The Platform Thread Problem Every Java thread since Java 1.0 maps 1:1 to an OS thread. OS threads are heavy: Stack memory: 512 KB – 2 MB per thread (configurable with -Xss, default 512 KB on Linux) Context switch cost: ~1–10 μs per switch (kernel mode transition + cache invalidation) Hard limit: A 64-bit machine with 8 GB RAM can support roughly 8,000–16,000 OS threads before running out of stack memory This limit shapes how Java servers are built.