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From Steve Blackburn <Steve.Blackb...@anu.edu.au>
Subject Re: Threading
Date Sun, 22 May 2005 11:09:20 GMT
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

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