Comment by lucms_ | Hacker News Reader

Comment by lucms_ | Hacker News Reader

Parent Full thread

Isn't data-oriented design about organizing the data in a way that reflects the most common access patterns of the program? The approach of placing all numbers in a big number vector, all Arrays into a big Array vector, and so on, would be "data-oriented design" if it actually reflects the most common access patterns. So, is it the case that when you read a number you also want all those other numbers that come together with it in the cache line? Is that the case for Arrays? For DataViews? In other words, does this approach to allocating memory reflect the most common data access patterns in JavaScript programs? I'm not saying it's a bad approach, and I'm not even trying to imply that it's not DOD, I'm genuinely asking.

Edit: maybe a better question is: does it reflect the most common data access patterns of a JavaScript Engine?

Excellent question: In a theoretical sense the answer would be that we cannot know since it depends on the JavaScript being run. But: In practice I think that is indeed the case. Especially for the more common an object is, the likelier it is that it is used in conjunction with others around it. At the same time, the more important their memory placement becomes.

eg. Say you have a JS programs that has about a 100 DataViews: I'd say it's unlikely these are used in conjunction with others very often, but they're also only a small part of the program, so their placement is mostly whatever.

Now what if that number is a million instead? Now I'm betting they're mostly all created together, used together, and that their placement is critical to the program's performance.

So, I'm betting that making random memory access performance worse while guaranteeing that data created together stays together and improving linear memory performance will be an overall win.

Whether this is true data-oriented design is then in the eye of the beholder: Maybe someone will think I'm wrong, my assumptions are wrong, and I'm thus not doing things in a data-oriented way.

Reply on news.ycombinator.com