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Comment by papercrane | Hacker News Reader

The issue here is currently virtual threads don't work well with the 'sychronized' keyword. Right now synchronized will pin the carrier thread. The fix was to switch to a higher-level abstraction that works with virtual threads.

My understanding is there is work to make synchronized not pin the carrier thread, but that's some pretty complex and important code to change.

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From a relatively brief skim and past Go and Java experience: synchronized blocks the current normal thread, so that doesn't really seem any different to me. If you starve your threads, you starve your threads.

It definitely leaves room to optimize by not pinning that thread, which would be great, but that shouldn't change semantics at all. Or is there something actually screwed up in the implementation of virtual threads that makes this a much bigger issue?

It’s a thread that supports n virtual threads. You want synchronized in virtual thread a and not the carrier thread which will block all the virtual threads.

Been away from Java land for a while. How did something like that even get into release? That’s like a pretty big loaded shotgun to leave lying around with lots of kids playing, no?

I agree. Seems like a huge Java design error.

It's an explicitly documented shortcoming of the existing implementation that will be fixed soon. I knew immediately from the title of TA what probably happened. The other similar limitations (CPU-bound tasks, native calls) seem much more severe, but are ultimately unsolvable. Meanwhile, the issue with synchronized is regarded as a scalability bottleneck since the JDK is supposed to temporarily spawn additional platform threads. This behavior can be controlled via the system property `jdk.virtualThreadScheduler.maxPoolSize`.

Also, this is a benchmark. It's not surprising that they managed to produce a situation where more than n_cores virtual threads would actually start waiting.

Appreciate the reply. Hope you get it out soon since many /g do not read documentation and synchronized semantics changing is a ‘surprise’, specially since this one of those bugs that is a nursery for heisenbugs.

It spawns a new carrier thread in place, up to a certain, configurable limit. But starving carrier threads will also result in effectively live locks, so that’s not a solution.

So I don’t see the big fuss about it - don’t spawn a million virtual thread that all just spams synchronized?

Java's virtual threads are supposed to be a drop-in replacement for real threads. But using virtual threads means you get a far smaller number of real threads, and things that were safe back when you had an unlimited number of real threads available (or at least, a larger number than your database connection pool) are no longer safe.

Shouldn't you be able to use the same number of real threads though, plus some additional effectively-threads for the virtual threads that are not pinned? Doesn't seem like this should change semantics there, so the risk would be code that changes because of perceived advantages which are not true in edge cases - that's new behavior that wasn't possible before, there aren't really any existing semantics to break.

If they're, like, limiting to CPU cores * 2 threads: yeah that would be Bad™. Unambiguously. I haven't been able to find anything conclusive about this though.

That sort of what happens, there is just a configurable hard limit on how much new thread may be created that was hit by this benchmark.

As mentioned in another comment: jdk.virtualThreadScheduler.maxPoolSize

Is there no limit (ignoring outside limits, e.g. from the OS) for normal threads? I know people usually use limited size thread pools for a variety of reasons, but I can't say that I've actually tried to exceed limits in a Java process yet...

That would indeed be a problem if it's not similarly unlimited by default. Configurable makes perfect sense, as does attempting to be conservative, but small hard-capped defaults are very obviously going to cause problems, especially while synchronized locks the carrier.

Since it's too late to edit, Java docs say:

>The maximum number of platform threads available to the scheduler. It defaults to 256.

Yeah, that's pretty small. >256 simultaneous synchronized calls doesn't seem particularly extreme, given how common its use is.

Tho now I wonder if you can just set this to max-int and resume like normal, or if giant values do awful things internally...

In this case the synchronized blocks are released by calls to Object.wait, so that code would not deadlock with normal threads.

The issue is that Object.wait doesn't suspend virtual threads, so you get deadlocks. The answer is to reimplement usages of the wait/notify pattern to use locks or concurrent collections (for example, using a concurrent message queue for the producer/consumer pattern, which is a common use case for synchronized/wait/notify).

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