[Vm-dev] Primitive to set an identityHash

[Mariano Martinez Peck]

On Wed, Jan 18, 2012 at 8:02 PM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

>
>
>
> On Wed, Jan 18, 2012 at 2:25 AM, Henrik Johansen <
> henrik.s.johansen at veloxit.no> wrote:
>
>>
>> I really don't see what good could come of it being available in general…
>>
>
> I can think of one good use, which my file tried to illustrate.  If Symbol
> instances identity hashes were derived from their string hash then they
> would be hashed the same in all images.  One can take advantage of this in
> e.g. method dictionary layout and hence binary class loading.  This happens
> in two steps.
>
> With modern machines, where linear search through a dictionary is fast,
> and with a JIT with inline cacheing (and even an interpreter with a large
> method lookup cache), where method dictionaries are not looked at much, one
> can save a significant amount of space by making method dictionaries flat
> pair-wise arrays of selector, method.  Most method dictionaries are small
> and linear search is faster than fetching the Symbol's identity hash and
> doing a hash probe.  By ordering method dictionaries  by selector
> identityHash, very large method dictionaries such as Object's are indexed
> using binary search.  We saved about 8% of the image size in VisualWorks by
> moving to this representation (one saves on eliminating the nils in the
> selector vector and the value vector, and in eliminating the value
> vector/method array, hence saving its header space; you still need the
> space for the method).  [The savings in Squeak look to be much less; I just
> found that he same overhead in the 4.3 trunk image is only ~ 1.7%].
>
> Now, if in addition selector identityHashes are deterministic, derived
> from their string hash, then one does not need to rehash/reorder a method
> dictionary when loading it from a binary stream (e.g. Fuel), which is again
> a win.
>

Indeed, in Fuel we would save the rehash of MethodDictionaries.


>
> Now, I'm not suggesting we do either of these things now, but making
> Symbol identity hashes deterministic, derived from their string hash, can
> enable significant optimisation further down the road.
>
>
>> Cheers,
>> Henry
>>
>> On Jan 18, 2012, at 8:16 26AM, stephane ducasse wrote:
>>
>> >
>> > After the discussions we got and with a really big comment I would add
>> it.
>> >
>> > Stef
>> > On Jan 17, 2012, at 11:02 PM, Mariano Martinez Peck wrote:
>> >
>> >>
>> >>
>> >> On Tue, Jan 17, 2012 at 8:04 PM, Eliot Miranda <
>> eliot.miranda at gmail.com> wrote:
>> >>
>> >>
>> >>
>> >> On Tue, Jan 17, 2012 at 8:43 AM, Mariano Martinez Peck <
>> marianopeck at gmail.com> wrote:
>> >>
>> >> Hi guys. Becuase of some work I am doing with proxies, I would like to
>> be able to set a specific identityHash to a proxy instance. I can add this
>> primitive in my VM, but I was thinking if this could be of a general
>> interest also?
>> >>
>> >> It already exists.  See primitiveSetIdentityHash in
>> InterpreterPrimitives.  Primitive # 161.
>> >>
>> >> Thank you so much Eliot. You even save my time of coding it ;)  I
>> should have checked before...I always forget about InterpreterPrimitives
>> hahaha
>> >> So...Pharaoers... do you want the image side of the primitive and some
>> tests?  I can provide that if desired (in my opinion I would include it)
>> >>
>> >> Cheers
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> Cheers
>> >>
>> >> --
>> >> Mariano
>> >> http://marianopeck.wordpress.com
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> --
>> >> best,
>> >> Eliot
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> --
>> >> Mariano
>> >> http://marianopeck.wordpress.com
>> >>
>> >
>>
>>
>
>
> --
> best,
> Eliot
>
>
>


-- 
Mariano
http://marianopeck.wordpress.com
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