Hacker News new | ask | show | jobs
by lmm 885 days ago
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.
1 comments
Groxx 885 days ago
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.

link
kaba0 885 days ago
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

link
Groxx 884 days ago
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.

link
Groxx 884 days ago
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...

link