[Vm-dev] Primitive to set an identityHash

Wed Jan 18 20:04:07 UTC 2012

On Wed, Jan 18, 2012 at 8:58 PM, Mariano Martinez Peck <
marianopeck at gmail.com> wrote:

>
>
> 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.
>>
>
 I have another one but I am not sure if I can make it clear. In my case, I
have original which I do #becomeForward: to proxies. Original objects are
then swapped out to disk and garbage collected. Then after, I materialize
from disk, and I do proxies  #becomeForward: materialized objects.  I have
no idea whether original objects were stored in hashed collections. Not
even in which collections. So...to avoid rehashing all instances of all
hashed collections, what I do is the becomeForward: that copies
identityHash.  Problem is...that when I do original objects becomeForward:
proxies ... it change the identityHash of proxies, and what happens if
proxies were stored also in hashed collections?  would need a rehash...

So...with this new primitive I can directly set the same identityHash to
the proxy when I create it, since I know who he will proxify :)

> 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
>
>

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