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Reference counting isn't garbage collection. It's the opposite of garbage collection. With reference counting, which is optional in C++ and can be opted out of

Reference counting isn't garbage collection. It's the opposite of garbage collection. With reference counting, which is optional in C++ and can be opted out of in Swift, you keep track of your allocations and release them when you're done with them. With garbage collection an outside program decides when you're done with memory and deals with it for you on its terms. It's true that there can be unlimited work done in reference counting. Of course, it's true there can be unlimited work in any function of any program. [0] With reference counting a programmer can fix the amount of work to be done and when in deterministic fashion. With garbage collection, there is no such power. The gc decides when it will work and you cannot control or predict the interruption. You cannot produce serious interactive or time sensitive control software like that. You certainly can't write system software like that. [0] https://en.wikipedia.org/wiki/Halting_problem https://en.wikipedia.org/wiki/Halting_problem

lossolo 10y ago
> Reference counting isn't garbage collection. "Reference counting is a form of garbage collection"[1] [1] https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)#Reference_counting https://en.wikipedia.org/wiki/Garbage_collection_(computer_s...
prodigal_erik 10y ago
If the runtime mechanically detects that an object will never be used again, without you having to explicitly say so (and woe unto you if you're mistaken), that's what's meant by garbage collection. Reference counting does have a deterministic cost, but it's extremely high and applies to everything you do. Copying collection benefits from deferring and batching the work when most objects die quickly and can be ignored with zero effort (just recycle the arena containing all of them).
- WildUtah 10y ago
  You can write a reference counter in three short lines of code that works the same as shared_ptr<T> or Swift. A modern automatic garbage collector requires a threading library plus tens of thousands of lines of intricate code. We're not comparing like with like here.
  - prodigal_erik 10y ago
    The three line version is really easy to write. And every time you assign to a pointer, it requires uncached atomic updates to the refcounts of the old and new objects, which is staggeringly expensive compared to a block of code that can go unused for minutes at a time. And all it gets you is learning that a small piece of memory is available slightly sooner, but you'll probably want to coalesce your free memory anyway for better locality.
    - WildUtah 10y ago
      I hope that the 100-man years of top engineers version has some benefits over the 3-line newbie approach.
    - lokedhs 10y ago
      People tend to think that refcounting must be faster than a tracing GC because you don't have to scan the memory. The second half of the previous sentence is true, but what they forget is that refcounting needs to do a lot of things that a GC doesn't have to: Like you said, need to update the reference every time it's moved. malloc becomes much slower, compared to a GC language (with a compacting GC) where malloc is effectively just a single add instruction. free also needs to do a lot of bookkeeping while the GC language doesn't need to do anything. Multithreading introduces another set of issues when dealing with refcounting that is not a problem with a GC.
      - je42 10y ago
        Multithreading also introduces issues for GC. It needs to stop all threads. If you look at how much work has been delivered towards the GC's of for example JVM, Go and V8. It is a really tough problem to get right for all application types. Reference counting is very predictable. And if you see a bottle neck you can start fixing it. And with ARC (Swift/ObjC) you get even compiler support to optimize all ref counting administration. Further malloc doesn't have anything to do with ref counting as ref counting sits on top if it. Further, GC languages also need to do book keeping: Generations and Compaction.
- wahern 10y ago
  In order to know whether an arena is empty, you need to have walked, at a minimum, each object that was in the arena. That applies for copying collections too as during the copy phase you've walked and inspected each object that might be copied into the arena at least once. Arenas do not reduce the cost of garbage collection to O(1). And in any event arenas are not solely the preserve of sweeping or sweeping+copying collectors. The algorithmic complexity of a sweeping collector is identical to that of a reference counting collector that can also break cycles. See A Unified Theory of Garbage Collection at http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.146.3973 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.146.... A reference counting collector that _cannot_ break cycles (like in Swift) might have lower algorithmic complexity. I'm not sure. The biggest bone of contention with reference counting is the constant adjustment of the reference count of objects on the stack. OTOH, the bone of contention with sweeping collectors is constantly walking all extant objects, including ones not directly on the stack. There are various optimizations you can make to one or the other or both; many arguments about how real-world scenarios tax one or the other model; and many arguments about which is easier to optimize. But at the end of the day reference counting is no worse in terms of algorithmic complexity than sweeping. What matters most will be the quality of implementation. Is the compiler smart enough to know that an object passed 3 routines deep doesn't really escape the stack scope? If it can do that, it can elide all the cost of either approach.
  - WildUtah 10y ago
    What matters most will be the quality of implementation. Is the compiler smart enough to know that an object passed 3 routines deep doesn't really escape the stack scope? If it can do that, it can elide all the cost of either approach. This is the plan with Swift and what makes it zippy. bone of contention with sweeping collectors is constantly walking The real problem is that you cannot determine when the program will pause. It makes interactive and soft realtime impossible with gc. (Hard realtime should not run a generic allocator at all.) You can't write anything but servers and unpleasantly unresponsive apps. Android developers have to carefully jump through hoops to avoid allocating any RAM at all if they don't want to bug their users. For years when the chips were slow, ref counting was a major competitive advantage for iPhone. A hundred-billion dollar win. And then there's system software. You can't afford a live, unpredictable runtime messing with you in systems code so anything that touches low level code can't use gc. There's a disincentive to really invest in a language that simply can't ever do a full stack, even if you're unlikely to need to write a device driver anytime soon. That's why no major browsers, performant ACID databases, or fast app servers depend on RAM gc.
pjmlp 10y ago
> Reference counting isn't garbage collection. Reference books in computer science, acknowledged by the experts in the field, happen to disagree. http://gchandbook.org/ http://gchandbook.org/
- khedoros1 10y ago
  How about: Reference counting is one form of garbage collection, but "Garbage Collection" without any additional qualifiers or context usually implies "tracing garbage collection", with or without the aid of other GC algorithms. In common usage, I don't hear of languages like C++ described as a "garbage collecting language". That usually describes Java, Python, etc.
  - pjmlp 10y ago
    C++ has a GC API and a few dialects that support GC like C++/CLI. Having RC library types doesn't make a language GC, because their correct use is not enforced by language semantics.
- mannykannot 10y ago
  I could say this in response to a dozen posts here, but arguing over the strict definition of the term is largely beside the point, which is the actual capabilities of the various languages. In fact, the argument itself shows that the distinction is no longer either as straightforward or useful as it once was, making the dichotomy moot.
  - pjmlp 10y ago
    Well, those that review CS papers about GC algorithms to ACM, OOPSLA and SIGPLAN, think differently.
    - mannykannot 10y ago
      Elsewhere in this discussion: hellofunk: Then C++ is also garbage collected. yourself: Kind of, but only because C++11 defined a pluggable GC ABI. This is the sort of discussion that is far more fruitful than a pedantic argument over the precise meaning of a term of art in a field that is rapidly advancing. And if the academic community really is obsessed with this, then that is unfortunate.
nostrademons 10y ago
They're duals, actually, and most practical GC algorithms used in production are hybrids of the two: http://www.cs.virginia.edu/~cs415/reading/bacon-garbage.pdf http://www.cs.virginia.edu/~cs415/reading/bacon-garbage.pdf
- WildUtah 10y ago
  They're duals, actually No, actually, they're not. Just try allocating a lot of looped structure RAM and see.
simias 10y ago
I'm surprised by this heated debate regarding GC in this thread. Are you arguing that scripting languages that use reference counting are not garbage collected? PHP used reference counting for a long time (cycle detection only made it into the language in version 5.3 if I trust Wikipedia), it feels odd to consider PHP as not being garbage collected. For me the underlying technology is not very relevant, what matters is "do I have to worry about exactly when the resource is going to be dropped". I use the resource and when I stop using it it'll probably be deleted soon-ish. At any rate it's not really my problem (at least in a perfect world). You say "With reference counting [...] you keep track of your allocations and release them when you're done with them" but that's not really true, I don't keep track of anything, the language or library does. I can't really tell by looking at some code in isolation when or how the resource is destroyed. The GC takes care of that. Some languages like python, Java, Go or PHP have 1st party garbage collection that's active by default and other languages like C, C++, Rust and friends have optional 3rd party GC implemented as libraries (reference counting or otherwise). I really don't see what's so controversial about this.
- WildUtah 10y ago
  In C, C++, Rust, Swift, &c I can determine how and when references go into or out of scope and get freed. The defining characteristic of a garbage collected system is an active outside process with priority and control over my code that chooses when to inject bugs (to wit: arbitrary pauses) into my program outside my control.
- braveo 10y ago
  PHP is a bad example, it was designed around the idea of the OS doing the cleanup for you when the process exited at the end of the request.

lossolo 10y ago

> Reference counting isn't garbage collection. "Reference counting is a form of garbage collection"[1] [1] https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)#Reference_counting https://en.wikipedia.org/wiki/Garbage_collection_(computer_s...

prodigal_erik 10y ago

If the runtime mechanically detects that an object will never be used again, without you having to explicitly say so (and woe unto you if you're mistaken), that's what's meant by garbage collection. Reference counting does have a deterministic cost, but it's extremely high and applies to everything you do. Copying collection benefits from deferring and batching the work when most objects die quickly and can be ignored with zero effort (just recycle the arena containing all of them).

WildUtah 10y ago
You can write a reference counter in three short lines of code that works the same as shared_ptr<T> or Swift. A modern automatic garbage collector requires a threading library plus tens of thousands of lines of intricate code. We're not comparing like with like here.
- prodigal_erik 10y ago
  The three line version is really easy to write. And every time you assign to a pointer, it requires uncached atomic updates to the refcounts of the old and new objects, which is staggeringly expensive compared to a block of code that can go unused for minutes at a time. And all it gets you is learning that a small piece of memory is available slightly sooner, but you'll probably want to coalesce your free memory anyway for better locality.
  - WildUtah 10y ago
    I hope that the 100-man years of top engineers version has some benefits over the 3-line newbie approach.
  - lokedhs 10y ago
    People tend to think that refcounting must be faster than a tracing GC because you don't have to scan the memory. The second half of the previous sentence is true, but what they forget is that refcounting needs to do a lot of things that a GC doesn't have to: Like you said, need to update the reference every time it's moved. malloc becomes much slower, compared to a GC language (with a compacting GC) where malloc is effectively just a single add instruction. free also needs to do a lot of bookkeeping while the GC language doesn't need to do anything. Multithreading introduces another set of issues when dealing with refcounting that is not a problem with a GC.
    - je42 10y ago
      Multithreading also introduces issues for GC. It needs to stop all threads. If you look at how much work has been delivered towards the GC's of for example JVM, Go and V8. It is a really tough problem to get right for all application types. Reference counting is very predictable. And if you see a bottle neck you can start fixing it. And with ARC (Swift/ObjC) you get even compiler support to optimize all ref counting administration. Further malloc doesn't have anything to do with ref counting as ref counting sits on top if it. Further, GC languages also need to do book keeping: Generations and Compaction.
wahern 10y ago
In order to know whether an arena is empty, you need to have walked, at a minimum, each object that was in the arena. That applies for copying collections too as during the copy phase you've walked and inspected each object that might be copied into the arena at least once. Arenas do not reduce the cost of garbage collection to O(1). And in any event arenas are not solely the preserve of sweeping or sweeping+copying collectors. The algorithmic complexity of a sweeping collector is identical to that of a reference counting collector that can also break cycles. See A Unified Theory of Garbage Collection at http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.146.3973 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.146.... A reference counting collector that _cannot_ break cycles (like in Swift) might have lower algorithmic complexity. I'm not sure. The biggest bone of contention with reference counting is the constant adjustment of the reference count of objects on the stack. OTOH, the bone of contention with sweeping collectors is constantly walking all extant objects, including ones not directly on the stack. There are various optimizations you can make to one or the other or both; many arguments about how real-world scenarios tax one or the other model; and many arguments about which is easier to optimize. But at the end of the day reference counting is no worse in terms of algorithmic complexity than sweeping. What matters most will be the quality of implementation. Is the compiler smart enough to know that an object passed 3 routines deep doesn't really escape the stack scope? If it can do that, it can elide all the cost of either approach.
- WildUtah 10y ago
  What matters most will be the quality of implementation. Is the compiler smart enough to know that an object passed 3 routines deep doesn't really escape the stack scope? If it can do that, it can elide all the cost of either approach. This is the plan with Swift and what makes it zippy. bone of contention with sweeping collectors is constantly walking The real problem is that you cannot determine when the program will pause. It makes interactive and soft realtime impossible with gc. (Hard realtime should not run a generic allocator at all.) You can't write anything but servers and unpleasantly unresponsive apps. Android developers have to carefully jump through hoops to avoid allocating any RAM at all if they don't want to bug their users. For years when the chips were slow, ref counting was a major competitive advantage for iPhone. A hundred-billion dollar win. And then there's system software. You can't afford a live, unpredictable runtime messing with you in systems code so anything that touches low level code can't use gc. There's a disincentive to really invest in a language that simply can't ever do a full stack, even if you're unlikely to need to write a device driver anytime soon. That's why no major browsers, performant ACID databases, or fast app servers depend on RAM gc.

pjmlp 10y ago

> Reference counting isn't garbage collection. Reference books in computer science, acknowledged by the experts in the field, happen to disagree. http://gchandbook.org/ http://gchandbook.org/

khedoros1 10y ago
How about: Reference counting is one form of garbage collection, but "Garbage Collection" without any additional qualifiers or context usually implies "tracing garbage collection", with or without the aid of other GC algorithms. In common usage, I don't hear of languages like C++ described as a "garbage collecting language". That usually describes Java, Python, etc.
- pjmlp 10y ago
  C++ has a GC API and a few dialects that support GC like C++/CLI. Having RC library types doesn't make a language GC, because their correct use is not enforced by language semantics.
mannykannot 10y ago
I could say this in response to a dozen posts here, but arguing over the strict definition of the term is largely beside the point, which is the actual capabilities of the various languages. In fact, the argument itself shows that the distinction is no longer either as straightforward or useful as it once was, making the dichotomy moot.
- pjmlp 10y ago
  Well, those that review CS papers about GC algorithms to ACM, OOPSLA and SIGPLAN, think differently.
  - mannykannot 10y ago
    Elsewhere in this discussion: hellofunk: Then C++ is also garbage collected. yourself: Kind of, but only because C++11 defined a pluggable GC ABI. This is the sort of discussion that is far more fruitful than a pedantic argument over the precise meaning of a term of art in a field that is rapidly advancing. And if the academic community really is obsessed with this, then that is unfortunate.

nostrademons 10y ago

They're duals, actually, and most practical GC algorithms used in production are hybrids of the two: http://www.cs.virginia.edu/~cs415/reading/bacon-garbage.pdf http://www.cs.virginia.edu/~cs415/reading/bacon-garbage.pdf

WildUtah 10y ago
They're duals, actually No, actually, they're not. Just try allocating a lot of looped structure RAM and see.

simias 10y ago

I'm surprised by this heated debate regarding GC in this thread. Are you arguing that scripting languages that use reference counting are not garbage collected? PHP used reference counting for a long time (cycle detection only made it into the language in version 5.3 if I trust Wikipedia), it feels odd to consider PHP as not being garbage collected. For me the underlying technology is not very relevant, what matters is "do I have to worry about exactly when the resource is going to be dropped". I use the resource and when I stop using it it'll probably be deleted soon-ish. At any rate it's not really my problem (at least in a perfect world). You say "With reference counting [...] you keep track of your allocations and release them when you're done with them" but that's not really true, I don't keep track of anything, the language or library does. I can't really tell by looking at some code in isolation when or how the resource is destroyed. The GC takes care of that. Some languages like python, Java, Go or PHP have 1st party garbage collection that's active by default and other languages like C, C++, Rust and friends have optional 3rd party GC implemented as libraries (reference counting or otherwise). I really don't see what's so controversial about this.

WildUtah 10y ago
In C, C++, Rust, Swift, &c I can determine how and when references go into or out of scope and get freed. The defining characteristic of a garbage collected system is an active outside process with priority and control over my code that chooses when to inject bugs (to wit: arbitrary pauses) into my program outside my control.
braveo 10y ago
PHP is a bad example, it was designed around the idea of the OS doing the cleanup for you when the process exited at the end of the request.