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

Levente Uzonyi leves at elte.hu
Sun Mar 28 21:28:45 UTC 2010

On Sun, 28 Mar 2010, Chris Muller wrote:

> Hi Levente, thanks a lot for all of your great help with the hashed
> collections.  I would like to test your LargeIdentitySet and
> LargeIdentityDictionary with Magma and some of my proprietary
> applications.  May I assume use of them under a MIT license?

Sure. Note that LargeIdentityDictionary cannot have nil as key at the 


> - Chris
> 2010/3/23 Levente Uzonyi <leves at elte.hu>:
>> On Mon, 22 Mar 2010, Chris Muller wrote:
>>> 4.1 hashed collections, across the board, small to large, are slower
>>> by a factor of 2?!  I just don't think we can keep doing this; getting
>>> slower and slower and slower, like molasses..  I'm sorry, but I really
>>> care about this and I know you do too because speed was the whole
>>> premise of putting these changes in.
>>> What went wrong?  More importantly, how can we fix this?
>> What went wrong?
>> I think nothing. :) IdentitySet and IdentityDictionary wasn't ment to
>> support really large collections. The main reason is that there are only
>> 4096 different hash values. So practically these collections are growing
>> 4096 lists in a single array. In 3.9 and 3.10 these collections used a hash
>> expansion technique which distributed the elements uniformly. This was
>> changed when we integrated Andrés' hash changes. As you noticed some of the
>> primes didn't work well with #scaledIdentityHash, it's far better now,
>> though there may be even better primes. Finding such primes is a
>> computationally intensive task and the current ones (up to 10000000) are
>> pretty close to optimal.
>> Other than that there are two things that cause slowdown:
>>  +1 extra message send/scanning: #scaledIdentityHash (the new hash expansion
>> scheme, but we save one by not using #findElementOrNil:)
>>  +k (worst case) extra message send/scanning: #enclosedSetElement (OO nil
>> support, this only applies for IdentitySet)
>> Where k is the length of the list. Since there are only 4096 different
>> identity hash values for n = 250000 k will be ~61 (if the identity hashes
>> have a uniform distribution). For n = 1000000 it will be ~244. Note that
>> your benchmark exploits the worst case.
>> The long lists are bad, because HashedCollection is optimized for O(1) list
>> length. In this case the length of the list is not O(1), but O(n) (with a
>> very small constant).
>> How can we fix this?
>> I see two possible solutions for the problem:
>> 1) use #largeHash instead of #identityHash, which is the identity hash of
>> the object mixed with the identity hash of its class. This helps if there
>> are objects from different classes in the set, but it doesn't help with your
>> benchmark. SystemTracer uses this method.
>> 2) use differently implemented collections which are optimized for your use
>> case. For example I wrote LargeIdentitySet which probably has the best
>> performance you can have:
>> http://leves.web.elte.hu/squeak/LargeIdentitySet.st
>> (note that it's hardly tested, probably contains bugs)
>> Levente

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