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by chrisseaton 2230 days ago
> Which would, yes, prevent pre-emption, AND require indication of intent

I don't understand why. If you have a maths-intensive operation like matrix-multiplication using untagged maths, why does that prevent pre-emption? Why does it require indication of intent?

And there's already a basically zero-overhead way to implement pre-emption - safepoints - that's what the JVM does when it wants to pre-empt in user-space.
1 comments
lostcolony 2230 days ago
Uh...no. A safepoint is when all threads in the JVM have blocked (which is purely cooperative, and happens during thread transitions), and, importantly, when OS threads running native code still are running, but can't return/respond to the JVM. The JVM doesn't pause those threads.

Which is the point. You can't preempt crunching those numbers if it's not within the BEAM. Which might be fine. Or it might not. Making it invisible to the user is not really a good idea when going for soft realtime properties; at least with a NIF and dirty scheduler you're being explicit about it.

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chrisseaton 2230 days ago
> A safepoint is when all threads in the JVM have blocked

And having blocked them, you can then pre-empt them.

> You can't preempt crunching those numbers if it's not within the BEAM.

I still don't see why sorry. If you had a JIT and you compiled maths intensive code to native code, it could run efficiently in BEAM and still be pre-emptible by having a safepoint in the generated code.

How do you think Java is doing optimised numerical code that is pre-emptible from user-space? Safepoints! BEAM could do the same thing.

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lostcolony 2230 days ago
You, uh, realize that's not actually preemptive right? Like, having to thread in checks, that are cooperative, is by definition not preemptive?
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chrisseaton 2230 days ago
Yes I think that's a reasonable definition of pre-emption because they don't interrupt the numerical pipeline. They cause zero data or control dependencies. But even if you don't think it fits the definition, what do you think the practical difference is when you argue about this terminology?

What did we want to achieve? We wanted to be able to run a tight loop of highly optimised, untagged numerical code but still be able interrupt it to switch threads on demand from user-space if needed.

Safepoints let us do that.

What else did we need that this doesn't cover?

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