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comp.lang.ruby

sizeof(Class)/sizeof(Object

Nikolai Weibull

12/31/2004 1:19:00 PM

I would like to figure out how much memory a certain class or object
will/is consuming. Is there an easy way? Is there a hard way?
The reason I ask is because I want to keep several hundred thousand
instances of an object much like the following one:

class A
def initialize
@a = @b = @c = @d = nil
@e = 1
end
end

nikolai

--
::: name: Nikolai Weibull :: aliases: pcp / lone-star / aka :::
::: born: Chicago, IL USA :: loc atm: Gothenburg, Sweden :::
::: page: www.pcppopper.org :: fun atm: gf,lps,ruby,lisp,war3 :::
main(){printf(&linux["\021%six\012\0"],(linux)["have"]+"fun"-97);}
2 Answers

Charles Mills

12/31/2004 5:59:00 PM

0

Nikolai Weibull wrote:
> I would like to figure out how much memory a certain class or object
> will/is consuming. Is there an easy way? Is there a hard way?
> The reason I ask is because I want to keep several hundred thousand
> instances of an object much like the following one:
>
> class A
> def initialize
> @a = @b = @c = @d = nil
> @e = 1
> end
> end
>

Instance variables are stored in a hash table associated with each
object. See rb_ivar_set() in variable.c. So basically each instance
will take up the size of struct RObject - which is 20 bytes on a 32bit
machine, plus the size of the hash table (see st.h), plus something
else I am probably missing. As far as I know st.h doesn't provide any
means of calcing the number of bytes used by the hash.
Is there a way to do this?

Also, objects with no instance variables don't create a hash table to
store instance variables...

-Charlie

Robert Klemme

12/31/2004 6:43:00 PM

0


"Charles Mills" <cmills@freeshell.org> schrieb im Newsbeitrag
news:1104515965.487836.265250@c13g2000cwb.googlegroups.com...
> Nikolai Weibull wrote:
>> I would like to figure out how much memory a certain class or object
>> will/is consuming. Is there an easy way? Is there a hard way?
>> The reason I ask is because I want to keep several hundred thousand
>> instances of an object much like the following one:
>>
>> class A
>> def initialize
>> @a = @b = @c = @d = nil
>> @e = 1
>> end
>> end
>>
>
> Instance variables are stored in a hash table associated with each
> object. See rb_ivar_set() in variable.c. So basically each instance
> will take up the size of struct RObject - which is 20 bytes on a 32bit
> machine, plus the size of the hash table (see st.h), plus something
> else I am probably missing. As far as I know st.h doesn't provide any
> means of calcing the number of bytes used by the hash.
> Is there a way to do this?
>
> Also, objects with no instance variables don't create a hash table to
> store instance variables...

So if there was only one instance var an optimization could be to store them
in a hash in the instance's class. This could work for multiple ivars as
well but then there is the array overhead. Hm...

class Foo
@ivar = {}

def initialize
cl = self.class
ObjectSpace.define_finalizer(self) {|id| cl.remove id}
end

def bar=(x) self.class.set_val(id, x) end
def bar; self.class.get_val(id) end

def self.set_val(id, val)
@ivar[id] = val
end

def self.get_val(id)
@ivar[id]
end

def self.remove(id)
puts "removing #{id}"
@ivar.delete id
end
end

Just some thoughts in code...

robert