IME, relational database mappings typically restrict the classes of mapped objects to some hierarchy of classes. The top of the hierarchy adds an instance variable such as id, which typically holds values from the primary key of the table in question. Please excuse my ignorance, but is this how Magma works? If not, how does it do things?
On 3/30/10 17:55 , Levente Uzonyi wrote:
On Tue, 30 Mar 2010, Andres Valloud wrote:
What is the purpose of adding an id to so many objects? Is this a real application problem? Can you be more specific as to the context?
AFAIK Magma uses it to assign unique ids to objects.
Levente
On 3/30/10 9:21 , Levente Uzonyi wrote:
On Tue, 30 Mar 2010, Andres Valloud wrote:
I mentioned implementing identityHash as hash as an example given. I also think I mentioned the rule x == y => x identityHash = y identityHash, so I hope it's clear that one shouldn't just blindly move ahead in these matters. The point is that nothing prevents anybody from adding an instance variable called identityHash to their objects, storing arbitrary (but well chosen!) small integers in said instance variable, and then having the identityHash message just answer said values. If you do this for the cases in which you have significantly more than 4096 objects, then you only pay the price to hold better identityHash values for the objects that need them (as opposed to every single object in the image when the header is expanded instead). Or perhaps you don't really need identity for the cases being discussed in practice, and just using hash as identityHash is fine. It's hard to tell without concrete examples. One way or the other, I do not think the size of the identityHash field *must* result in poor hashed collection performance. Such an implication does not follow.
The original problem is: pick 1000000 or more objects and associate an id to them.
- Using #hash and #== doesn't work help, because the objects state may
change.
- Adding a slot to every object can't be done, because the class of the
objects can be anything. Adding a slot to Object would break lots of code.
- Using #largeHash (12 bits from the object + 12 bits from the object's
class) doesn't help, because there may be only a few different classes.
Levente
On 3/30/10 1:48 , Andreas Raab wrote:
On 3/30/2010 1:09 AM, Andres Valloud wrote:
Right, so provide a better identityHash implementation in the image (e.g.: implement hash and then have identityHash call hash instead), and problem solved...
Except that #hash is not constant over the lifetime of most objects but #identityHash is. So if you have a property associated with an object in a IDDict and the #hash depends on a value of a variable it may change over the lifetime of the object and your key gets invalid.
Cheers, - Andreas
On 3/26/10 3:37 , Levente Uzonyi wrote:
> On Thu, 25 Mar 2010, Andres Valloud wrote: > > > > >> If lookups find the sought object in mostly one attempt, the >> primitive is >> overkill... most of the time, the real issue is the quality of the >> hash >> function. >> >> >> > That's true, but this is not the case with the 4096 hash values. > > > Levente > > > > >> On 3/25/10 1:27 , Levente Uzonyi wrote: >> >> >> >>> On Thu, 25 Mar 2010, Igor Stasenko wrote: >>> >>> >>> >>> >>> >>>> i think that #pointsTo: is a cheat :), which you can use in Sets but >>>> not dictionaries, because >>>> it contains associations. Also, it works only for identity-based >>>> collections. >>>> >>>> >>>> >>>> >>> Dictionaries don't have to use associations (for example >>> MethodDictionary >>> doesn't use them), that's why #pointsTo: works (MethodDictionary also >>> uses it). >>> >>> >>> >>> >>> >>>>> I wonder how LargeIdentityDictionary compares to your >>>>> dictionaries'. >>>>> >>>>> >>>>> >>>>> >>>>> >>>> me too. >>>> >>>> >>>> >>>> >>> If you give me a pointer to the source code, I can run the >>> benchmarks. >>> >>> >>> Levente >>> >>> >>> >>> >>> >>> >> >
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