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+# Ruby's Garbage Collectors
+
+This directory contains implementations for Ruby's garbage collector (GC). The GC implementations use the Modular GC API to interact with Ruby. For more details about this API, see the [Modular GC API](#modular-gc-api) section.
+
+Two GC implementations are included in Ruby:
+
+- Default: The GC implementation that is used by default in Ruby. This GC is stable and production ready. The implementation uses a mark-sweep-compact algorithm.
+- MMTk: An experimental implementation using the [MMTk](https://www.mmtk.io/) framework. The code lives in the [ruby/mmtk](https://github.com/ruby/mmtk) repository and is synchronized here. MMTk provides a [wide variety of GC algorithms](https://www.mmtk.io/status#implemented-collectors) to choose from. For usage instructions and current progress, refer to the [ruby/mmtk](https://github.com/ruby/mmtk) repository.
+
+## Building guide
+
+> [!TIP]
+> If you are not sure how to build Ruby, follow the [Building Ruby](https://docs.ruby-lang.org/en/master/contributing/building_ruby_md.html) guide.
+
+> [!IMPORTANT]
+> Ruby's modular GC feature is experimental and subject to change. There may be bugs or performance impacts. Use at your own risk.
+
+1. Configure Ruby with the `--with-modular-gc=<dir>` option, where `dir` is the directory you want to place the built GC libraries into.
+2. Build Ruby as usual.
+3. Build your desired GC implementation with `make modular-gc MODULAR_GC=<impl>`. This will build the GC implementation and place the built library into the `dir` specified in step 1. `impl` can be one of:
+    - `default`: The default GC that Ruby ships with.
+    - `mmtk`: The GC that uses [MMTk](https://www.mmtk.io/) as the back-end. See Ruby-specific details in the [ruby/mmtk](https://github.com/ruby/mmtk) repository.
+4. Run your desired GC implementation by setting the `RUBY_GC_LIBRARY=<lib>` environment variable, where `lib` could be `default`, `mmtk`, or your own implementation (as long as you place it in the `dir` specified in step 1).
+
+## Modular GC API
+
+> [!WARNING]
+> The Modular GC API is experimental and subject to change without notice.
+
+GC implementations interact with Ruby via the Modular GC API. All implementations must provide the functions in [gc/gc_impl.h](https://github.com/ruby/ruby/blob/master/gc/gc_impl.h) for Ruby to hook into. GC implementations can use any public C API in Ruby, along with additional APIs defined in [gc/gc.h](https://github.com/ruby/ruby/blob/master/gc/gc.h).
+
+Additionally, create an extconf.rb file to build the GC library. This file must use [gc/extconf_base.rb](https://github.com/ruby/ruby/blob/master/gc/extconf_base.rb) and the `create_gc_makefile` method.