A Global Interpreter Lock (GIL) is a mutual exclusion

Mutual exclusion

Mutual exclusion algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections. A critical section is a piece of code in which a process or thread accesses a common resource...

 lock

Lock (computer science)

In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...

 held by a programming language

Programming language

A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

 interpreter

Interpreter (computing)

In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language...

 thread

Thread (computer science)

In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...

 to avoid sharing code that is not thread-safe

Thread-safe

Thread safety is a computer programming concept applicable in the context of multi-threaded programs. A piece of code is thread-safe if it only manipulates shared data structures in a thread-safe manner, which enables safe execution by multiple threads at the same time...

 with other threads. In languages with a GIL, there is always one GIL for each interpreter process

Process (computing)

In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...

. CPython

CPython

CPython is the default, most-widely used implementation of the Python programming language. It is written in C. In addition to CPython, there are two other production-quality Python implementations: Jython, written in Java, and IronPython, which is written for the Common Language Runtime. There...

 and CRuby use GILs.

Applications written in programming languages with a GIL can be designed to use separate processes to achieve full concurrency, as each process has its own interpreter and in turn has its own GIL. Otherwise, the GIL can be a significant barrier to concurrency—a price paid for having the dynamism of the language.

Benefits and drawbacks

Use of a Global Interpreter Lock in a language effectively limits the concurrency

Concurrency (computer science)

In computer science, concurrency is a property of systems in which several computations are executing simultaneously, and potentially interacting with each other...

 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 for long periods of time (which can release the lock on the GIL on that thread while it processes), there is likely to be very little increase in speed when running the process on a multiprocessor

Multiprocessor

Computer system having two or more processing units each sharing main memory and peripherals, in order to simultaneously process programs.Sometimes the term Multiprocessor is confused with the term Multiprocessing....

 machine. Due to signaling with a CPU-bound thread, it can cause a significant slowdown, even on single processors.

Reasons for employing such a lock include:

• increased speed of single-threaded programs (no necessity to acquire or release locks on all data structures separately)
• easy integration of C
  C (programming language)
  C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
  
   libraries that usually are not thread-safe.

Examples

Some language implementations that implement a Global Interpreter Lock are CPython

CPython

CPython is the default, most-widely used implementation of the Python programming language. It is written in C. In addition to CPython, there are two other production-quality Python implementations: Jython, written in Java, and IronPython, which is written for the Common Language Runtime. There...

, the most widely used implementation of Python

Python (programming language)

Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...

 and Ruby MRI

Ruby MRI

Matz's Ruby Interpreter or Ruby MRI is the reference implementation of the Ruby programming language. As there is currently no specification of the Ruby language, the MRI implementation is considered the de facto reference. The RubySpec project has created a large test suite that captures...

, the reference implementation of Ruby

Ruby (programming language)

Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...

 (where it is called Global VM Lock).
JVM

Java Virtual Machine

A Java virtual machine is a virtual machine capable of executing Java bytecode. It is the code execution component of the Java software platform. Sun Microsystems stated that there are over 4.5 billion JVM-enabled devices.-Overview:...

-based equivalents of these languages (Jython

Jython

Jython, successor of JPython, is an implementation of the Python programming language written in Java.-Overview:Jython programs can seamlessly import and use any Java class. Except for some standard modules, Jython programs use Java classes instead of Python modules...

 and JRuby

JRuby

JRuby is a Java implementation of the Ruby programming language, being developed by the JRuby team. It is free software released under a three-way CPL/GPL/LGPL license...

) do not use Global Interpreter Locks. IronPython

IronPython

IronPython is an implementation of the Python programming language targeting the .NET Framework and Mono. Jim Hugunin created the project and actively contributed to it up until Version 1.0 which was released on September 5, 2006. Thereafter, it was maintained by a small team at Microsoft until...

 and IronRuby

IronRuby

IronRuby is an implementation of the Ruby programming language targeting Microsoft .NET framework. It is implemented on top of the Dynamic Language Runtime , a library running on top of the Common Language Infrastructure that provides dynamic typing and dynamic method dispatch, among other things,...

 are implemented on top of Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

's Dynamic Language Runtime

Dynamic Language Runtime

The Dynamic Language Runtime from Microsoft is an ongoing effort to bring a set of services that run on top of the Common Language Runtime and provides language services for several different dynamic languages...

and also avoid using a GIL.