6 ms·

ARC (if ‘A’ means atomic) is simply not a good fit for modern hardware. With traditional tracing GCs you get blazing fast allocation and get to amortize the cos

by kaba0 4y ago

ARC (if ‘A’ means atomic) is simply not a good fit for modern hardware. With traditional tracing GCs you get blazing fast allocation and get to amortize the cost of deallocation, while you pay a price for ref counting on every usage (or at best on almost every usage with a good compiler).

magicalhippo 4y ago
> ARC (if ‘A’ means atomic) A is for Automatic, as in the compiler adds the calls needed to manage the reference counting. If the language and compiler can guarantee a reference is strictly contained within a thread, it doesn't have to be atomic. Usually that is not the case, so atomic operations are used.
- rwmj 4y ago
  You need more than that. Reference counting amplifies reads into writes. To ameliorate that (somewhat) you need to keep the references separate from the data (a particular problem with Rust). Otherwise you end up with lots of writes scattered all over memory, causing false sharing and lots of unnecessary communication over buses between your cores. One way to do this is to keep the references together in separate cache lines or pages, away from the data. And this only partly alleviates the problem.
  - zarzavat 4y ago
    While this is true on a microscopic scale, the motivation for using RC is macroscopic: to release memory ASAP to get predictable, low memory usage. From the RC point of view the real enemy is swap and the memory management system's primary objective is to eliminate swap.
    - rwmj 4y ago
      To summarise: - your application is using close to 100% available RAM - you don't care about the impact of all those extra writes clogging up CPU caches and bouncing cache lines back and forth across internal buses - you can't afford to pay for a small amount of extra RAM In that case reference counting sounds ideal. Good luck!
      - zarzavat 4y ago
        This was exactly Apple's reasoning for using refcounting in Swift. Although, as I understand it they worked around problem (2) by building their own silicon. If you're on the server then just buy more RAM and use a GC, I agree. But if you're making consumer software then buying more RAM isn't an option. Additionally, RAM costs battery life even when it isn't in use.
    - JonChesterfield 4y ago
      Prompt is even better than predictable. In particular, reliably knowing the refcount on immutable data (in the persistent structure sense) has reached one means you can mutate it undetectably.
- zarzavat 4y ago
  FWIW there is a clash of terminology here. - ARC (upper case): Automatic Reference Counting (in Swift/ObjC) - Arc (title case): Atomic reference counting (in Rust) I assume OP intended the former.
  - magicalhippo 4y ago
    Ah good to know. Never used Rust so haven't come across Arc, only ARC in various other languages.
  - celeritascelery 4y ago
    I don't think there is really a distinction here. Swift uses atomic reference counting. Granted the details are different between approaches (swift tries to elide references counts when it can), but there are not two different "arcs" being discussed here, they refer to the same concept.
    - tsimionescu 4y ago
      Atomic references counting is a necessary technology when you want to do reference counting for an object shared by multiple threads. This applies both when doing manual reference counting (Rust's Arc, C++'s std::shared_ptr) or when doing automatic reference counting (objects in Swift, Objective C, Python). Choosing to use automatic reference counting in a language is a design choice, with positives and negatives. Atomic reference counting is an implementation detail, a strict necessity whenever you can't be sure an object will only be referenced by one thread (in essence, non-atomic reference counting is just an optimization).
CyberDildonics 4y ago
you pay a price for ref counting on every usage What do you mean by 'usage'? References don't change when reading or writing the data being reference counted, they change when being passed to a function or returned from one. In C++ it's rare that you would even use reference counting, since that essentially means you don't know the lifetime of your value. Most variables are going to be on the stack and the vast majority of dynamic allocation is going to be referenced from a single scope at one time. The reality is that it takes gross incompetence to have a speed impact from reference counting.
- kaba0 4y ago
  Yeah I meant usage as “getting it into/out of scope”. > In C++ it's rare that you would even use reference counting, since that essentially means you don't know the lifetime of your value Sure, because it is a manual memory managed language with RC being an escape hatch only. But there are plenty of problems/programs where you simply can’t know the lifetime of your objects, e.g. Chrome uses a proper GC for C++ as well.
  - CyberDildonics 4y ago
    with RC being an escape hatch only I don't know what you mean by escape hatch, but it generally just isn't necessary and memory is managed automatically by scope. The bigger point here is that it just isn't a significant part of execution time. But there are plenty of problems/programs where you simply can’t know the lifetime of your objects Like what? I can only think of one, which is passing memory allocated in one thread to another thread. Chrome uses a proper GC for C++ as well This is an anecdote, it doesn't prove or disprove anything in the bigger picture.
    - kaba0 4y ago
      > managed automatically by scope That’s just an implementation detail. The point is that the object’s lifetime is only known at runtime and will be reclaimed when a counter reaches zero, this is reference counting. Whether you have to manually inc/dec that counter, or the language does it for you through some abstraction is besides the point, it is automatic memory management either way, as it.. manages memory automatically. > Like what? I can only think of one, which is passing memory allocated in one thread to another thread Any programming language, both parsing into an AST, AST manipulations, interpretation (and that is a very wide category, not only for things you would think of as proper languages). But even some games may want to use GC for some in-game objects, as the lifetime of those is fundamentally dependent on user action. Would the litany of managed languages and their widespread usage be less anecdotal?

by kaba0 4y ago

magicalhippo 4y ago

> ARC (if ‘A’ means atomic) A is for Automatic, as in the compiler adds the calls needed to manage the reference counting. If the language and compiler can guarantee a reference is strictly contained within a thread, it doesn't have to be atomic. Usually that is not the case, so atomic operations are used.

rwmj 4y ago
You need more than that. Reference counting amplifies reads into writes. To ameliorate that (somewhat) you need to keep the references separate from the data (a particular problem with Rust). Otherwise you end up with lots of writes scattered all over memory, causing false sharing and lots of unnecessary communication over buses between your cores. One way to do this is to keep the references together in separate cache lines or pages, away from the data. And this only partly alleviates the problem.
- zarzavat 4y ago
  While this is true on a microscopic scale, the motivation for using RC is macroscopic: to release memory ASAP to get predictable, low memory usage. From the RC point of view the real enemy is swap and the memory management system's primary objective is to eliminate swap.
  - rwmj 4y ago
    To summarise: - your application is using close to 100% available RAM - you don't care about the impact of all those extra writes clogging up CPU caches and bouncing cache lines back and forth across internal buses - you can't afford to pay for a small amount of extra RAM In that case reference counting sounds ideal. Good luck!
    - zarzavat 4y ago
      This was exactly Apple's reasoning for using refcounting in Swift. Although, as I understand it they worked around problem (2) by building their own silicon. If you're on the server then just buy more RAM and use a GC, I agree. But if you're making consumer software then buying more RAM isn't an option. Additionally, RAM costs battery life even when it isn't in use.
  - JonChesterfield 4y ago
    Prompt is even better than predictable. In particular, reliably knowing the refcount on immutable data (in the persistent structure sense) has reached one means you can mutate it undetectably.
zarzavat 4y ago
FWIW there is a clash of terminology here. - ARC (upper case): Automatic Reference Counting (in Swift/ObjC) - Arc (title case): Atomic reference counting (in Rust) I assume OP intended the former.
- magicalhippo 4y ago
  Ah good to know. Never used Rust so haven't come across Arc, only ARC in various other languages.
- celeritascelery 4y ago
  I don't think there is really a distinction here. Swift uses atomic reference counting. Granted the details are different between approaches (swift tries to elide references counts when it can), but there are not two different "arcs" being discussed here, they refer to the same concept.
  - tsimionescu 4y ago
    Atomic references counting is a necessary technology when you want to do reference counting for an object shared by multiple threads. This applies both when doing manual reference counting (Rust's Arc, C++'s std::shared_ptr) or when doing automatic reference counting (objects in Swift, Objective C, Python). Choosing to use automatic reference counting in a language is a design choice, with positives and negatives. Atomic reference counting is an implementation detail, a strict necessity whenever you can't be sure an object will only be referenced by one thread (in essence, non-atomic reference counting is just an optimization).

CyberDildonics 4y ago

you pay a price for ref counting on every usage What do you mean by 'usage'? References don't change when reading or writing the data being reference counted, they change when being passed to a function or returned from one. In C++ it's rare that you would even use reference counting, since that essentially means you don't know the lifetime of your value. Most variables are going to be on the stack and the vast majority of dynamic allocation is going to be referenced from a single scope at one time. The reality is that it takes gross incompetence to have a speed impact from reference counting.

kaba0 4y ago
Yeah I meant usage as “getting it into/out of scope”. > In C++ it's rare that you would even use reference counting, since that essentially means you don't know the lifetime of your value Sure, because it is a manual memory managed language with RC being an escape hatch only. But there are plenty of problems/programs where you simply can’t know the lifetime of your objects, e.g. Chrome uses a proper GC for C++ as well.
- CyberDildonics 4y ago
  with RC being an escape hatch only I don't know what you mean by escape hatch, but it generally just isn't necessary and memory is managed automatically by scope. The bigger point here is that it just isn't a significant part of execution time. But there are plenty of problems/programs where you simply can’t know the lifetime of your objects Like what? I can only think of one, which is passing memory allocated in one thread to another thread. Chrome uses a proper GC for C++ as well This is an anecdote, it doesn't prove or disprove anything in the bigger picture.
  - kaba0 4y ago
    > managed automatically by scope That’s just an implementation detail. The point is that the object’s lifetime is only known at runtime and will be reclaimed when a counter reaches zero, this is reference counting. Whether you have to manually inc/dec that counter, or the language does it for you through some abstraction is besides the point, it is automatic memory management either way, as it.. manages memory automatically. > Like what? I can only think of one, which is passing memory allocated in one thread to another thread Any programming language, both parsing into an AST, AST manipulations, interpretation (and that is a very wide category, not only for things you would think of as proper languages). But even some games may want to use GC for some in-game objects, as the lifetime of those is fundamentally dependent on user action. Would the litany of managed languages and their widespread usage be less anecdotal?