Getting an MRI heap dump

heap_dump.rb

require 'objspace'

# a class and instance
class A
end
a = A.new


# dump the heap
outfile = 'dump.json'
File.open(outfile, "w") { |f| ObjectSpace.dump_all output: f }


# look through the dump to find the object
system 'bash', '-c', <<SH
function find_class {
  # may need to $ brew install jq
  jq 'select(.type == "CLASS" and .name == "'"$1"'")' #{outfile}
}

echo ----- The class -----
find_class A

echo

echo ----- The instance -----
find_class A | jq .address | xargs -I {addr} jq 'select(.class == "{addr}")' #{outfile}
SH

output

----- The class -----
{
  "address": "0x7fc9db815ce8",
  "type": "CLASS",
  "class": "0x7fc9db815cc0",
  "name": "A",
  "references": [
    "0x7fc9db81c958",
    "0x7fc9db8c6c78"
  ],
  "memsize": 712,
  "flags": {
    "wb_protected": true
  }
}

----- The instance -----
{
  "address": "0x7fc9db815c70",
  "type": "OBJECT",
  "class": "0x7fc9db815ce8",
  "ivars": 0,
  "memsize": 40,
  "flags": {
    "wb_protected": true
  }
}