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Garbage collection does not have to mean GC as we know it from Java. It's not that the objects created by the app are garbage collected. The whole memory management in modern systems resembles a GC (it pretty much is a GC). Just instead of managing liveness of the objects, you manage the block mapping. Sometimes you have to write them back to the disk, sometimes you have no memory left and you have to swap them out...

That's pretty much the GC algorithm. There's nothing wrong with that mention.

When he says "the garbage collection algorithm may require that all of a program’s data be in physical RAM before collection can happen," it sure sounds like he's talking about in-app heap collection. Does "may require that all of a program's data be in physical RAM" really apply to the kernel level of memory management? It makes no sense to me when applied that way.

I don't see why this was downvoted. There are probably still many applications in the wild that use Objective-C 2 garbage collection. The garbage collector scans the stack and global memory for references, and recursively follows strong references to find out which objects are in the reachable set. This may touch a lot of memory pages that were paged out. So, even if the garbage collection algorithm does not require application to be in physical RAM, probably many pages will faulted to physical RAM as a part of garbage collection.

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