> In contrast, in a GC system where you do _not_ have access to the explicit refcounting, you tend to always copy the node, just because you don't know if the o

> In contrast, in a GC system where you do _not_ have access to the explicit refcounting, you tend to always copy the node, just because you don't know if the original node might be shared through another tree or not. Even if sharing ends up not being the most common case. So you do a lot of extra work, and you end up with even more cache pressure. It's possible that things were different in 2002, but I don't really think this is the case now. In general, I make the node immutable and never copy it (copying an immutable object makes no sense). In a well-designed code base, mutations happen within the function where the data is created (read: on the stack, where cache locality is a given). Immutability also addresses Linus' concerns with thread-safety. And that's not accounting for concerns which Linus DOESN'T mention, such as increased development speed and correct program behavior. I'm not the only one saying this. Josh Bloch, for example, recommends immutability and cites cache reasons (http://www.ibm.com/developerworks/java/library/j-jtp02183/index.html http://www.ibm.com/developerworks/java/library/j-jtp02183/in...). And many languages (Haskell, Clojure) are designed heavily around avoiding mutation and sharing nodes within data structures. This talk of copying nodes to avoid your objects changing out from under you sounds a lot like what I call "writing C in Java". Linus is looking at this from the perspective of, "If they took away explicit memory management from C, this is how I would do it." But OF COURSE if you just bolt a feature like GC into a language that didn't have it before, it won't work well. Effective cache usage in a GCed system requires other language constructs (like immutability). Now, after all that, I won't make the claim that immutability in a GCed language like Java or C# is faster or even as fast as C with explicit memory management: it would take a lot of profiling code and comparing its functionality to make that claim with any kind of certainty. But it doesn't seem like Linus has done that profiling and comparison either.