How does Ruby's Global VM Lock (GVL) impact multi-threaded performance and concurrency?

The GVL guarantees that only a single OS thread can execute Ruby bytecode at any given moment, preventing true parallel multi-core execution for CPU-bound tasks but allowing high concurrency for I/O-bound processes.

MRI (Matz's Ruby Interpreter) utilizes a Global VM Lock (GVL) to safeguard its internal data structures and memory allocations against race conditions. Consequently, even on multi-core server processors, standard Ruby threads cannot execute computation instructions in parallel. However, during I/O blocks (such as database queries, HTTP lookups, or file writes), the GVL is unlocked by the interpreter, allowing other threads to run. For CPU-intensive operations, threads add execution scheduling overhead without any performance improvements.