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Thank you very much for your well-wishes <3

> Yes, but note this is still a different pattern of access (array of "entities").

I was referring to the `[foo, foo2, foo3]` objects themselves; V8 does use an "cache local" placement for those so you'll find them laid out in memory as:

> [foo_proto, foo_elems, foo_props, foo_name, foo_lastName, foo2_proto, foo2_elems, foo2_props, foo2_name, foo2_lastName, ...]

For what it's worth, I am interested in laying object properties out in an ECS like manner in Nova, so the properties would be laid out as `[foo.name, foo2.name, foo3.name, ...]`, but currently the properties are laid out similarly to V8, `[foo.name, foo.lastName]`. The only difference is that we do not have "in object properties".

That being said: I am obviously biased, but I do wonder if an ECS-like layout wouldn't be nearly universally beneficial. Thinking of the `foo.name` and `foo.lastName` access: If those are on the same cache line then accessing the two only reads one cache line. This is nice. But if there are more properties in the objects (and there often are), then those will pollute the cache. If you do this access once, it doesn't matter. If you do this a million times, now the cache pollution becomes a real issue: In Node.js even the optimal case would be that you read read 625,000 cache lines worth of data, only to discard 250,000 cache lines of it.

If instead we use an ECS-like layout, then accessing these two properties reads two 10100cache lines: That's bad, but on the other hand if this happens once then it won't even make a blip on the screen. If a million of these accesses are done, you could think that we'd suddenly be slow as molasses but now the ECS-like layout is probably going to help you: You're more likely reading the next `name` and `lastName` property values on each access. If you have it bad and only half of the property data you read is actually the `name` and `lastName` properties you want, then you read 750,000 cache lines and lose out to the traditional engine by 100,000 cache lines. If you get 67% "hit rate" then you break even. And that's comparing to the case where the objects only contain `name` and `lastName` and nothing more.

It of course all comes down to statistics but... I'm very interested in the potential benefits here :)

Again, thank you for your comments, I've enjoyed discussing and pondering this <3

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dinfuehr 7 days ago | parent [-]

Would this mean that each shape/structure/map get its own vector for each field in order for this to work?

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aapoalas 7 days ago | parent [-]

That would be one way; it would offer the best theoretical memory layout for well-behaved programs. But, I expect it to be very painful to work with and to come with some performance penalties in mixed use cases due to the extra indirection required.

No, my thinking is that properties would be stored into tables based on their size class: All objects that have 4-7 properties are in the same table, and all of their first property would be in the same slice, second property in another etc.

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munificent 7 days ago | parent [-]

That sounds to me like you'll end up getting little benefit from ECS then. Let's say the JavaScript program is iterating over a hundred thousand instances of some Foo class which happens to have 6 properties. You'd ideally want good use of cache, but if your object vector that has all of the Foo objects also happens to have all sorts of instances of other types that have the same field count mixed in there, then you're going to spend a lot of time skipping over those unrelated objects and refreshing the cache.

I know that ECS is treated as a silver bullet by a lot of people, but my experience is that it really only works well when the data you're working with is statically typed so that you can actually partition into arrays where each array does represent a single meaningful type.

	▲	aapoalas 7 days ago \| parent [-]
		That is definitely possible: If it turns out to not make sense then I can at least always go back go the current system where all properties of an object as stored in a single array. It's not as ECS'ssy as one would hope but it's at least proven technology :D