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by brabel 695 days ago
OS Threads do not suck, they're great. But they are expensive to create as they require a syscall, and they're expensive to maintain as they consume quite a bit of memory just to exist, even if you don't need it (due to how they must pre-allocate a stack which apparently is around 2MB initially, and can't be made smaller as in most cases you will need even more, so it would make most cases worse).

Virtual Threads are very fast to create and allocate only the memory needed by the actual call stack, which can be much less than for OS Threads.

Also, blocking code is very simple compared to the equivalent async code. So using blocking code makes your code much easier to follow. Check out examples of reactive frameworks for Java and you will quickly understand why.
1 comments
kllrnohj 695 days ago
> and they're expensive to maintain as they consume quite a bit of memory just to exist, even if you don't need it (due to how they must pre-allocate a stack which apparently is around 2MB initially,

I'm not familiar with windows, but this certainly isn't the case on Linux. It only costs 2mb-8mb of virtual address space, not actual physical memory. And there's no particular reason to believe the JVM can have a list of threads and their states more efficiently than the kernel can.

All you really save is the syscall to create it and some context switching costs as the JVM doesn't need to deal with saving/restoring registers as there's no preemption.

The downside though is you don't have any preemption, which depending on your usage is a really fucking massive downside.

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brabel 694 days ago
> And there's no particular reason to believe the JVM can have a list of threads and their states more efficiently than the kernel can.

Of course there is. The JVM is able to store the current stack for the Thread efficiently in the pre-allocated heap. Switching execution between Virtual Threads is very cheap. Experiments show you can have millions of VTs, but only a few thousand OS Threads.

I don't know why you think preemption is a big downside?! The JVM only suspends a Thread at safe points and those are points where it knows exactly when to resume. I don't believe there's any downsides at all.

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Someone 694 days ago
> The downside though is you don't have any preemption, which depending on your usage is a […] massive downside.

Nobody is taking OS threads away, so you can choose to use them when they better fit your use case.

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