[Vm-dev] Direct object pointers vs indirect ones pros and cons

[Igor Stasenko]

On 12 November 2010 15:24, Levente Uzonyi <leves at elte.hu> wrote:
>
> On Fri, 12 Nov 2010, Igor Stasenko wrote:
>
>>
>> Here a 'simulated' kind of indirect pointers.
>>
>> The Wrapper class having an 'object' ivar
>> and in this way, we simulating indirection.
>>
>>
>> | objects wrapped t1 t2 t3 |
>> objects := (1 to: 1000) collect: [:i | Object new ].
>> wrapped := objects collect: [:each | Wrapper new object: each ].
>>
>> t1 := [ 100000 timesRepeat: [ objects do:[ :each |  each yourself ] ]
>> ] timeToRun.
>> t2 := [ 100000 timesRepeat: [ wrapped do:[ :each |  each object
>> yourself ] ] ] timeToRun.
>> t3 := [ 100000 timesRepeat: [ wrapped do:[ :each |  ] ] ] timeToRun.
>> {t1. t2. t3}
>>
>> Running on Cog it gives:
>>
>> #(3241 3498 2793)
>
> Single measurement is probably inaccurate. This benchmark creates lots of
> blocks, which means GC noise. The size of the "object table" is too small to
> be realistic, this hides cache related performance hits. Why don't you use
> an Array instead of Wrapper? IIRC Cog has optimization for the #at:
> primitive, also Arrays are compact. Why do you send #yourself and
> #timesRepeat:? You should slightly shuffle the objects to be more realistic
> about cache usage.
>

I'm inviting you to make own version of benchmark, which could
simulate an extra level of indirection
for accessing object field(s).

actually i was trying:
 t1 := [ 100000 timesRepeat: [ objects do:[ :each |  each yourself ] ]
vs
 t2 := [ 100000 timesRepeat: [ objects do:[ :each |  each object ] ]

where #yourself there is to compensate an extra message send
(#object), so it will compare
'read nothing, return self' , and 'read ivar', which is an indirection.
But what i found that this gives no difference, and actually t2 < t1
sometimes :)

>
> Levente
>
>>
>> the first bench is kind-of 'measure time to access directly to objects'
>> the second one is 'measure indirect access'
>> and third is measure a loop overhead.
>>
>> So, by taking this naive benchmark, we got:
>>
>> (3498 - 2793) / (3241 - 2793) asFloat
>> 1.573660714285714
>>
>> so, 50% slower.
>>
>> But actually this benchmarks shows a cost of extra message send rather
>> than impact of extra level of indirection.
>> Well, a message is a kind of indirection..  :)
>>
>> --
>> Best regards,
>> Igor Stasenko AKA sig.
>>
>



-- 
Best regards,
Igor Stasenko AKA sig.