[squeak-dev] 4.1 - hashed collections still a problem
Levente Uzonyi
leves at elte.hu
Wed Mar 24 08:47:07 UTC 2010
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 -
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>> .
>>>
>>>
>>>
>> .
>>
>>
>
>
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