Re: Native Threads in the RTS

[Wolfgang Thaller]

I'll write up a new version of the proposal tomorrow. For now, here are some answers and [at the end] a question about the current FFI specification.

Simon Peyton Jones wrote:

You don't say (but you do mean)

	A bound Haskell thread can be executed only by its associated
native thread

No I don't mean that, and I've been avoiding to say that. This is how the GHC implementation will probably handle it, but I do not want to put that in the general specification. Alastair remarked a while ago:

For that matter, I'd like it to be possible to implement this spec in
Hugs.  Hugs is internally single-threaded but this spec is concerned
with what happens when Haskell calls out to C and we could arrange to
switch into the appropriate native thread when Hugs calls out to C.

After all, the only thing which needs to be guaranteed is that foreign functions called by the Haskell thread are executed in the associated native thread.

You don't say (and I'm not sure if you mean)

	If a bound native thread blocks, all of its associated Haskell
	threads are blocked too

	If a bound Haskell thread blocks, its associate native thread
and all its
	associated Haskell threads also block.

Does this sound clearer:

*) Exactly one Haskell thread associated with the native thread is executing. All other associated Haskell threads are blocked. No foreign code is being executed by the native thread.
*) The native thread is executing foreign code. No Haskell code is executing in any of the associated Haskell threads.
*) The native thread and all Haskell threads associated with it are blocked.

| The thread that main runs in, threads created using forkIO and threads
| created for running finalizers or signal handlers are not necessarily
| associated with a native thread. However, an implementation might
| choose to do so.

But the impl may *not* choose a bound native thread. These must be kept
inviolate.

[...]
Again, it must not be a bound native thread.

Good point, I had overlooked that.

If a bound Haskell thread
	calls a foreign import that is not labelled 'threadsafe'
	which calls a bound foreign export
does that work?  What if the foreign export was not bound?

Similarly, if the foreign import was labelled 'threadsafe', would it
work?  It's not obvious to me.  Some kind of semantics would be good.

Good question. I reread Section 3.3 of the FFI document (RC7), and now I think I cannot clarify my specification in this respect without first asking others to clarify the current specs - can someone explain the distinction between unsafe, safe and threadsafe in the current FFI to me? I think I know what it does in GHC, but what's the general definition? I've read the description in the FFI document, but it's not clear to me. Is there any reason why "safe" is the default and not "threadsafe"? After all, "safe" is less safe (it might cause the whole program to block). To me, "safe" seems to be an odd middle ground between speed and safety. What is "safe" guaranteed/allowed to do? Is it _guaranteed_ to block other Haskell threads under certain conditions? Or is that only an artifact of current implementations? Why are implementations allowed to _silently_ fall back to "safe" when "threadsafe" is not supported? Isn't that dangerous?

If I'm not mistaken, "threadsafe" calls from bound Haskell threads would have exactly the same overhead as "safe" calls. Should we make sure that "safe" calls somehow block other threads? If so, why?


Thats all for now....

Wolfgang

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