Time to reconsider m:n or green threading options?

[John Rose]

On Jun 21, 2008, at 3:18 AM, Patrick Wright wrote:

> Big drawback appears to have
> been a) blocking I/O could cause the whole VM to wait and b) there's
> only 1 OS thread underneath, meaning no use of multiple processors.

Yep, those are the reasons.  The transition away from green threads  
occupied a year or two of my early career on the JVM.  It was  
necessary and most unpleasant; some of the worst bugs I've ever seen.

Both kernel threads and green threads are designed around the idea  
that you get a rich virtual processor with all the OS trimmings and  
stack space for as many stack frames as you are ever likely to need.   
This is inherently expensive to instantiate.  You want the JVM to be  
able to pass control between unrelated actors without completely  
switching out the whole register file, etc.

It would be great if we had really lightweight continuations, with a  
JVM scheduler (Scheme calls them engines, I think) which keeps  
running the next one.  The part I can't see yet is how to make stack- 
based and heap-based activation records play together efficiently.   
(Maybe you JIT two versions of every method, with inlining to remove  
overheads as usual?)

-- John
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