On Tue, 23 Mar 2010, Andres Valloud wrote:

> (with a good hash function, the primitive will almost always find the 
> required object in the first try, negating the benefits of the primitive)

With 4096 different hash values and 1000000 objects that won't happen.


Levente

>
> On 3/23/10 18:20 , Andres Valloud wrote:
>> As soon as you get a JIT VM, you will be surprised at how expensive
>> primitives that require calling a C function can be.  You might be
>> better off without the primitive and with a more streamlined hashed
>> collection instead.  Also, the presence of the primitive will allow
>> little to no flexibility...
>> 
>> On 3/23/10 16:47 , Levente Uzonyi wrote:
>> 
>>> On Wed, 24 Mar 2010, Bert Freudenberg wrote:
>>> 
>>>
>>> 
>>>> On 23.03.2010, at 23:57, Levente Uzonyi wrote:
>>>>
>>>> 
>>>>> On Tue, 23 Mar 2010, Bert Freudenberg wrote:
>>>>> 
>>>>>
>>>>> 
>>>>>> On 23.03.2010, at 16:01, Lukas Renggli wrote:
>>>>>>
>>>>>> 
>>>>>>>
>>>>>>> 
>>>>>>>>> Just an idea: we could get rid of compact classes, which would give 
>>>>>>>>> us
>>>>>>>>> additional 6 bits (5 bits from the compact class index plus 1 bit 
>>>>>>>>> from the
>>>>>>>>> header type because there would only be 2 header types left). This 
>>>>>>>>> would
>>>>>>>>> increase the identity hash values from 4096 to 262144. In a 
>>>>>>>>> PharoCore1.0
>>>>>>>>> image there are 148589 instances of compact classes, hence this 
>>>>>>>>> would cost
>>>>>>>>> 580k. Or, we could just add an additional word and use the spare 
>>>>>>>>> bits from
>>>>>>>>> the old identity hash for other stuff, e.g., immutability ;)
>>>>>>>>>
>>>>>>>>> 
>>>>>>>> I like the first idea, we could even have the 17 continuous bits for
>>>>>>>> identity hash the 1 separate bit for immutability.
>>>>>>>>
>>>>>>>> 
>>>>>>> Yes please, I love it :-)
>>>>>>> 
>>>>>>> Lukas
>>>>>>>
>>>>>>> 
>>>>>> Well, someone should code it up, and then lets's see macro benchmarks 
>>>>>> :)
>>>>>>
>>>>>> 
>>>>> That's a great idea, but I'm sure it'll take a while until that happens. 
>>>>> Fortunately identityhash related benchmarks can be done without changing 
>>>>> the vm. I rewrote a bit the benchmark from Chris, created three classes 
>>>>> which have 17, 18 and 30 bits for #scaledIdentityHash. Ran the benchmark 
>>>>> with these three classes + Object, collected the data and created some 
>>>>> diagrams. I'm sure most people don't care about the code/data[1], so 
>>>>> here are the diagrams:
>>>>> http://leves.web.elte.hu/identityHashBits/identityHashBits.png
>>>>> http://leves.web.elte.hu/identityHashBits/identityHashBits2.png
>>>>> http://leves.web.elte.hu/identityHashBits/identityHashBits3.png
>>>>> 
>>>>> The first one contains the four graphs. It clearly shows that 12 bits 
>>>>> (Object) are insufficient for #identityHash. Even 5 more bits gives 8-9x 
>>>>> speedup and a dramatic change in behavior.
>>>>> 
>>>>> The second is the same as the first, but it shows only the 17, 18 and 30 
>>>>> bits case. Note that the primes (hashtable sizes) are now optimized for 
>>>>> 12 bits. If they are optimized for 17/18 bits then the results can be 
>>>>> better for larger set sizes (130+/260+) where they show worse behavior 
>>>>> compared to the 18/30 bits case.
>>>>> 
>>>>> The third graph shows how an optimized data structure (LargeIdentitySet) 
>>>>> compares to the 17, 18 and 30 bits case using only 12 bits.
>>>>> 
>>>>> [1] All the code/data that were used to generate these graphs can be 
>>>>> found here http://leves.web.elte.hu/identityHashBits
>>>>> 
>>>>> 
>>>>> Levente
>>>>> 
>>>>> P.S. I also created a 12 bit version of the 17-18-30 bit classes and 
>>>>> found that it's 1.2-2.0x slower than Object, so the values after the vm 
>>>>> changes are expected to be even better than what these graphs show.
>>>>>
>>>>> 
>>>> So this seems to indicate your specialized data structure beats all VM 
>>>> hash bits extension?
>>>>
>>>> 
>>> For IdentitySet - probably yes, up to a few million elements, but
>>> I expect the difference to be smaller with the vm support and optimal
>>> table sizes. (note that a "normal" image contains less than 500000 
>>> objects).
>>> For IdentityDictionary - probably not, because we don't have a fast
>>> primitive that can be used for the lookups.
>>> 
>>> 
>>> Levente
>>> 
>>>
>>> 
>>>> Also, we don't know yet how getting rid of compact classes would affect 
>>>> performance.
>>>> 
>>>> - Bert -
>>>> 
>>>> 
>>>> 
>>>> 
>>>>
>>>> 
>>> .
>>> 
>>>
>>> 
>> .
>>
>> 
>
>