The explanation in the video is good, here is a transcription for convenience (emphasis added): > OpenJDK implements threads as thin wrappers around the OS thr

The explanation in the video is good, here is a transcription for convenience (emphasis added): > OpenJDK implements threads as thin wrappers around the OS thread [...] OS threads are expensive, by which I mean that we cannot have too many of them. Maybe a few thousand active ones, maybe ten thousand active ones, but that's about it. And the reason for that is that the OS doesn't know the various languages, and the most costly component of a thread is its stack, and the operating system doesn't know how the various languages it might support makes use of the stack. So it must address the worst case and allocate a very large stack. It's usually on a megabyte scale. And even though we use virtual memory and not all of it is committed to physical memory, once we touch a page it becomes committed – it can't be uncommitted again because the OS doesn't know how the stack is used. And in any event, all the operations are done at a page granularity which is 4k, it's also not that small. There may also be arbitrary OS-specific limits. For example on Linux there is system-wide limit on the number of threads, controlled by /proc/sys/kernel/threads-max (docs: https://www.kernel.org/doc/html/latest/admin-guide/sysctl/kernel.html#threads-max https://www.kernel.org/doc/html/latest/admin-guide/sysctl/ke..., see also: https://stackoverflow.com/a/8849114 https://stackoverflow.com/a/8849114). On my system it's ~60,000 (with 8G RAM).