feat: virtual thread executor support

[johanandren]

This adds a new executor config which will make a dispatcher run each scheduled task as a separate, new, virtual thread. This is very useful for blocking logic, for example in futures, but less so for running actors on and has considerable overhead for message intense systems.

Somewhat related issue #31613

[patriknw]

How would you write this without reflection?

[patriknw]

isn't the public way like this?

ThreadFactory virtualThreadFactory = Thread.ofVirtual().factory();

[johanandren]

Yep, that's what we are calling here, with a potential name inbetween Thread.ofVirtual().name(virtualThreadName).factory()

[johanandren]

To clarify the comment, the reason why we can't just do ofVirtual.getClass.getMethod("name") is that it is of a package private type, so we need to look up the interface it implements that has the name method instead.

[patriknw]

I was thinking we could use Thread.Builder.OfVirtual, which is the public type. I think this should work:

val ofVirtual = ofVirtualMethod.invoke(null)
val nameMethod = classOf[Thread.Builder.OfVirtual].getMethod("name", classOf[String])
nameMethod.invoke(ofVirtual, virtualThreadName)

[johanandren]

Thread.Builder.OfVirtual only exists on JDK 21 and later so we can't do classOf[Thread.Builder.OfVirtual] and have the code still compile on 11 and 17

[patriknw]

sorry, above comment about ThreadBuilders.VirtualThreadBuilder was misleading me. I thought you reached out for that one. You are already using the one that I suggested.

val ofVirtualInterface = dynamicAccess.getClassFor[AnyRef]("java.lang.Thread$Builder$OfVirtual").get

[johanandren]

Ah, I'll drop the comment, it's not needed and worse if it is confusing :)

[johanandren]

For the record, here are some results of ActorBenchmark comparing direct JFP with virtual threads to see the overhead (Linux, 12 core x86 intel i5):

akka-bench-jmh/jmh:run -f 1 -wi 10 -i 10 .*actor\.ActorBenchmark
[info] Benchmark            (batchSize)    (dispatcher)                                         (mailbox)  (tpt)   Mode  Cnt          Score         Error  Units
[info] ActorBenchmark.echo           50  fjp-dispatcher  akka.dispatch.SingleConsumerOnlyUnboundedMailbox     50  thrpt   10  202 427 509.824 ± 4177250.804  ops/s
[info] ActorBenchmark.echo           50   vt-dispatcher  akka.dispatch.SingleConsumerOnlyUnboundedMailbox     50  thrpt   10  145 382 846.167 ±  288754.289  ops/s

I've also run a benchmark looking at gc, sadly did not save the output, but alloc rate up from 0.001 B per op to 0.309 B, not scaled with troughput: 27mb/s for FJP up to 48mb/s with virtual thread per task, this leads to around 10% more time spent on GC.

This proves that it is maybe not a good default default dispatcher for actors, however for a blocking io dispatcher that will anyway have a much lower throughput, mostly waiting for some blocking task, it seems like a reasonable tradeoff.

[patriknw]

I was thinking we could use Thread.Builder.OfVirtual, which is the public type. I think this should work:

val ofVirtual = ofVirtualMethod.invoke(null)
val nameMethod = classOf[Thread.Builder.OfVirtual].getMethod("name", classOf[String])
nameMethod.invoke(ofVirtual, virtualThreadName)

[patriknw]

LGTM, with possibly changing misleading comment

[patriknw]

sorry, above comment about ThreadBuilders.VirtualThreadBuilder was misleading me. I thought you reached out for that one. You are already using the one that I suggested.

val ofVirtualInterface = dynamicAccess.getClassFor[AnyRef]("java.lang.Thread$Builder$OfVirtual").get