Issue #19783 has been updated by peterzhu2118 (Peter Zhu).
[Bug #19436] is fixed by checking inline method cache data structures.
Thank you for fixing the bug. An issue that was brought up to me by the [MMTk team](https://www.mmtk.io/) (the people who are working on implementing alternate GC in Ruby) is that the current implementation of #19436 only works for the mark-sweep garbage collector. This is causing issues for them because some collectors do not mark the whole heap, so we cannot always determine the liveliness of objects. Here are quotes from our discussions:
There are well-known solutions for handling such fields. We re-visit those fields after tracing and clear them. It is important that we either treat them like strong references and trace those fields during tracing, or handle those fields after tracing and clear them if they point to unreachable objects. But in either way, we always have to handle them. We can't just ignore those fields.
How about to pass the data structure like that?
Using a linked list was my original implementation. However, @byroot pointed out that using a linked list is bad for Copy-on-Write performance because these objects are usually long-lived, and so we should avoid writing into them.
Could you make such benchmark?
I will look into benchmarking this. ---------------------------------------- Feature #19783: Weak References in the GC https://bugs.ruby-lang.org/issues/19783#change-104219 * Author: peterzhu2118 (Peter Zhu) * Status: Open * Priority: Normal ---------------------------------------- GitHub PR: https://github.com/ruby/ruby/pull/8113 I'm proposing support for weak references in the Ruby garbage collector. This feature adds a new function called `void rb_gc_mark_weak(VALUE *ptr)` which marks `*ptr` as weak, meaning that if no other object strongly marks `*ptr` (using `rb_gc_mark` or `rb_gc_mark_movable`), then it will be overwritten with `*ptr = Qundef`. Weak references are implemented using a buffer in `objspace` that stores all the `ptr` in the latest marking phase. After marking has finished, we iterate over the buffer and check if the `*ptr` is a dead object. If it is, then we set `*ptr = Qundef`. Weak references are implemented on the callable method entry (CME) of callcaches, which fixes issue #19436. Weak references are also implemented on `ObjectSpace::WeakMap` and `ObjectSpace::WeakKeyMap`, which have: - Significantly simpler implementations because we no longer need to have multiple tables and do not need to define finalizers on the objects. - Support for compaction because finalizers pin objects and we no longer need to define finalizers on the objects. - Much faster performance (see [benchmarks](#microbenchmarks)). ## Metrics This patch also adds two metrics, `GC.latest_gc_info(:weak_references_count)` and `GC.latest_gc_info(:retained_weak_references_count)`. These two metrics returns information about the number of weak references registered and the number of weak references retained (references that did not point to a dead object) in the last GC cycle. ## Benchmark results ### YJIT-bench We see largely no change in performance or memory usage after this feature. ``` -------------- --------- ---------- --------- ----------- ---------- --------- -------------- ----------- bench base (ms) stddev (%) RSS (MiB) branch (ms) stddev (%) RSS (MiB) branch 1st itr base/branch activerecord 72.3 2.2 51.9 72.9 2.2 51.9 0.99 0.99 chunky-png 889.2 0.3 43.9 874.5 0.3 42.5 1.02 1.02 erubi-rails 21.2 13.5 90.7 21.0 13.3 90.9 1.01 1.01 hexapdf 2557.0 0.8 157.1 2559.2 0.7 197.1 1.01 1.00 liquid-c 65.2 0.4 34.5 65.4 0.4 34.5 0.99 1.00 liquid-compile 62.5 0.4 30.9 62.2 0.4 31.0 1.00 1.01 liquid-render 164.6 0.4 33.1 162.6 0.3 33.1 1.01 1.01 mail 133.3 0.1 46.4 134.4 0.2 46.4 1.03 0.99 psych-load 2066.6 0.2 31.6 2083.6 0.1 31.6 0.99 0.99 railsbench 2027.0 0.5 88.8 2019.4 0.5 89.0 1.01 1.00 ruby-lsp 65.6 3.0 90.1 65.4 3.1 88.5 1.00 1.00 sequel 73.1 1.1 36.6 73.1 1.1 36.6 1.00 1.00 -------------- --------- ---------- --------- ----------- ---------- --------- -------------- ----------- ``` ### Microbenchmarks We can see signficantly improved performance in `ObjectSpace::WeakMap`, with `ObjectSpace::WeakMap#[]=` being nearly 3x faster. Base: ``` ObjectSpace::WeakMap#[]= 1.037M (± 0.5%) i/s - 5.262M in 5.072833s ObjectSpace::WeakMap#[] 12.367M (± 0.9%) i/s - 62.479M in 5.052365s ``` Branch: ``` ObjectSpace::WeakMap#[]= 3.054M (± 0.3%) i/s - 15.448M in 5.058783s ObjectSpace::WeakMap#[] 15.796M (± 4.8%) i/s - 79.245M in 5.028583s ``` Code: ```ruby require "bundler/inline" gemfile do source "https://rubygems.org" gem "benchmark-ips" end wmap = ObjectSpace::WeakMap.new key = Object.new val = Object.new wmap[key] = val Benchmark.ips do |x| x.report("ObjectSpace::WeakMap#[]=") do |times| i = 0 while i < times wmap[Object.new] = Object.new i += 1 end end x.report("ObjectSpace::WeakMap#[]") do |times| i = 0 while i < times wmap[key] wmap[val] # does not exist i += 1 end end end ``` -- https://bugs.ruby-lang.org/