Global interpreter lock


A global interpreter lock is a mechanism used in computer-language interpreters to synchronize the execution of threads so that only one native thread can execute at a time. An interpreter that uses GIL always allows exactly one thread to execute at a time, even if run on a multi-core processor. Some popular interpreters that have GIL are CPython and Ruby MRI.

Technical background concepts

A global interpreter lock is a mutual-exclusion lock held by a programming language interpreter thread to avoid sharing code that is not thread-safe with other threads. In implementations with a GIL, there is always one GIL for each interpreter process.
Applications running on implementations with a GIL can be designed to use separate processes to achieve full parallelism, as each process has its own interpreter and in turn has its own GIL. Otherwise, the GIL can be a significant barrier to parallelism.

Drawbacks

Use of a global interpreter lock in a language effectively limits the amount of parallelism reachable through concurrency of a single interpreter process with multiple threads. If the process is almost purely made up of interpreted code and does not make calls outside of the interpreter which block for long periods of time, there is likely to be very little increase in speed when running the process on a multiprocessor machine. Due to signaling with a CPU-bound thread, it can cause a significant slowdown, even on single processors.

Advantages

Reasons for employing a global interpreter lock include:
A way to get around a GIL is creating a separate interpreter per thread, which is too expensive with most languages.

Examples

Some language implementations that implement a global interpreter lock are CPython, the most widely-used implementation of Python, and Ruby MRI, the reference implementation of Ruby.
JVM-based equivalents of these languages do not use global interpreter locks. IronPython and IronRuby are implemented on top of Microsoft's Dynamic Language Runtime and also avoid using a GIL.
An example for an interpreted language not suffering from GIL is Tcl, which is used in the benchmarking tool HammerDB.