Re: Native Threads in the RTS

[Wolfgang Thaller]

I've postponed writing up a new proposal again...

But I'm going to sum up some requirements that I would like to see fulfilled - to make it clearer to others why I'm proposing such strange things...

*) It should be possible for Haskell code to arrange that a sequence of calls to a given library are performed by the same native thread and that if an external library calls into Haskell, then any outgoing calls from Haskell are performed by the same native thread.

*) The specification should be implementable in a way that allows a lot of foreign calls to be made with no additional overhead with respect to GHC's current "unsafe" foreign calls.

*) The good performance of the existing lightweight "green" threads in GHC should not be sacrificed. Performance should still OK when using the new features with only a few threads (i.e. not more than commonly used from multithreaded C programs).

*) The specification shouldn't explicitly require lightweight "green" threads to exist. The specification should be implementable in a simple and obvious way in haskell systems that always use a 1:1 correspondence between Haskell threads and OS threads.

*) The specification shouldn't specify which particular OS thread should be used to execute Haskell code. It should be possible to implement it with e.g. a Haskell interpreter running in one OS thread that just uses other OS threads for foreign calls.

*) There should be no unexpected blocking. Especially, threadsafe calls should never cause other threads to block.

I'm currently stuck thinking about one particular problem that I discovered in my current version of the spec. What happens when an unbound Haskell thread calls a threadsafe foreign function which in turn calls a bound foreign exported function? Well, I think my current proposal says relatively clearly what's supposed to happen, but I discovered it's not as easy to fit that in the current implementation of GHC as I thought... in fact it might be quite difficult. I'll have to do more thinking before I can be sure, though.


That's it for today,

Regards,

Wolfgang

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