task in front of us is
designing and implementing a VM core which can support the ambitious
goals of Harmony---this is a challenge.
As I've said in previous posts, I think that looking at clean, simple
VMs such as Jam VM will be invaluable to this project.
However, clean abstractions made in the context of a simple VM goes to
pieces once the complexities of aggressive optimizations and pluggable
generality come into the picture. A great example of this is the
hardness of retrofitting GC maps into a VM. GC maps are essential to
high performance GC, but are often overlooked in simple VM designs and
yet retro designing them is an enormous task. Likewise object model
optimizations, method dispatch, stack frame organization, etc etc all
wreak havoc on more basic abstractions.
I don't think we're that far away from being able to build a new VM
core. We have so much at the table. My hope is that we could have our
own VM core with a simple interpreter or JIT and perhaps a naive GC
could be up and working in a matter of months. I am also a strong
believer in being prepared to throw early implementations away. If we
try to get our first implementation perfect first shot, we're sure never
to have a first implementation. On the other hand, we should certainly
try to make the most of the experience we do have at hand, so that our
first implementation is not too wildly off the mark.
A few more detailed responses:
- it is important to distinct between innovation/research and good
engineering.(eg mono
Innovation/research and good engineering are absolutely not in
opposition!! Good engineering is KEY to good research.
well engineered framework
shall make research easier after all; that is why I'm rather a
C-camper
What is the connection between good engineering and the use of C? I
don't follow.
make sure to make it efficient as some operations (write
barriers, for example) are critical when it comes to performance;
Please see previous posts on this subject. Fast barriers probably need
to be written in Java or compiler IR, not C (this is because they are
injected into user code and need to be optimized in context).
- avoid prematurely sacrificing design for the sake of performance;
Agree 100%
- Java-C/C-Java (internal) interface doesn't necessarily have to be
slow;
Anything that involves a function call is "slow" in the context of many
aspects of VM implementation. This is fine for coarse grained
interfaces, but unacceptable for fine grained interfaces such as the
allocation and barrier paths. Careful design should make the
integration of C & Java modules possible (just avoid interface
transitions on the fast path).
- having some kind of 'frankenVM' consisting of various pieces doesn't
have to be inherently bad; did Mono emerge this way or am I wrong ?
A VM that draws on the enormous investment at hand by taking a JIT from
here and a memory manager from there is a very wise choice.
- someone has to build a 'big picture' and split it into parts to make
people working on details (don't start with one or two interfaces,
start with a big picture first: several main modules, how shall they
interact with other modules, where are potential problems); that HAS
to be done by a person with extensive experience in VM construction
(engineer rather than researcher); newbies like myself fall short in
this mainly because of not having dealt with details;
I agree with this very strongly, however I don't think it should be one
person---it should be done collectively.
- sorry for being a bit offensive on researchers ;) it wasn't my
intention, I just think that we need to have a proven set of things
first, then may do some good research on it;
OK. I'm offended ;-)
In all seriousness, you need to understand that the state of the art in
VM design and implementation is coming out of researchers and research
labs. Research and proven results are not and must not be mutually
exclusive. To the contrary! As far as I know, as far as "proven"
results go, the two best performing open VMs are ORP and Jikes RVM, both
of which have come out of research labs. Good engineering is at the
heart of good research---I believe the opposite to be true too.
--Steve
