[ruby-core:121519] [Ruby Bug#21214] VmRSS consumption increase in Ruby 3.4.2 vs Ruby 3.3.6

Issue #21214 has been reported by mood_vuadensl (LOIC VUADENS).

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Bug #21214: VmRSS consumption increase in Ruby 3.4.2 vs Ruby 3.3.6
https://bugs.ruby-lang.org/issues/21214

* Author: mood_vuadensl (LOIC VUADENS)
* Status: Open
* ruby -v: ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
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Hello,

After updating Ruby from 3.3.6 to 3.4.2, our batch-style (not based on rails) application exceed its memory limit.
Below is an example script that runs on both versions and demonstrates that 'ObjectSpace.memsize_of_all' does not vary significantly, but the OS 'VmRSS' increases significantly.

Do you have any information on what might have caused this increase or any lead to reduce this peak?

Here is the result on a Linux 5.15.167.4-microsoft-standard-WSL2:
with Ruby 3.3.6:
```text
ruby 3.3.6 (2024-11-05 revision 75015d4c1f) [x86_64-linux]
On start on 3.3.6
- OS VmRSS:        20628 kB
- ObjectSpace.memsize_of_all: 1.73MB

 On first full workload: 0
- OS VmRSS:       521504 kB
- ObjectSpace.memsize_of_all: 289.74MB

....
After workload
- OS VmRSS:       573484 kB
- ObjectSpace.memsize_of_all: 251.59MB

After data released
- OS VmRSS:       487404 kB
- ObjectSpace.memsize_of_all: 1.73MB
```
and 3.4.2:
```text
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-linux]
On start on 3.4.2
- OS VmRSS:        13132 kB
- ObjectSpace.memsize_of_all: 1.71MB

 On first full workload: 0
- OS VmRSS:       612508 kB
- ObjectSpace.memsize_of_all: 315.47MB

....
After workload
- OS VmRSS:       725356 kB
- ObjectSpace.memsize_of_all: 251.57MB


After data released
- OS VmRSS:       616364 kB
- ObjectSpace.memsize_of_all: 1.71MB
```

and the associated script:
```ruby
require 'objspace'

BYTES_TO_MB = 1024 * 1024

$stdout.sync = true
srand(1)

# Declare supporting code
def print_info(context)
  puts context
  GC.start

  os_mem_metric = File.readlines("/proc/#{Process.pid}/status").find { |line| line.start_with?('VmRSS:') }
  puts "- OS #{os_mem_metric}"
  puts "- ObjectSpace.memsize_of_all: #{(ObjectSpace.memsize_of_all.to_f/BYTES_TO_MB).round(2)}MB"
  puts ''
end

def random_string = Array.new(10) { rand(99) }.join

class A
  def initialize
    @a = random_string
    @b = rand(1000000000000)
  end
end


# Main
print_info "On start on #{RUBY_VERSION}"

objects = Array.new(1_000_000) { A.new }
hashes =  Array.new(250_000) { { a: rand(100_000), b: rand(100_000), c: random_string } }
arrays =  Array.new(250_000) { [rand(100_000), rand(100_000), random_string] }

keep_if = ->(index) { index.even? }
0.upto(3) do |i_loop|
  objects = objects.map.with_index { |obj, index| keep_if.call(index) ? obj : A.new }
  hashes = hashes.map.with_index { |obj, index| keep_if.call(index) ? obj : { a: rand(10_000), b: rand(10_000) } }
  arrays = arrays.map.with_index { |obj, index| keep_if.call(index) ? obj : [rand(10_000), rand(10_000)] }

  print_info " On first full workload: #{i_loop}" if i_loop.zero?

  keep_if = ->(index) { index.odd? } if i_loop == 1
  keep_if = ->(index) { index%5 == 0 } if i_loop == 2
  keep_if = ->(index) { index.even? } if i_loop == 3
  print '.'
end
puts ''

print_info 'After workload'

objects.clear
hashes.clear
arrays.clear

print_info 'After data released'
```

Regards




-- 
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