The Jikes RVM experience is kind of interesting...
From the outset, one of the key goals of the project was to achieve
much greater levels of scalability than the commercial VMs could deliver
(BTW, the project was then known as Jalapeno). The design decision
was to use a multiplexed threading model, where the VM scheduled its own
"green" threads on top of posix threads, and multiple posix threads were
supported. One view of this was that it was pointless to have more than
one posix thread per physical CPU (since multiple posix threads would
only have to time slice anyway). Under that world view, the JVM might
be run on a 64-way SMP with 64 kernel threads onto which the user
threads were mapped. This resulted in a highly scalable system: one of
the first big achievements of the project (many years ago now) was
enormously better scalability than the commercial VMs on very large SMP
boxes.
I was discussing this recently and the view was put that really this
level of scalability was probably not worth the various sacrifices
associated with the approach (our load balancing leaves something to be
desired, for example). So as far as I know, most VMs these days just
rely on posix style threads. Of course in that case your scalability
will largely depend on your underlying kernel threads implementation.
As a side note, I am working on a project with MITRE right now where
we're implementing coroutine support in Jikes RVM so we can support
massive numbers of coroutines (they're using this to run large scale
scale simulations). We've got the system pretty much working and can
support > 100000 of these very light weight threads. This has been
demoed at MITRE and far outscales the commercial VMs. We achieve it
with a simple variation of cactus stacks. We expect that once
completed, the MITRE work will be contributed back to Jikes RVM.
Incidentally, this is a good example of where James Gosling misses the
point a little: MITRE got involved in Jikes RVM not because it is
"better" than the Sun VM, but because it was OSS which meant they could
fix a limitation (and redistribute the fix) that they observed in the
commercial and non-commercial VMs alike.
--Steve
