[Cuis-dev] Small dictionaries

[Jan Vraný]

On Mon, 2026-01-12 at 21:19 +0700, Luciano Notarfrancesco wrote:
> Great to know! I think you mentioned this before… you used it for method
> dictionaries, right?

Yes, except in case of MD this was merely to keep the full lookup in the VM
simpler and more independent of the rest, assuming this is fairly rare not to
matter much.

> I’ll make a note to look into this further, because in
> modern computers it might have less of an impact, I should do benchmarks.
> 
I'd think on modern computers it'd be even more pronounced as the gap between
CPU and memory speeds widens so every cache miss is becoming more of a pain. 
Of course all this assumes all the code is properly jitted.

Jan

> 
> On Mon, Jan 12, 2026 at 21:01 Jan Vran <Jan.Vrany at labware.com> wrote:
> 
> > On Mon, 2026-01-12 at 10:35 -0300, Juan Vuletich via Cuis-dev wrote:
> > > Hi Luciano,
> > > 
> > > On 2026-01-12 4:52 AM, Luciano Notarfrancesco via Cuis-dev wrote:
> > > > In the software Im currently programming Im using the events
> > > > subsystem a lot, and Im subclassing from ActiveModel. Each model
> > > > typically only triggers 2-3 types of events, so the action dictionary
> > > > is very small. And I have another similar case with a small dictionary
> > > > (also typically 2-3 entries, at most 5). Lookups and event triggering
> > > > happen at about 300 Hz. Im wondering, wouldnt be more efficient to
> > > > just use arrays and a linear lookup instead of dictionaries? Or, has
> > > > anyone implemented dictionaries optimized for a small number of
> > > > entries? Any advice?
> > > > 
> > > > In general, I think we dont have models that trigger many different
> > > > types of events, so it seems this optimization could be used in
> > > > ActionModel. But perhaps it hasnt been a priority to optimize this
> > > > because we dont usually fire events at high rates, I guess.
> > > > 
> > > > Thanks,
> > > > Luciano
> > > 
> > > 
> > > I think such optimization would belong in Dictionary. I mean, if a
> > > Dictionary is small, it could use an alternative lookup method if that
> > > improves performance. Having a different class would make use way more
> > > complicated, and we'd need to keep an eye on whether we're using the
> > > correct kind (small / large) all the time.
> > > 
> > > Still, I doubt there would be any performance improvement. Especially in
> > > IdentityDictionaries where the hash and equality are both quick
> > > primitives, it looks like it would perform the same as today.
> > > 
> > > Some benchmarking is in order, if you want to investigate.
> > 
> > I did the benchmarking in the past. Expecially for small identity
> > dictionaries
> > it made a difference.
> > 
> > It makes sense. If you organize small identity dictionary as a an array of
> > alternating key/value, then you can squeeze 4 elements into a single
> > cacheline.
> > 
> > In that respect, the traditional smalltalk dictionary implementation using
> > an
> > array of associations is somewhat suboptimal. Simply one big array or
> > having two arrays one with keys and other with values seems more
> > cache-friendly.
> > 
> > Jan
> > 
> > 
> > > 
> > > Thanks,
> > > 
> > > --
> > > Juan Vuletich
> > > http://www.cuis.st
> > > http://github.com/jvuletich
> > > http://researchgate.net/profile/Juan-Vuletich
> > > http://independent.academia.edu/JuanVuletich
> > > http://patents.justia.com/inventor/juan-manuel-vuletich
> >