7 ms·

What exactly is the argument for garbage collection over RAII-style freeing? Even in garbage-collected languages, you still need to do things like close file ob

What exactly is the argument for garbage collection over RAII-style freeing? Even in garbage-collected languages, you still need to do things like close file objects; the runtime doesn't do so automatically when it detects that there are no stack references to the file. Same goes for unlocking mutexes, or any of the other things that RAII tends to address. Is it just that maintaining ownership semantics for heap memory tends to be more complicated than doing the same for file objects (much more rarely shared, I'd guess) or mutexes (not sure if "sharing" is even sensibly defined here).

crawshaw 13y ago
It is hard to manage ownership when allocations are passed around between concurrent threads. Garbage collection in Go makes goroutines and channels easy.
- agwa 13y ago
  Though arguably you shouldn't be accessing the same allocation from concurrent threads at the same time anyways, because that requires mutexes, which are ugly. Rust requires you to transfer ownership of objects when passing them between threads, which both enforces mutex-free programming and makes RAII work. Only one thread owns an object at a time, and that's the only thread that can use the object, and is also the thread that is responsible for freeing it when it goes out of scope.
  - codex 13y ago
    This implies that an object will be unavailable while it is being passsed between threads. How fast is that? With a mutex, it's as fast as shared memory.
    - agwa 13y ago
      Under the hood, you still use shared memory in this style of programming. But the programmer never uses shared memory directly. Instead he passes references to the shared memory to other threads, and the act of passing a reference clears the reference in the sending thread so the sending thread can no longer access it. In Rust, this style is enforced by the language. In C++11 you can enforce it by the convention of using std::unique_ptr and passing those between threads via a shared data structure like a blocking queue.
    - pcwalton 13y ago
      Message passing is a couple of atomic operations, and there are also L4-esque optimizations that allow a message send to be optimized into a direct context switch without a pass through the scheduler. Also, you can use shared memory in Rust. If you do, the type system ensures there can be no data races: either the data is immutable or you must take the lock before you mutate the data.
      - mratzloff 13y ago
        That's pretty nice. I've been waiting for Rust to stabilize before I bother learning it... It's been taking awhile, though. :-)
  - rbehrends 13y ago
    Mutexes are only ugly because few languages properly support them. It is not difficult to ensure at the language level that no object is accessed without owning the the lock associated with that object (read/write locks can be a bit trickier), but for some reason hardly any language implements something along these lines. It's not fundamentally different from having bound checks for array indices. You can either do that pretty easily at runtime or use a simple type system to do it at compile time (e.g., Boyapati's [1] or Flanagan's [2] work). It's been done, e.g. for Cyclone [3]. This is decades old stuff, not rocket science. Honestly, Monitors had the basic idea right in the 1970s, except as Per Brinch Hansen lamented [4], language designers keep screwing it up. [1] http://www.pmg.lcs.mit.edu/~chandra/publications/oopsla02.pdf http://www.pmg.lcs.mit.edu/~chandra/publications/oopsla02.pd... [2] http://users.soe.ucsc.edu/~cormac/papers/esop99.pdf http://users.soe.ucsc.edu/~cormac/papers/esop99.pdf [3] https://homes.cs.washington.edu/~djg/papers/cycthreads.pdf https://homes.cs.washington.edu/~djg/papers/cycthreads.pdf [4] http://brinch-hansen.net/papers/1999b.pdf http://brinch-hansen.net/papers/1999b.pdf
deleted 13y ago
[deleted]
rbehrends 13y ago
There are a number of reasons why you may want garbage collection/automatic memory management over RAII: - RAII-style freeing means a lot of reference counting. Reference counting and multiple threads don't mix well. You can circumvent that by having multiple separate heaps, but that's not always a practical solution. - Reference counting is slow; if you want GC-like speed and RC-like pause time guarantees, you're probably better off with deferred reference counting, but that's not easy to engineer into C++; it's essentially a form of garbage collection. - Lack of fully automatic memory management may affect modularity adversely; see Jones/Lins for the gory details. In brief, wholly or partly manual memory management creates leaky abstractions (this is if you eschew RC for some of your code/types for speed reasons or because you need to deal with cyclic references). - It is not difficult to have scoped deallocation for resources (scope statement in D, using statement in C#, etc., destructor pragma in Nimrod). Resource lifetime, in any event, does not always coincide with the lifetime of a variable, so this is an incomplete solution. Using GC does not mean not using RAII where its downsides do not matter.
- hershel 13y ago
  There are some non-defered reference counting that can offer pretty good performance(within 19% of defered reference counting), while still offering determinism[1]. [1]http://www.hpl.hp.com/personal/Pramod_Joisha/Publications/ismm06.pdf http://www.hpl.hp.com/personal/Pramod_Joisha/Publications/is...
  - rbehrends 13y ago
    Yes, but they have the same problem that they require compiler smarts; i.e., they require more than a template to implement shared_ptr<T>. When you're doing that, you may just as well implement something more sophisticated for automated memory management. The attraction of RAII is that it's a pretty simple mechanism that doesn't require compiler intervention outside of basic, already existing optimizations such as inlining.
- agwa 13y ago
  RAII does not mean a lot of reference counting. In the vast majority of cases in typical programs, objects have exactly one owner, and managing that ownership is very easy if your language provides good facilities for move semantics (as C++11 and Rust do). You can't say reference counting is bad for multi-threaded programs without mentioning that GC is also bad for multi-threaded programs, since the garbage collector has to pause all other threads, either for the entirety of the GC run or (if you're lucky and using a good GC) for parts of it. The modularity argument is reasonable, and I've been annoyed by this aspect several times, though in the majority of cases it doesn't seem to be an issue.
pkolaczk 13y ago
Lack of GC makes concurrent lockless programming extremely hard (and most of the examples on the internets are subtly broken). RAII doesn't work at all besides trivial cases when there is no sharing at all. Reference counting incurs additional contention, due to interlocked increments/decrements and is also very hard to do properly. Where in a GCed language a single CAS is often enough, in a non-GCed language you need to resort to less efficient and often completely unsupported things like DCAS or CAS2.
mratzloff 13y ago
Take a look at Go's `defer` keyword. I prefer it over RAII; `defer` is both more explicit and more flexible.
- WalterBright 13y ago
  You might be interested in D's scope guard statement: http://dlang.org/statement.html#ScopeGuardStatement http://dlang.org/statement.html#ScopeGuardStatement and an article about it: http://dlang.org/exception-safe.html http://dlang.org/exception-safe.html
- agwa 13y ago
  defer is great for "one off" cleanups, but in so many cases (closing file handles, releasing locks, etc.) cleanup is not one off but is repeated over and over again. So the price of explicitness is redundant code that is easy to forget or get wrong. Note that if your language supports RAII and lambdas (e.g. C++11), it's trivial to implement defer using a class that takes a function as an argument to its constructor and calls that function in its destructor. So you can still have the flexibility of defer in a language with RAII.
  - Shorel 13y ago
    Reminds me of UNWIND-PROTECT in CL
  - mratzloff 13y ago
    My preferred method of handling this is how Ruby does it with, e.g., File.open. The function yields the file handle and then closes it once the supplied block (think lambda function) ends. This is similar to what you refer to, but without the class. The class is useful for heap objects, of course.
- pcwalton 13y ago
  "defer" has really weird semantics; it's based on imperatively pushing cleanups onto implicit per-function mutable state. (This shows up if you call "defer" in a loop, for example.) It's also possible to screw up and forget to use "defer", causing things like leaks of file handles. RAII, on the other hand, is based on lexical scope, without hidden mutation, and also makes it harder to forget to do things like close files.
jmdavisProg 13y ago
There have been papers on how GCs can be more efficient than manual memory management in some situations, so it's not always the case that RAII is better from a performance standpoint (though I think that it frequently is). But regardless, a big reason for the GC in D is memory safety. Using the GC, you can make memory safety guarantees that cannot be made with malloc and free (e.g. you can guarantee that a pointer won't be invalid with a GC but can't with manual memory management), and D is big on making memory safety guarantees. Also, some features (such as array slices or closures) work very well when you have a GC and become much more difficult to pull off without one due to a lack of automatic memory management that the compiler can use and a lack of clear ownership in some cases (e.g. the runtime owns the memory for array slices rather than there being a particular array which owns the memory). That being said, D does not require the GC. It's easier if you use it for at least some of the features, but D makes it very easy to use RAII and manual memory management, which also helps a lot in making the GC work better, because it's much easier to avoid making a lot of garbage for it to collect when you don't need to. A lot of stuff in D ends up on the stack rather than the heap, and D's GC ends up having a lot less work to do than the GC does in languages like C# or Java. That being said, the current GC implementation needs some work (and effort is definitely put forth in that area), but if you don't want to use the GC, you can minimize its use or even outright avoid it (though _completely_ avoiding it can be a bit annoying since that means avoiding a few language features that require it; that list is short though). So, while D definitely uses the GC and promotes its use where appropriate, you have full control over memory just like you would in C++. And the few features that are hampered by avoiding the GC don't even exist in C++ in the first place.
- bad_user 13y ago
  > A lot of stuff in D ends up on the stack rather than the heap, and D's GC ends up having a lot less work to do than the GC does in languages like C# or Java This is true, however there's a compromise involved. Because D also allows for manual memory management and unsafe memory access, it means that the GC is not free to move stuff in memory at will ... which really means that garbage collectors, like the ones available for Java (precise, generational, non-blocking, fairly predictable and compacting) are very hard to develop, probably next to impossible. This is the price you pay for using a lower level language and it's not a price that will go away easily. I've been using Scala a lot for the past two years, using it to build high-traffic web services and while Scala is pretty wasteful in terms of allocating short-lived objects, I've been surprised at how well the JVM handles it. In terms of throughput for example, Java GCs are much better than manual memory management. Allocating memory usually involves just incrementing a pointer, so it's basically as cheap as stack allocation. Deallocating short-lived objects is also fairly cheap, since it happens in bulk and so the amortized cost is pretty similar to dealocating stuff on the stack (!!!). The JVM can do some pretty neat things, like for example if it detects that certain references do not escape their local context, it can decide to allocate those objects straight on the stack. What really sucks about garbage collection is the unpredictability. Java's CMS for example, awesome as it is, still blocks the world from time to time. And when it does, you have no real control over how much time it keeps the process hostage. The new G1 in JDK7 is much better and if you want the state of the art in terms of near-real-time GCs, you can buy into Azul's pauseless GC. But they still suck for certain apps. Allocating objects on the heap also means you have to pay the price of boxing/unboxing the references involved. This sucks too. On the other hand, by having a good GC at disposal, it's much easier to build multi-threaded architectures. In C++ for example, it's so freaking painful to deal with non-blocking concurrent algorithms, or really, multi-threading of any kind.
com2kid 13y ago
For reference, a lot of embedded programmers still use static allocations only, not even trusting heap allocations at all. Took awhile to get used to, but it is a damned fine way to avoid memory leaks of any kind! But even that aside, if you are writing something like a render loop with hard timing guarantees, (or anything else with timing needs!) having non-deterministic allocation costs can easily blow your cycle budget.
59nadir 13y ago
> Even in garbage-collected languages, you still need to do things like close file objects; the runtime doesn't do so automatically when it detects that there are no stack references to the file. I'd just like to say there are sane solutions out there already to things like this. With the proper abstractions you get things like custodians[0] in Racket and several alternatives in terms of "call-with-..."[1] functions that do handling for you. These might not be the languages that are being discussed here, but they offer things like these precisely because they are not the languages discussed here right now. [0] http://docs.racket-lang.org/reference/eval-model.html?q=custodians#%28part._custodian-model%29 http://docs.racket-lang.org/reference/eval-model.html?q=cust... [1] http://docs.racket-lang.org/reference/file-ports.html?q=call-with#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._call-with-input-file%29%29 http://docs.racket-lang.org/reference/file-ports.html?q=call...

crawshaw 13y ago

It is hard to manage ownership when allocations are passed around between concurrent threads. Garbage collection in Go makes goroutines and channels easy.

agwa 13y ago
Though arguably you shouldn't be accessing the same allocation from concurrent threads at the same time anyways, because that requires mutexes, which are ugly. Rust requires you to transfer ownership of objects when passing them between threads, which both enforces mutex-free programming and makes RAII work. Only one thread owns an object at a time, and that's the only thread that can use the object, and is also the thread that is responsible for freeing it when it goes out of scope.
- codex 13y ago
  This implies that an object will be unavailable while it is being passsed between threads. How fast is that? With a mutex, it's as fast as shared memory.
  - agwa 13y ago
    Under the hood, you still use shared memory in this style of programming. But the programmer never uses shared memory directly. Instead he passes references to the shared memory to other threads, and the act of passing a reference clears the reference in the sending thread so the sending thread can no longer access it. In Rust, this style is enforced by the language. In C++11 you can enforce it by the convention of using std::unique_ptr and passing those between threads via a shared data structure like a blocking queue.
  - pcwalton 13y ago
    Message passing is a couple of atomic operations, and there are also L4-esque optimizations that allow a message send to be optimized into a direct context switch without a pass through the scheduler. Also, you can use shared memory in Rust. If you do, the type system ensures there can be no data races: either the data is immutable or you must take the lock before you mutate the data.
    - mratzloff 13y ago
      That's pretty nice. I've been waiting for Rust to stabilize before I bother learning it... It's been taking awhile, though. :-)
- rbehrends 13y ago
  Mutexes are only ugly because few languages properly support them. It is not difficult to ensure at the language level that no object is accessed without owning the the lock associated with that object (read/write locks can be a bit trickier), but for some reason hardly any language implements something along these lines. It's not fundamentally different from having bound checks for array indices. You can either do that pretty easily at runtime or use a simple type system to do it at compile time (e.g., Boyapati's [1] or Flanagan's [2] work). It's been done, e.g. for Cyclone [3]. This is decades old stuff, not rocket science. Honestly, Monitors had the basic idea right in the 1970s, except as Per Brinch Hansen lamented [4], language designers keep screwing it up. [1] http://www.pmg.lcs.mit.edu/~chandra/publications/oopsla02.pdf http://www.pmg.lcs.mit.edu/~chandra/publications/oopsla02.pd... [2] http://users.soe.ucsc.edu/~cormac/papers/esop99.pdf http://users.soe.ucsc.edu/~cormac/papers/esop99.pdf [3] https://homes.cs.washington.edu/~djg/papers/cycthreads.pdf https://homes.cs.washington.edu/~djg/papers/cycthreads.pdf [4] http://brinch-hansen.net/papers/1999b.pdf http://brinch-hansen.net/papers/1999b.pdf

deleted 13y ago

[deleted]

rbehrends 13y ago

There are a number of reasons why you may want garbage collection/automatic memory management over RAII: - RAII-style freeing means a lot of reference counting. Reference counting and multiple threads don't mix well. You can circumvent that by having multiple separate heaps, but that's not always a practical solution. - Reference counting is slow; if you want GC-like speed and RC-like pause time guarantees, you're probably better off with deferred reference counting, but that's not easy to engineer into C++; it's essentially a form of garbage collection. - Lack of fully automatic memory management may affect modularity adversely; see Jones/Lins for the gory details. In brief, wholly or partly manual memory management creates leaky abstractions (this is if you eschew RC for some of your code/types for speed reasons or because you need to deal with cyclic references). - It is not difficult to have scoped deallocation for resources (scope statement in D, using statement in C#, etc., destructor pragma in Nimrod). Resource lifetime, in any event, does not always coincide with the lifetime of a variable, so this is an incomplete solution. Using GC does not mean not using RAII where its downsides do not matter.

hershel 13y ago
There are some non-defered reference counting that can offer pretty good performance(within 19% of defered reference counting), while still offering determinism[1]. [1]http://www.hpl.hp.com/personal/Pramod_Joisha/Publications/ismm06.pdf http://www.hpl.hp.com/personal/Pramod_Joisha/Publications/is...
- rbehrends 13y ago
  Yes, but they have the same problem that they require compiler smarts; i.e., they require more than a template to implement shared_ptr<T>. When you're doing that, you may just as well implement something more sophisticated for automated memory management. The attraction of RAII is that it's a pretty simple mechanism that doesn't require compiler intervention outside of basic, already existing optimizations such as inlining.
agwa 13y ago
RAII does not mean a lot of reference counting. In the vast majority of cases in typical programs, objects have exactly one owner, and managing that ownership is very easy if your language provides good facilities for move semantics (as C++11 and Rust do). You can't say reference counting is bad for multi-threaded programs without mentioning that GC is also bad for multi-threaded programs, since the garbage collector has to pause all other threads, either for the entirety of the GC run or (if you're lucky and using a good GC) for parts of it. The modularity argument is reasonable, and I've been annoyed by this aspect several times, though in the majority of cases it doesn't seem to be an issue.

pkolaczk 13y ago

Lack of GC makes concurrent lockless programming extremely hard (and most of the examples on the internets are subtly broken). RAII doesn't work at all besides trivial cases when there is no sharing at all. Reference counting incurs additional contention, due to interlocked increments/decrements and is also very hard to do properly. Where in a GCed language a single CAS is often enough, in a non-GCed language you need to resort to less efficient and often completely unsupported things like DCAS or CAS2.

mratzloff 13y ago

Take a look at Go's `defer` keyword. I prefer it over RAII; `defer` is both more explicit and more flexible.

WalterBright 13y ago
You might be interested in D's scope guard statement: http://dlang.org/statement.html#ScopeGuardStatement http://dlang.org/statement.html#ScopeGuardStatement and an article about it: http://dlang.org/exception-safe.html http://dlang.org/exception-safe.html
agwa 13y ago
defer is great for "one off" cleanups, but in so many cases (closing file handles, releasing locks, etc.) cleanup is not one off but is repeated over and over again. So the price of explicitness is redundant code that is easy to forget or get wrong. Note that if your language supports RAII and lambdas (e.g. C++11), it's trivial to implement defer using a class that takes a function as an argument to its constructor and calls that function in its destructor. So you can still have the flexibility of defer in a language with RAII.
- Shorel 13y ago
  Reminds me of UNWIND-PROTECT in CL
- mratzloff 13y ago
  My preferred method of handling this is how Ruby does it with, e.g., File.open. The function yields the file handle and then closes it once the supplied block (think lambda function) ends. This is similar to what you refer to, but without the class. The class is useful for heap objects, of course.
pcwalton 13y ago
"defer" has really weird semantics; it's based on imperatively pushing cleanups onto implicit per-function mutable state. (This shows up if you call "defer" in a loop, for example.) It's also possible to screw up and forget to use "defer", causing things like leaks of file handles. RAII, on the other hand, is based on lexical scope, without hidden mutation, and also makes it harder to forget to do things like close files.

jmdavisProg 13y ago

There have been papers on how GCs can be more efficient than manual memory management in some situations, so it's not always the case that RAII is better from a performance standpoint (though I think that it frequently is). But regardless, a big reason for the GC in D is memory safety. Using the GC, you can make memory safety guarantees that cannot be made with malloc and free (e.g. you can guarantee that a pointer won't be invalid with a GC but can't with manual memory management), and D is big on making memory safety guarantees. Also, some features (such as array slices or closures) work very well when you have a GC and become much more difficult to pull off without one due to a lack of automatic memory management that the compiler can use and a lack of clear ownership in some cases (e.g. the runtime owns the memory for array slices rather than there being a particular array which owns the memory). That being said, D does not require the GC. It's easier if you use it for at least some of the features, but D makes it very easy to use RAII and manual memory management, which also helps a lot in making the GC work better, because it's much easier to avoid making a lot of garbage for it to collect when you don't need to. A lot of stuff in D ends up on the stack rather than the heap, and D's GC ends up having a lot less work to do than the GC does in languages like C# or Java. That being said, the current GC implementation needs some work (and effort is definitely put forth in that area), but if you don't want to use the GC, you can minimize its use or even outright avoid it (though _completely_ avoiding it can be a bit annoying since that means avoiding a few language features that require it; that list is short though). So, while D definitely uses the GC and promotes its use where appropriate, you have full control over memory just like you would in C++. And the few features that are hampered by avoiding the GC don't even exist in C++ in the first place.

bad_user 13y ago
> A lot of stuff in D ends up on the stack rather than the heap, and D's GC ends up having a lot less work to do than the GC does in languages like C# or Java This is true, however there's a compromise involved. Because D also allows for manual memory management and unsafe memory access, it means that the GC is not free to move stuff in memory at will ... which really means that garbage collectors, like the ones available for Java (precise, generational, non-blocking, fairly predictable and compacting) are very hard to develop, probably next to impossible. This is the price you pay for using a lower level language and it's not a price that will go away easily. I've been using Scala a lot for the past two years, using it to build high-traffic web services and while Scala is pretty wasteful in terms of allocating short-lived objects, I've been surprised at how well the JVM handles it. In terms of throughput for example, Java GCs are much better than manual memory management. Allocating memory usually involves just incrementing a pointer, so it's basically as cheap as stack allocation. Deallocating short-lived objects is also fairly cheap, since it happens in bulk and so the amortized cost is pretty similar to dealocating stuff on the stack (!!!). The JVM can do some pretty neat things, like for example if it detects that certain references do not escape their local context, it can decide to allocate those objects straight on the stack. What really sucks about garbage collection is the unpredictability. Java's CMS for example, awesome as it is, still blocks the world from time to time. And when it does, you have no real control over how much time it keeps the process hostage. The new G1 in JDK7 is much better and if you want the state of the art in terms of near-real-time GCs, you can buy into Azul's pauseless GC. But they still suck for certain apps. Allocating objects on the heap also means you have to pay the price of boxing/unboxing the references involved. This sucks too. On the other hand, by having a good GC at disposal, it's much easier to build multi-threaded architectures. In C++ for example, it's so freaking painful to deal with non-blocking concurrent algorithms, or really, multi-threading of any kind.

com2kid 13y ago

For reference, a lot of embedded programmers still use static allocations only, not even trusting heap allocations at all. Took awhile to get used to, but it is a damned fine way to avoid memory leaks of any kind! But even that aside, if you are writing something like a render loop with hard timing guarantees, (or anything else with timing needs!) having non-deterministic allocation costs can easily blow your cycle budget.

59nadir 13y ago

> Even in garbage-collected languages, you still need to do things like close file objects; the runtime doesn't do so automatically when it detects that there are no stack references to the file. I'd just like to say there are sane solutions out there already to things like this. With the proper abstractions you get things like custodians[0] in Racket and several alternatives in terms of "call-with-..."[1] functions that do handling for you. These might not be the languages that are being discussed here, but they offer things like these precisely because they are not the languages discussed here right now. [0] http://docs.racket-lang.org/reference/eval-model.html?q=custodians#%28part._custodian-model%29 http://docs.racket-lang.org/reference/eval-model.html?q=cust... [1] http://docs.racket-lang.org/reference/file-ports.html?q=call-with#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._call-with-input-file%29%29 http://docs.racket-lang.org/reference/file-ports.html?q=call...