From: "nobu (Nobuyoshi Nakada) via ruby-core" <ruby-core@...>
Date: 2025-01-29T08:08:24+00:00
Subject: [ruby-core:120818] [Ruby master Feature#20470] Extract Ruby's Garbage Collector
Issue #20470 has been updated by nobu (Nobuyoshi Nakada).
This change still keeps `GC_DEBUG_STRESS_TO_CLASS` feature, but removed `GC.add_stress_to_class` method.
Is it intentional?
----------------------------------------
Feature #20470: Extract Ruby's Garbage Collector
https://bugs.ruby-lang.org/issues/20470#change-111686
* Author: peterzhu2118 (Peter Zhu)
* Status: Closed
----------------------------------------
# Extract Ruby's Garbage Collector
## Background
As described in [[Feature #20351]](https://bugs.ruby-lang.org/issues/20351), we are working on the ability to plug alternative garbage collector implementations into Ruby. Our goal is to allow developers and researchers to create and experiment with new implementations of garbage collectors in Ruby in a simplified way. This will also allow experimentation with different GC implementations in production systems so users can choose the best GC implementation for their workloads.
## Implementation
GitHub PR: [10721](https://github.com/ruby/ruby/pull/10721)
In this patch, we have split the current `gc.c` file into two files: `gc.c` and `gc_impl.c`.
`gc.c` now only contains code not specific to Ruby GC. This includes code to mark objects (which the GC implementation may choose not to use) and wrappers for internal APIs that the implementation may need to use (e.g. locking the VM).
`gc_impl.c` now contains the implementation of Ruby's GC. This includes marking, sweeping, compaction, and statistics. Most importantly, `gc_impl.c` only uses public APIs in Ruby and a limited set of functions exposed in `gc.c`. This allows us to build `gc_impl.c` independently of Ruby and plug Ruby's GC into itself.
## Demonstration
After [checking out the branch](https://github.com/ruby/ruby/pull/10721), we can first configure with `--with-shared-gc`:
```bash
$ ./configure --with-shared-gc
...
$ make -j
...
```
Let's now change the slot size of the GC to 64 bytes:
```bash
$ sed -i 's/\(#define BASE_SLOT_SIZE\).*/\1 64/' gc_impl.c
```
We can compile `gc_impl.c` independently using the following commands for clang or gcc (you may have to change the last `-I` to match your architecture and platform):
```bash
$ clang -Iinclude -I. -I.ext/include/arm64-darwin23 -undefined dynamic_lookup -g -O3 -dynamiclib -o libgc.dylib gc_impl.c
$ gcc -Iinclude -I. -I.ext/include/x86_64-linux -Wl,-undefined,dynamic_lookup -fPIC -g -O3 -shared -o libgc.so gc_impl.c
```
We can see that by default, the slot size is 40 bytes and objects are 40 bytes in size:
```bash
$ ./ruby -e "puts GC.stat_heap(0, :slot_size)"
40
$ ./ruby -robjspace -e "puts ObjectSpace.dump(Object.new)"
{"address":"0x1054a23f0", "type":"OBJECT", "shape_id":3, "slot_size":40, "class":"0x10528fd38", "embedded":true, "ivars":0, "memsize":40, "flags":{"wb_protected":true}}
```
We can now load our new GC using the `RUBY_GC_LIBRARY_PATH` environment variable (note that you may have to change the path to the DSO):
```bash
$ RUBY_GC_LIBRARY_PATH=./libgc.dylib ./ruby -e "puts GC.stat_heap(0, :slot_size)"
64
$ RUBY_GC_LIBRARY_PATH=./libgc.dylib ./ruby -robjspace -e "puts ObjectSpace.dump(Object.new)"
{"address":"0x1038de440", "type":"OBJECT", "shape_id":3, "slot_size":64, "class":"0x10355fc00", "embedded":true, "ivars":0, "memsize":64, "flags":{"wb_protected":true}}
```
## Benchmark
Benchmarks were ran on commit [c78cebb](https://github.com/ruby/ruby/commit/c78cebb469fe56b45ee5daad16ae976b7760497c) on Ubuntu 22.04 using [yjit-bench](https://github.com/Shopify/yjit-bench/) on commit [cc5a76e](https://github.com/Shopify/yjit-bench/commit/cc5a76ef6240113650547bdfb260a87934208321).
Compiling gc_impl branch without `--with-shared-gc` (i.e. how the default Ruby is built), the benchmarks show little to no decrease in performance, with most of it being 0% to 1% slower:
```
-------------- ----------- ---------- ------------ ---------- --------------- --------------
bench master (ms) stddev (%) gc_impl (ms) stddev (%) gc_impl 1st itr master/gc_impl
activerecord 73.9 0.3 74.6 0.3 1.00 0.99
chunky-png 911.1 0.2 937.2 0.2 0.97 0.97
erubi-rails 1582.4 0.1 1583.5 0.0 1.00 1.00
hexapdf 2716.2 1.1 2760.2 0.7 1.00 0.98
liquid-c 68.9 0.5 68.6 0.4 1.00 1.00
liquid-compile 67.9 0.1 68.2 0.2 0.99 1.00
liquid-render 172.8 0.1 174.9 0.1 0.99 0.99
lobsters 1033.9 0.4 1036.0 0.3 1.08 1.00
mail 135.1 0.2 136.5 0.2 0.99 0.99
psych-load 2250.8 0.1 2274.9 0.3 0.99 0.99
railsbench 2499.2 0.2 2502.9 0.1 1.00 1.00
rubocop 178.3 0.5 179.8 0.4 1.00 0.99
ruby-lsp 116.8 0.1 118.5 0.2 1.00 0.99
sequel 75.4 0.2 76.2 0.3 0.99 0.99
-------------- ----------- ---------- ------------ ---------- --------------- --------------
```
Compiling gc_impl branch with `--with-shared-gc` and loading Ruby's current GC using `RUBY_GC_LIBRARY_PATH`, the benchmarks are still fairly good with performance decrease of only around 1% to 2%:
```
-------------- ----------- ---------- ------------ ---------- --------------- --------------
bench master (ms) stddev (%) gc_impl (ms) stddev (%) gc_impl 1st itr master/gc_impl
activerecord 74.2 0.2 75.4 0.5 0.98 0.98
chunky-png 916.3 0.3 933.2 0.1 0.98 0.98
erubi-rails 1597.6 0.1 1586.3 0.2 1.01 1.01
hexapdf 2731.4 0.5 2776.8 0.7 1.00 0.98
liquid-c 68.5 0.1 68.9 0.4 0.97 0.99
liquid-compile 67.4 0.4 68.3 0.2 0.95 0.99
liquid-render 171.8 0.1 175.6 0.2 0.97 0.98
lobsters 1031.9 0.3 1041.4 0.3 0.94 0.99
mail 135.5 0.4 136.7 0.1 0.99 0.99
psych-load 2246.0 0.1 2281.3 0.1 0.99 0.98
railsbench 2490.9 0.0 2490.0 0.1 1.01 1.00
rubocop 179.8 2.3 180.0 0.4 0.94 1.00
ruby-lsp 117.3 0.1 118.5 0.1 0.99 0.99
sequel 75.8 0.5 76.3 0.2 0.99 0.99
-------------- ----------- ---------- ------------ ---------- --------------- --------------
```
## Limitations
We recognize that our current implementation does not yet offer the flexibility required for a generic plug-in GC. Specifically, the set of APIs that the plug-in GC has to implement is relatively large, at around 70 functions. Additionally, some of these functions are specific to the current GC.
We would like to emphasize that the API is NOT stable and is subject to change. We will be working on improving this API and reducing the surface area. This will be future work and we're not working on it in this phase.
## Future plans
- Refactor and improve `gc_impl.c`.
- Implement alternate GC implementations, such as the Epsilon GC and [MMTk](https://www.mmtk.io/) to prove that this API allows for alternate implementations of the GC.
- Reduce and improve the API of the GC implementation.
- Benchmark and improve performance of the DSO API.
--
https://bugs.ruby-lang.org/
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