How does Ruby's Global VM Lock (GVL) actually work at the execution level?

GVL ensures only one Ruby thread executes Ruby bytecode at a time, even on multi-core systems.

In CRuby, the GVL (Global VM Lock) is a mutex protecting the Ruby VM state. Even though threads are OS-level, execution of Ruby bytecode is serialized. Threads alternate execution by acquiring the GVL in time slices or when blocking operations occur. When a thread performs I/O, it releases the GVL, allowing another thread to run. This design simplifies memory management and GC safety but prevents true CPU parallelism in Ruby code.