From: "matteo.folin@... (Matteo Folin) via ruby-core" <ruby-core@...>
Date: 2026-10-19T14:57:24+00:00
Subject: [ruby-core:127173] [Ruby Bug#22454] Heaps of exited Ractors are reclaimed only by a global GC, so memory grows without bound with short-lived Ractors

Issue #22454 has been updated by matteo.folin@gmail.com (Matteo Folin).

Subject changed from exited Ractors' memory is reclaimed only by a global GC, so memory grows without bound. to Heaps of exited Ractors are reclaimed only by a global GC, so memory grows without bound with short-lived Ractors
ruby -v changed from 4.1.0-preview1 to ruby 4.1.0preview1 (2026-10-11 master 2d08b5f42f)

### Description

With the per-Ractor GC in 4.1.0-preview1, a program that keeps creating
short-lived Ractors grows its RSS by ~140 MB per 1,000 exited Ractors.
As I understand the NEWS, a dead Ractor's heap is reclaimed by a global
GC, but nothing starts one when Ractors exit, so the heaps pile up.
In 8,000 Ractors only one global GC ran, and memory kept growing after
it. Calling `GC.start` after each round of 1,000 keeps RSS flat, so
the memory is reclaimable. The automatic trigger doesn't seem to
account for it.

A server that runs a Ractor per connection hits this directly: it ran
out of memory (killed by the kernel's OOM killer in an 8 GB container)
a few seconds after its first few thousand connections closed. On 4.0
the same server stays flat.

### Reproduction

```ruby
# Rounds of 1,000 short-lived Ractors, each allocating a little and exiting.
# Pass "gc" to run GC.start after each round.
Warning[:experimental] = false
def rss_mb = File.read("/proc/self/status")[/VmRSS:\s+(\d+)/, 1].to_i / 1024
def gc_stat = (GC.stat(scope: :global) rescue GC.stat)

8.times do |round|
  Array.new(1000) { Ractor.new { Array.new(900) { "x" * 30 }.size } }.each(&:join)
  GC.start if ARGV[0] == "gc"
  stat = gc_stat
  puts "round #{round}: rss #{rss_mb} MB, Ractor.count #{Ractor.count}, " \
    "GCs #{stat[:count]}, global GCs #{stat.fetch(:global_gc_count, "-")}"
end
```

### Results

Linux aarch64 (Docker on macOS, official `ruby` images), pinned to 4
cores.

4.1.0-preview1, `ruby dead_ractor_heaps.rb`:

```
round 0: rss 159 MB, Ractor.count 1, GCs 1000, global GCs 0
round 1: rss 302 MB, Ractor.count 1, GCs 2000, global GCs 0
round 2: rss 448 MB, Ractor.count 1, GCs 3000, global GCs 0
round 3: rss 596 MB, Ractor.count 1, GCs 4000, global GCs 0
round 4: rss 733 MB, Ractor.count 1, GCs 5001, global GCs 1
round 5: rss 860 MB, Ractor.count 1, GCs 6001, global GCs 1
round 6: rss 988 MB, Ractor.count 1, GCs 7001, global GCs 1
round 7: rss 1125 MB, Ractor.count 1, GCs 8001, global GCs 1
```

4.1.0-preview1, `ruby dead_ractor_heaps.rb gc`:

```
round 0: rss 162 MB, Ractor.count 1, GCs 1001, global GCs 1
round 1: rss 292 MB, Ractor.count 1, GCs 2002, global GCs 2
round 2: rss 435 MB, Ractor.count 1, GCs 3003, global GCs 3
round 3: rss 435 MB, Ractor.count 1, GCs 4004, global GCs 4
...
round 7: rss 436 MB, Ractor.count 1, GCs 8008, global GCs 8
```

4.0.7, either way: RSS 34 ��� 39 MB over the 8 rounds (44 MB with `gc`).

### Expected

RSS bounded without an explicit `GC.start`, for example a global GC (or
reclaiming dead heaps without one) triggered by the memory or the count
of exited Ractors' heaps waiting to be reclaimed.

Also worth a look: even with `GC.start` every round, RSS levels off at
~435 MB, against ~44 MB on 4.0.7.

----------------------------------------
Bug #22454: Heaps of exited Ractors are reclaimed only by a global GC, so memory grows without bound with short-lived Ractors
https://bugs.ruby-lang.org/issues/22454#change-119588

* Author: matteo.folin@gmail.com (Matteo Folin)
* Status: Open
* ruby -v: ruby 4.1.0preview1 (2026-10-11 master 2d08b5f42f)
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
- 8 rounds of 1,000 short-lived Ractors on 4.1.0-preview1: RSS climbs from 159 MB to 1,125 MB, with a single global GC.
- With a GC.start after each round, it stops at ~435 MB.
- On 4.0.7 it stays at 34���44 MB.
- It tells the server story: OOM-killed in 8 GB once the first sockets closed.
- It asks for a trigger based on the exited Ractors still waiting to be reclaimed, and notes that even 435 MB is ~10�� 4.0's figure.



-- 
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