[squeak-dev] Association >> #hash performance

christoph.thiede at student.hpi.uni-potsdam.de christoph.thiede at student.hpi.uni-potsdam.de
Sat Jul 9 18:57:24 UTC 2022


Hi Levente, Hi Marcel,

thank you for your feedback. Revised and merged via System-ct.1367.

(I decided against "UserInterfaceTheme cleanUpAndReset" because I used myself to modify my themes from time to time and would not like to revert such changes when avoidable.)

Best,
Christoph

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On 2022-07-07T10:23:22+02:00, marcel.taeumel at hpi.de wrote:

> Hi Christoph --
> 
> I agree with Levente in that we should not change Association >> #hash.
> 
> +1 on your proposed changes to UserInterfaceTheme.
> 
> Maybe just use "UserInterfaceTheme cleanUpAndReset" as a postscript. :-) At this point, we still do not expect custom stuff in there.
> 
> Best,
> Marcel
> Am 26.05.2022 23:32:27 schrieb Levente Uzonyi <leves at caesar.elte.hu>:
> Hi Christoph,
> 
> On Thu, 26 May 2022, christoph.thiede at student.hpi.uni-potsdam.de wrote:
> 
> > Hi all!
> >
> > while playing around with code simulation recently, I found out that the following dictionary lookup consumes extremely many bytecodes:
> >
> >     UserInterfaceTheme current get: #return for: SHTextStylerST80.
> >
> > Dictionary >> #scanFor: takes 59 (sic!) attempts to find the right item in the dictionary. In other words, almost 20% of the property table is traversed, which does not conform my idea of a hashtable.
> >
> > The reason for this can be found in Association >> #hash: Since Collections-ar.304, it does not take the value of an association into consideration for the hash any longer. As UserInterfaceTheme uses Associations as keys in
> > the Dictionary, we have a lot of "hash misses". Even though you might not notice this in practice, I noticed this indeed during debugging und simulating, I can tell you. :-)
> >
> > On the other hand, reverting Association >> #hash to honor the value slows down Unicode class compileAll indeed (significantly: 60 ms vs. 2100 ms), as noted in Andreas' original comment:
> >
> >     Name: Collections-ar.304
> >     Author: ar
> >     Time: 11 February 2010, 11:52:58.959 pm
> >     UUID: 9e97b360-adf1-f745-8f8c-562aa08d566e
> >     Ancestors: Collections-ar.303
> >     
> >     Fix an age-old bug in Association>>hash which causes extreme slowdowns in compiling Unicode methods. Association>>hash does not need to hash the value; it's slow and useless.
> >
> > So my question is: What would be the right fix for this situation? Is it
> > a) Do not use Associations in Dictionarys when its value matters for the lookup; or
> > b) Do only use Associations in Dictionarys when its value maters for the lookup?
> 
> It's the user's (the developer who decided to use a hashed collection)
> responsibility to provide keys that have a good hash distribution.
> If it's not possible with the default hash function, a PluggableDictionary
> should be used with a better one.
> 
> In this case, however, I would use Arrays as tuples instead of
> Associations because not all the keys are Associations in that dictionary,
> and Array's #hash is good enough here.
> Changing Association >> #hash would have too many side effects for such a
> localized problem to fix.
> 
> > For a), the dictionary layout of UserInterfaceTheme needs to be changed, for which I am attaching a changeset that replaces Associations by Arrays there.* For b), the layout of litIndSet in Encoder needs to be changed, for
> > instance by using Bindings instead of Associations for classPool entries. However, I'm not familiar enough with Encoder etc. to answer this question.
> >
> > *The changeset will automatically update all UserInterfaceTheme instances. Benchmark:
> >
> >     theme := UserInterfaceTheme current.
> >     random := Random seed: 20220526.
> >     properties := (1 to: 10000) collect: [:i | theme properties keys atRandom: random].
> >     [properties collect: [:ea | theme get: ea]] bench.
> >
> > Old: 45 ms | New: 10 ms
> >
> > Looking forward to your opinion: How should we treat Associations in Dictionarys? Of course, this is not release-critical. :-)
> >
> > Best,
> > Christoph
> >
> > =============== Postscript (accelerateUserInterfaceTheme.2.cs) ===============
> >
> > "Postscript: Update UserInterfaceTheme instances"
> >
> > UserInterfaceTheme allSubInstancesDo: [:theme |
> >     | newProperties |
> >     newProperties := theme properties copyEmpty.
> >     theme properties keysAndValuesDo: [:key :value |
> >         newProperties
> >             at: ((key isKindOf: Association)
> >                 ifTrue: [{key key. key value}]
> >                 ifFalse: [key])
> >             put: value].
> >     theme instVarNamed: 'properties' put: newProperties].
> 
> There's a method named #atomicUpdate:. I'm not sure if it's needed here,
> but the name suggests that it's better to use that to update the
> properties. The rest of the changes looks good to me. +1
> 
> 
> Levente
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