[ruby-core:121651] [Ruby Bug#21214] VmRSS consumption increase in Ruby 3.4.2 vs Ruby 3.3.6

Issue #21214 has been updated by k0kubun (Takashi Kokubun).

Backport changed from 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONTNEED, 3.4: REQUIRED to 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONTNEED, 3.4: WONTFIX

We've discussed whether we backport this to Ruby 3.4 or not with @peterzhu2118. Since he's too worried about potential side effects in corner cases, we've decided we won't backport this to Ruby 3.4. This will be available in the Ruby 3.5 release.

----------------------------------------
Bug #21214: VmRSS consumption increase in Ruby 3.4.2 vs Ruby 3.3.6
https://bugs.ruby-lang.org/issues/21214#change-112705

* Author: mood_vuadensl (LOIC VUADENS)
* Status: Closed
* ruby -v: ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-linux]
* Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONTNEED, 3.4: WONTFIX
----------------------------------------
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:        20616 kB
- ObjectSpace.memsize_of_all: 1.7MB

 On first full workload: 0
- OS VmRSS:       559212 kB
- ObjectSpace.memsize_of_all: 327.86MB

....
After workload
- OS VmRSS:       711776 kB
- ObjectSpace.memsize_of_all: 327.86MB

After data released
- OS VmRSS:       616364 kB
- ObjectSpace.memsize_of_all: 1.71MB
```
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:        13076 kB
- ObjectSpace.memsize_of_all: 1.7MB

 On first full workload: 0
- OS VmRSS:       674324 kB
- ObjectSpace.memsize_of_all: 353.6MB

....
After workload
- OS VmRSS:      1000628 kB
- ObjectSpace.memsize_of_all: 327.85MB

After data released
- OS VmRSS:       843636 kB
- ObjectSpace.memsize_of_all: 1.7MB
```

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), c: random_string } }
  arrays = arrays.map.with_index { |obj, index| keep_if.call(index) ? obj : [rand(10_000), rand(10_000), random_string] }

  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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