[squeak-dev] 4.1 - hashed collections still a problem

Levente Uzonyi leves at elte.hu
Tue Mar 23 22:57:50 UTC 2010

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:

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


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.

> - Bert -

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