Java Virtual Threads: A Case Study

Post by riku_iki » Fri, Jul 19, 2024, 5:30 AM UTC

stoperaticless wrote: post #191

riku_iki wrote: post #179

Earlier quoted context omitted.

> Some usage types don’t care, some do. I suspect if you care about threads overhead, you won't pick Java, because there will be overhead in other areas too > From what I gather virtual threads are an alternative to “callback-hell” (js) or async coloring (python). there is also existing ExecutorService and futures in Java

> there is also existing ExecutorService and futures in Java Yes, virtual threads are an alternative also to those. (Kind of)

Post by cstrahan » Fri, Jul 19, 2024, 8:46 PM UTC

Jtsummers wrote: post #146

nimish wrote: post #145

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This is a really unfortunate gotcha that's not at all obvious. Does it kick preemption up a layer to the OS then?

The "not at all obvious" gotcha is described in the documentation near the top, under the heading "What is a Virtual Thread?": https://docs.oracle.com/en/java/javase/21/core/virtual-threa... > Like a platform thread, a virtual thread is also an instance of java.lang.Thread. However, a virtual thread isn't tied to a specific OS thread. A virtual thread still runs code on an OS thread. However, when code running in a v…

Post by MaxBarraclough » Sat, Jul 20, 2024, 10:07 AM UTC

troupo wrote: post #140

binary132 wrote: post #125

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Greenlets ultimately have to be scheduled onto system threads at the end of the day unless you have a lightweight thread model of some sort supported by the OS, so it’s a little bit misleading depending on how far down the stack you want to think about optimizing for greenlets. You could potentially have a poor implementation of task scheduling for some legacy compatibility reason, however. I guess I’d be curious abo…

Even though yes, in the end you have to map onto system threads, there are still quite a fee things you can do. But this is infeasible for Java, unfortunately. For example, in Erlang the entire VM is built around green threads with a huge amount of guarantees and mechanisms: https://news.ycombinator.com/item?id=40989995 When your entire system is optimized for green threads, the question of "it still needs to map ont…

Post by SureshG » Sun, Jul 21, 2024, 6:45 AM UTC

tomp wrote: post #141

jayd16 wrote: post #137

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Virtual threads could be scheduled pre-emptively but currently the scheduler will wait for some kind of thread sleep to schedule another virtual thread. That's just a scheduler implementation detail and the spec is such that a time slice scheduler could be implemented.

Yes, but the problem is that the spec is such that preemptive blocking doesn't need to be implemented. That means that Java programmers have to be very careful when writing code, lest they block the entire underlying (OS) thread! Again, Go already went through that experience. It was painful. Java should have learned and implemented it from the start