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pizlonator is correct. I didn't mean the layout of individual structs (that is its fields). What I meant is the layout of large numbers of objects in memory (re

pizlonator is correct. I didn't mean the layout of individual structs (that is its fields). What I meant is the layout of large numbers of objects in memory (relative to each other) which is what the article was benchmarking. And it's not just that I don't need the GC to help me with that in a language that has value types. It's also that the GC might get it completely wrong if I'm not accessing the objects in creation order. The generational hypothesis will hold in general, but I don't think it is very relevant when we're talking about processing large arrays filled with small objects. They're all going to be old gen.

pcwalton 9y ago
> The generational hypothesis will hold in general, but I don't think it is very relevant when we're talking about processing large arrays filled with small objects. They're all going to be old gen. Most apps aren't doing that. The generational hypothesis is one of the strongest empirical observations we've ever seen in computer science, and those are very hard to come by. Every attempt to deny the generational hypothesis that I've ever seen has been proven to be wrong in the end.
- fauigerzigerk 9y ago
  Most apps aren't doing that, which is exactly why I said that the generational hypothesis will hold in general. I don't need convincing that optimizing for that case is generally a good idea. It just has very little to do with the point I was trying to make (apparently in a less than clear way)
- pizlonator 9y ago
  I think you're overstating the meaning of the empirical observation. The observation as I understand it is that if you take the average over a large set of programs, then the hypothesis holds. The observation is _not_ that if you take any program, then the hypothesis holds. I'm hedging that it's the average over a large population of programs. I'm not making a claim about every program, only most programs, or rather - "most programs most of the time". I think that this is important because without a doubt, some programs are not generational at all. Great example: go navigate from this page to another one. You want a full GC, not an eden GC, at this point. If your primary activity when browsing is navigating around and you're not spawning a new process for each navigation, then you are violating the generational hypothesis big time. Note that this is somewhat orthogonal to the question of whether you should implement a generational GC. Generational GCs are great in part because they are rarely a regression on non-generational workloads. Classic example: in the MMTk project they built this goofy "only for testing" non-generational collector that allocates in a bump nursery and then during GC it simultaneously evacuates the nursery and marks the "old" space. This non-generational GC, which has some traits of a generational GC, was slightly faster than their mark-sweep collector. It's often the case that generational GCs are really about a lot more than just exploiting the generational hypothesis.
  - pcwalton 9y ago
    Yes, there are some workloads that violate the generational hypothesis. But the vast majority of real-life programs adhere to it. I'm not arguing that the generational hypothesis holds over the entire universe of all possible programs; that would obviously be wrong. Rather I'm arguing against the implication expressed by some in this thread that generational GC isn't worth implementing because the generational hypothesis is wrong for most programs in X language.
    - pizlonator 9y ago
      I'm super puzzled why you think that the generational hypothesis should hold in every language, to the point that you're willing to reject empirical findings in other languages that contradict this presumption. If some language did not obey the generational hypothesis then this would not contract any single - not even one - prior empirical finding, unless there had also already been experiments on this subject in that particular language. The field of GC is way too young to start extrapolating findings from some languages to all languages.
      - pcwalton 9y ago
        I'm not rejecting empirical findings that contradict the generational hypothesis. I simply don't believe anybody has actually done any rigorous measurements of object mortality in Go and how it compares to other languages. In the absence of such measurements, I'm going to speculate similar allocation patterns to C#, which has a very similar model and the generational hypothesis certainly does hold for it. I'd be more than happy to discard my speculation if it happens to be wrong!

pcwalton 9y ago

> The generational hypothesis will hold in general, but I don't think it is very relevant when we're talking about processing large arrays filled with small objects. They're all going to be old gen. Most apps aren't doing that. The generational hypothesis is one of the strongest empirical observations we've ever seen in computer science, and those are very hard to come by. Every attempt to deny the generational hypothesis that I've ever seen has been proven to be wrong in the end.

fauigerzigerk 9y ago
Most apps aren't doing that, which is exactly why I said that the generational hypothesis will hold in general. I don't need convincing that optimizing for that case is generally a good idea. It just has very little to do with the point I was trying to make (apparently in a less than clear way)
pizlonator 9y ago
I think you're overstating the meaning of the empirical observation. The observation as I understand it is that if you take the average over a large set of programs, then the hypothesis holds. The observation is _not_ that if you take any program, then the hypothesis holds. I'm hedging that it's the average over a large population of programs. I'm not making a claim about every program, only most programs, or rather - "most programs most of the time". I think that this is important because without a doubt, some programs are not generational at all. Great example: go navigate from this page to another one. You want a full GC, not an eden GC, at this point. If your primary activity when browsing is navigating around and you're not spawning a new process for each navigation, then you are violating the generational hypothesis big time. Note that this is somewhat orthogonal to the question of whether you should implement a generational GC. Generational GCs are great in part because they are rarely a regression on non-generational workloads. Classic example: in the MMTk project they built this goofy "only for testing" non-generational collector that allocates in a bump nursery and then during GC it simultaneously evacuates the nursery and marks the "old" space. This non-generational GC, which has some traits of a generational GC, was slightly faster than their mark-sweep collector. It's often the case that generational GCs are really about a lot more than just exploiting the generational hypothesis.
- pcwalton 9y ago
  Yes, there are some workloads that violate the generational hypothesis. But the vast majority of real-life programs adhere to it. I'm not arguing that the generational hypothesis holds over the entire universe of all possible programs; that would obviously be wrong. Rather I'm arguing against the implication expressed by some in this thread that generational GC isn't worth implementing because the generational hypothesis is wrong for most programs in X language.
  - pizlonator 9y ago
    I'm super puzzled why you think that the generational hypothesis should hold in every language, to the point that you're willing to reject empirical findings in other languages that contradict this presumption. If some language did not obey the generational hypothesis then this would not contract any single - not even one - prior empirical finding, unless there had also already been experiments on this subject in that particular language. The field of GC is way too young to start extrapolating findings from some languages to all languages.
    - pcwalton 9y ago
      I'm not rejecting empirical findings that contradict the generational hypothesis. I simply don't believe anybody has actually done any rigorous measurements of object mortality in Go and how it compares to other languages. In the absence of such measurements, I'm going to speculate similar allocation patterns to C#, which has a very similar model and the generational hypothesis certainly does hold for it. I'd be more than happy to discard my speculation if it happens to be wrong!