Why is this unused string not garbage collected? - ruby

Question

Why does unused_variable_2 and unused_variable_3 get garbage collected, but not unused_variable_1?
# leaky_boat.rb
require "memprof"
class Boat
def initialize(string)
unused_variable1 = string[0...100]
puts unused_variable1.object_id
#string = string
puts #string.object_id
end
end
class Rocket
def initialize(string)
unused_variable_2 = string.dup
puts unused_variable_2.object_id
unused_variable_3 = String.new(string)
puts unused_variable_3.object_id
#string = string
puts #string.object_id
end
end
Memprof.start
text = "a" * 100
object_id_message = "Object ids of unused_variable_1, #string, unused_variable_2, unused_variable_3, and another #string"
before_gc_message = "Before GC"
after_gc_message = "After GC"
puts object_id_message
boat = Boat.new(text)
rocket = Rocket.new(text)
puts before_gc_message
Memprof.stats
ObjectSpace.garbage_collect
puts after_gc_message
Memprof.stats
Memprof.stop
Running the program:
$ uname -a
Linux [redacted] 3.2.0-25-generic #40-Ubuntu SMP Wed May 23 20:30:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
$ ruby --version # Have to use Ruby 1.8 - memprof doesn't work on 1.9
ruby 1.8.7 (2011-06-30 patchlevel 352) [x86_64-linux]
$ ruby -rubygems leaky_boat.rb
Object ids of unused_variable_1, #string, unused_variable_2, unused_variable_3, and another #string
70178323299180
70178323299320
70178323299100
70178323299060
70178323299320
Before GC
2 leaky_boat.rb:6:String
2 leaky_boat.rb:26:String
1 leaky_boat.rb:9:String
1 leaky_boat.rb:7:String
1 leaky_boat.rb:32:Rocket
1 leaky_boat.rb:31:Boat
1 leaky_boat.rb:29:String
1 leaky_boat.rb:28:String
1 leaky_boat.rb:27:String
1 leaky_boat.rb:20:String
1 leaky_boat.rb:18:String
1 leaky_boat.rb:17:String
1 leaky_boat.rb:16:String
1 leaky_boat.rb:15:String
After GC
1 leaky_boat.rb:6:String
1 leaky_boat.rb:32:Rocket
1 leaky_boat.rb:31:Boat
1 leaky_boat.rb:29:String
1 leaky_boat.rb:28:String
1 leaky_boat.rb:27:String
1 leaky_boat.rb:26:String

Answer 1

This behavior is because the string implementation of your version of ruby for substr has a special case to save memory allocations when you take a substr that is the tail of the source string and the string length is large enough to not store the string value in the base object structure.
If you trace the code, you see the range subscript string[0...100] will go through this clause in rb_str_substr. So the new string will be allocated via str_new3 which allocates a new object struct (hence the differing object_id), but sets the string value ptr field as a pointer into the source object's extended storage and sets the ELTS_SHARED flag to indicate that the new object shares storage with another object.
In your code you take this new substring object and assign it to instance var #string which is still a live reference when you run garbage collection. Since there's a live reference to the allocated storage of the original string, it can't be collected.
In ruby trunk, this optimization to share storage on compatible tail substrings appears to still exist.
The two other vars unused_variable_2 and unused_variable_3 don't have this extended storage sharing issue because they're set via mechanisms that assure distinct storage, so they get garbage collected as expected when their references pass out of scope.
String#dup runs rb_str_replace (via initialize_copy binding) which replaces the contents of the source string with a copy of the contents of the source string and assures that the storage is not shared.
String#new(source_str) runs through rb_str_init which similarly assures distinct storage with rb_str_replace on the supplied initial value.