Issue #20794 has been updated by eightbitraptor (Matt V-H). I've updated [the proposed PR](https://github.com/ruby/ruby/pull/11872) to reflect feedback from [DevMeeting-2024-11-07](https://github.com/ruby/dev-meeting-log/blob/master/2024/DevMeeting-2024-11-...). That is: - `GC.active_gc_name` has been replaced with `GC.config[:implementation]`. - Attempting to set the GC name using `GC.config(implementation: "somethingelse")` will raise `ArgumentError` with the message `Attempting to set read-only key "Implementation"). - `RUBY_DESCRIPTION` has been shortened to `+GC` when built with shared GC support, and `+GC[gc_name]` when a GC module is explicitly loaded (eg. `+GC[mmtk]`). ---------------------------------------- Feature #20794: Expose information about the currently running GC module https://bugs.ruby-lang.org/issues/20794#change-110568 * Author: eightbitraptor (Matt V-H) * Status: Open ---------------------------------------- ## Summary [[Github PR #11872]](https://github.com/ruby/ruby/pull/11872) Currently it's not possible for the user to easily find out information about the currently running GC, and whether modular GC has been enabled. The linked PR provides a method for the user to query whether modular GC's are enabled, and whether one is active, as well as a Ruby method that exposes the currently active GC. ## Background Since Modular GC was introduced it's been possible to build Ruby with several different GC configurations: 1. Modular GC disabled. Existing GC is linked statically, and active - This is the default case 2. Modular GC enabled, but not loaded. Existing GC is linked statically and is active. 3. Modular GC enabled, and a GC implementation is loaded and overrides the static default GC. Currently there's not an accurate way of determining which of these states the current Ruby process is in. This makes writing tests that work accurately given different GC configurations complex. It also means that developers and admins running Ruby aren't able to get accurate information about their Ruby processes. ## Proposal This ticket proposes the following changes: * Add GC status to `RUBY_DESCRIPTION` so it's visible in the output of `ruby -v` * Add a method `GC.active_gc_name` that will return the name of the currently running GC as a String. ## Implementation The linked PR implements these proposals. #### No shared GC support This is the default case. Ruby statically links and uses the existing GC, with no means to override it. `RUBY_DESCRIPTION` is obviously unchanged ``` ❯ ruby -v ruby 3.4.0dev (2024-10-09T14:20:46Z mvh-active-gc-name 3cb4bd4d43) +PRISM [arm64-darwin24] ``` and `GC.active_gc_name` returns `nil` ``` ❯ ./ruby -e "p GC.active_gc_name" nil ``` #### Shared GC support enabled, Not loaded This is the case when Ruby has been configured with `--with-shared-gc=/path/to/gc` but no GC has been explicitly loaded using the `RUBY_GC_LIBRARY` environment `RUBY_DESCRIPTION` will contain `+MOD_GC` to indicate that shared GC support is built in. This is similar to how `+PRISM`, and `+YJIT` work. ``` ❯ ruby -v ruby 3.4.0dev (2024-10-09T14:38:38Z mvh-active-gc-name 3cb4bd4d43) +PRISM +MOD_GC [arm64-darwin24] ``` and `GC.active_gc_name` still returns `nil`. This is because the existing statically linked GC is in use, and has not been overridden. ``` ❯ ./ruby -e "p GC.active_gc_name" nil ``` #### Shared GC support enabled, and modular GC loaded This is the case when Ruby has been configured with `--with-shared-gc=/path/to/gc` **and** an alternative GC library has been explicitly loaded using the `RUBY_GC_LIBRARY` environment variable `RUBY_DESCRIPTION` will contain `+MOD_GC` to indicate that shared GC support is built in. This is similar to how `+PRISM`, and `+YJIT` work. In addition it will also contain a GC name, which is a human readable name that each concrete GC implementation needs to provide as part of its API. This is seen here as `+MOD_GC[mmtk]` - because in this case `librubygc.dylib` reports it's name as being `mmtk` ``` ❯ RUBY_GC_LIBRARY=librubygc.dylib ./ruby -v ruby 3.4.0dev (2024-10-09T14:38:38Z mvh-active-gc-name 3cb4bd4d43) +PRISM +MOD_GC[mmtk] [arm64-darwin24] ``` `GC.active_gc_name` returns the GC's name, as configured by the GC module (this will be identical to the reported name loaded into `RUBY_DESCRIPTION`). ``` ❯ RUBY_GC_LIBRARY=librubygc.dylib ./ruby -e "puts GC.active_gc_name" mmtk ``` ### Conclusion This should cover all states that the GC can now be in. Users can now query the state from the command line to see whether modular GC's are enabled and if so, what GC is active. From Ruby we can use `GC.active_gc_name` in conjunction with the pre-existing option `RbConfig::CONFIG['shared_gc_dir']` to find out this same information. This will now make it easier to run different Ruby processes with different GC builds, as well as make it easier for GC implementors to write tests that behave correctly in all these circumstances. -- https://bugs.ruby-lang.org/