By "cost-free" I mean that they use precise traces so that memory is always freed at the exact moment it is no longer used. So they underestimate (greatly IMHO) the memory used by malloc/free versus the garbage collector. It would be a superhuman programmer who always called free() on the line exactly following the last use of every piece of allocated memory.

The second problem I have with the paper is that it underestimates the kinds of structures which GC makes possible -- eg sets of trees sharing nodes. And the corollary to that is that to implement those structures with malloc/free, programmers tend to reach for ref-counting, which adds to memory usage and is generally a terrible form of garbage collection for many other reasons.

Nevertheless it's an interesting paper which does add to the debate (I studied it about 10 years ago). It is worth reading, even though I have concerns about the methodology and hence the results.

> By "cost-free" I mean that they use precise traces so that memory is always freed at the exact moment it is no longer used. So they underestimate (greatly IMHO) the memory used by malloc/free versus the garbage collector. It would be a superhuman programmer who always called free() on the line exactly following the last use of every piece of allocated memory.

Let's say you are correct, that the manual memory management presented in the paper is too unrealistic. What do you think real world manual memory management would bring the multiple to? 5x? 4x?

> The second problem I have with the paper is that it underestimates the kinds of structures which GC makes possible -- eg sets of trees sharing nodes. And the corollary to that is that to implement those structures with malloc/free, programmers tend to reach for ref-counting, which adds to memory usage and is generally a terrible form of garbage collection for many other reasons.

These data structures are extremely niche, and when used when sharing is not truly needed just lead to time/space inefficiencies.