RUBY_ENGINE == "ruby"

''Matz's Ruby Implementation'' or ''MRI'' refers to the original Ruby interpreter by Yukihiro Matsumoto, the inventor of Ruby. Matz and contributors wrote the interpreter in C language; MRI is also known as ''C Ruby'' or ''CRuby'', by analogy with CPython.

The term "MRI" excludes other Ruby engines (such as JRuby or Rubinius). For example, one can say that Ruby has Array#sort method, and MRI uses quicksort; other Ruby engines might use different sorting algorithm.

When code works with Ruby 1.8.7, this can be MRI 1.8.7, or anything else that implements the same language.

Advantages

• MRI is the reference implementation, and the first implementation of every new Ruby version.
• MRI can fork, though only for Unix clones (not for Windows).
• MRI can save and restore continuations, though the implementation is slow because it copies the call stack.

Disadvantages

• MRI has the ''Global VM Lock'', alias ''Giant VM Lock'' or ''GVL''. A thread, to run Ruby code, must hold this exclusive lock. Only one thread can hold the GVL; therefore, multiple threads can use only one CPU. (Contrast JRuby, where multiple threads can use multiple CPUs.)
• MRI 1.8 is slow. Programs for Ruby 1.8 often run faster in JRuby or Rubinius.

Features

MRI has

an interpreter,

a mark-and-sweep garbage collector, and

the core and standard libraries.

Most of the core library is in C. The standard library is a mix of Ruby code and C extensions.

MRI 1.8 has a somewhat slow interpreter. MRI 1.9 has a new interpreter called Yet Another Ruby VM (YARV); it translates Ruby source code to an internal bytecode, then interprets the bytecode. Ruby code can run a few times faster in MRI 1.9 than in MRI 1.8. Ruby code remains slower than C code. For example, MRI 1.9.3 changes its 'date' package from Ruby code to a C extension; this gives better performance.