Comment by moring
4 hours ago
The article shows nicely how "every byte matters" is false. First, it starts off by talking about the cost of a new field, when the actual topic is array-of-structs vs. struct-of-arrays. Then, this:
> How much of an impact can this have? > Reading is:alive (1 byte) Across 1M Monsters
You aren't reading one byte here, you are reading 1M bytes! Of course, optimizing the access to 1M bytes is something to consider. Optimizing the access to one byte isn't.
The article is definitely worth reading IMHO, but it really needs a better headline!
Even more so, it shows that SoA data structure means you can add fields to your 1M monsters with little impact.
This is valid for sequential scanning of the data. The CPU will fill whole cache lines at once with the arrays that do get used and the algorithm touches all the field instances in the array.
Now think about random access to single struct instances instead: the CPU loads a cache line worth of data for each field and uses only one element out of the whole cache line. This is much worse than a compact structure representation of the same data.
SoA is not universally better.
No it's not always better and I didn't mean to imply it was. I was simply saying that the article argues against its title.
In both cases you want to think about locality of the next read and structure the data accordingly.
> you can add fields to your 1M monsters with little impact.
Great for this access pattern, but I wouldn't make a general statement like that. This is the same thing as row-oriented vs column-oriented databases, OLTP vs OLAP. SoA is weak if you are adding/removing monsters more often than accessing a single "hot" field.
> SoA is weak if you are adding/removing monsters more often than accessing a single "hot" field.
Why is that? Genuinely curious. Does "weak" mean that it performs worse than AoS, or that the gains aren't as significant versus AoS?
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Yes. I think one of the big advantages of SoA is that you only pay for the fields you're currently using. If you need a field somewhere, you can add it and only pay the cost of iterating it where you need it.
Every Struct Matters