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0 pointshaxen5y ago0 comments

We use the same technique of cooperative multithreading under any name, but without the low-level support to be able to write plain sequential Java code. However, even though that changes our internal programming model, the behavior with respect to native threads, interactions with the OS scheduler, CPU caches, etc., should be identical.

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pron5y ago· 2 in thread

Loom's virtual threads aren't cooperative, and the implementation actually does interact with the GCs in non-trivial ways. ZGC support was only added to Loom last week, and might still be unstable, but it would be interesting to test. Please report any finding -- be it performance or stability issues -- to the Loom mailing list.

haxenOP5y ago

"Virtual threads" are fibers? When you say they aren't cooperative, you mean there's JVM infrastructure that actually pre-empts a fiber?

pron5y ago

> "Virtual threads" are fibers?

Yes, but we found that the name "virtual thread" works better. For one, these fibres are actually an implementation of java.lang.Thread, so it's the same abstraction; for another, when people hear "fibres" they sometimes compare it to other implementations of fibres that are implemented very differently, and that causes confusion.

> When you say they aren't cooperative, you mean there's JVM infrastructure that actually pre-empts a fiber?

First, I mean that they don't require any explicit yields. Sometimes people confuse "preemptive" with "time-sliced preemption", so I'd rather just say "non-cooperative".

Second, there is a capability to forcefully preempt a virtual thread even if it's in some computation loop, but that's up to a chosen scheduler, and the scheduler isn't part of the JVM. You can supply your own scheduler, written in Java, and you'll be able to choose to preempt a runaway thread. Currently, this capability is not exposed, but it will be eventually.

See http://cr.openjdk.java.net/~rpressler/loom/loom/sol1_part1.h...

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