[ruby-talk:444645] Question regarding strange GC stat total_allocated_objects behaviour

ruby-talk
1

Hi everyone,

I have a curiosity regarding a simple ruby program stolen from this talk https://youtu.be/ZE6F3drGhA8?t=1811

<<EOF
def allocations
  x = GC.stat(:total_allocated_objects)
  yield
  GC.stat(:total_allocated_objects) - x
end

p allocations { 1 }
p allocations { 1 }
EOF

Running this program with ruby version 2.7.8 works as I imagined reading it - both calls should print "0" to stdout (considering no allocations are happening in the provided block). However, running it with any ruby version starting from 3.0.7 (may not be the exact one introducing the behavior just what i tested with) I get strange results: first call to allocations outputs 1 and then any other future call to it outputs 0 as I initially expected. I get even stranger results with something like:

<<EOF
class A end
p allocations { A.new }
p allocations { A.new }
EOF

Ruby v2.7.8 outputs 1 for both calls, however from 3.0.7 onwards I see 5 being printed at the first call followed by 2 for future calls.

Any hints or learning materials to understand this behavior would be greatly appreciated !

Have a nice rest of the day / evening depending on your timezone

···

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2

Hey Vlad, nice to meet you :waving_hand:

I have a curiosity regarding a simple ruby program stolen from this talk

Any hints or learning materials to understand this behavior would be

greatly appreciated !

I suspect these might be internal objects that the Ruby VM creates (for
things like passing arguments, caches, etc).

I've been working on a gem to experiment with these kinds of things -- you
may want to look at

and

for a way to know exactly when Ruby objects get created (and what those
objects are -- just be careful that not all of them can be returned back to
Ruby code).

Hopefully this gives a starting point for your investigation? :wink:

-- Ivo

···

On Thu, Apr 3, 2025 at 10:16 PM vlad.a.brb--- via ruby-talk < ruby-talk@ml.ruby-lang.org> wrote:

Hi everyone,

I have a curiosity regarding a simple ruby program stolen from this talk
https://youtu.be/ZE6F3drGhA8?t=1811

<<EOF
def allocations
  x = GC.stat(:total_allocated_objects)
  yield
  GC.stat(:total_allocated_objects) - x
end

p allocations { 1 }
p allocations { 1 }
EOF

Running this program with ruby version 2.7.8 works as I imagined reading
it - both calls should print "0" to stdout (considering no allocations are
happening in the provided block). However, running it with any ruby version
starting from 3.0.7 (may not be the exact one introducing the behavior just
what i tested with) I get strange results: first call to allocations
outputs 1 and then any other future call to it outputs 0 as I initially
expected. I get even stranger results with something like:

<<EOF
class A end
p allocations { A.new }
p allocations { A.new }
EOF

Ruby v2.7.8 outputs 1 for both calls, however from 3.0.7 onwards I see 5
being printed at the first call followed by 2 for future calls.

Any hints or learning materials to understand this behavior would be
greatly appreciated !

Have a nice rest of the day / evening depending on your timezone
______________________________________________
ruby-talk mailing list -- ruby-talk@ml.ruby-lang.org
To unsubscribe send an email to ruby-talk-leave@ml.ruby-lang.org
ruby-talk info --
https://ml.ruby-lang.org/mailman3/postorius/lists/ruby-talk.ml.ruby-lang.org/