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Ask HN: Why do langs with alleged “automatic memory management manage tiny part

Java boasts it has automatic memory management. But you have to close files after using them. That's manual memory management!

tjansen 7y ago
Closing files manually is manual resource management, not memory management. C++ uses its memory management for resource management, but Java doesn't. Java improved resource management with "try with resources", but generally resource management really isn't an issue for most Java developers. In modern frameworks like Spring you rarely have to manage resources yourself. You let the framework manage them for you, and the framework will provide them to you via injection.
- AnimalMuppet 7y ago
  Your way of saying it isn't wrong, but I think I would say instead that C++ uses destructors both for memory management and management of other resources.
  - brianush1 7y ago
    If we're being pedantic here, destructors don't manage memory at all, they only free resources. Memory is freed by the caller of the destructor, either automatically if the memory was allocated on the stack, or manually by the delete operator if the memory was allocated on the heap.
    - Someone 7y ago
      Memory is a resource, too, and C++-style destructors do manage it, sometimes. Destructors do not free the memory of the object being destroyed, but they can free memory that its fields point to.
- ErotemeObelus 7y ago
  I want a language where the garbage collector not only frees inaccessible dynamically allocated blocks, but also frees all resources when they're inaccessible. Is it possible to design such a generalized GC?
  - jarjarbinks455 7y ago
    In c# you can implement IDisposable. The garbage collector will call a dispose method allowing you piggyback on the garbage collector for just about anything. But it's rarely a good idea to piggyback. Most resources are much more limited or time sensitive than memory. So you should free them immediately. Even when your resource -is- memory it sometimes is a very very very bad idea to free via garbage collection.
    - ErotemeObelus 7y ago
      What is even the point of garbage collection? By the time you manage all the resources of a 100k line application you're going to have to do as much work as if you wrote it in C.
      - jarjarbinks455 7y ago
        Well the point of gc is to allow the algorithm to focus on the problem domain. and not worry about mechanical implementation details of a computer. Everything has a trade off. An obvious win still must pay a cost elsewhere. In the case of GC it's simply not a good trade off for many resources. Sometimes it cannot be used even if you were willing to accept a trade.
        ErotemeObelus 7y ago
        Managing resources by hand is managing the mechanical implementation details of a computer. So in order to have garbage collection that manages more than just a sliver of memory so small that it counts as a rounding error by the same you get to the size of real world projects, you need a custom operating system and possibly even custom hardware too.
        AnimalMuppet 7y ago
        Your statement is false for many real world projects. (For some, you are correct. Those projects should not be written in a garbage-collected language, or should be written to run on a custom OS and custom hardware.)
brudgers 7y ago
The "file" abstraction covers many different types of storage: spinning and solid state disks, in-memory structures, whatever your cloud actually uses, optical medial like DVD's, and even magnetic tape. Typically, these are managed by the underlying operating system's IO interface not Java itself. Historically, operating systems use buffers for IO and closing the file includes flushing the buffers of in-flight data (in addition to things like deallocating pointers to files at the OS level). Without explicit instructions the operating system doesn't know when a file is no longer in use. A programming language/environment could open and close files on every read and every write automatically. But it would entail overhead for operating system calls to allocate/deallocate nodes in the file table. For some types of programs this might be ok. For others it might be problematic, particularly those with moderate or strong dependence on IO.
- ErotemeObelus 7y ago
  So in order to have complete automatic resource management you need to build a custom OS and possibly even custom hardware.
  - codegladiator 7y ago
    How would your new OS/hardware know when the "use" of the resource is complete ? Maybe the dev wanted to read just 10 bytes from a 1 GB file, maybe 100 bytes... maybe its a telnet connection which is never supposed to be closed and just kept in a pool ? For a very small benefit of "automatic closing of resources" this looks like a huge trade-off.
    - ErotemeObelus 7y ago
      Disagree that it is a small benefit. Resource management becomes so unwieldly that there's little difference between a 100k line application in Java and a 100k line application in C with regards to memory/resource management. Are you saying that moving beyond C is a "small benefit"? And it's easy to determine when the use of the resource is complete: when there are no active pointers to the resource (the circular reference problem I don't think can happen with external resources like files).
frou_dh 7y ago
That's what the concept of "Finalizers" is for. They indicate that something should be done to an object just before the runtime garbage collects it. https://en.wikipedia.org/wiki/Finalizer https://en.wikipedia.org/wiki/Finalizer Files specifically are a bit thorny because the OS generally places a limit on how many simultaneous open files a process can have, and you don't want to risk bumping in to that limit due to unpredictable garbage collection timing.

Java boasts it has automatic memory management. But you have to close files after using them. That's manual memory management!

tjansen 7y ago

Closing files manually is manual resource management, not memory management. C++ uses its memory management for resource management, but Java doesn't. Java improved resource management with "try with resources", but generally resource management really isn't an issue for most Java developers. In modern frameworks like Spring you rarely have to manage resources yourself. You let the framework manage them for you, and the framework will provide them to you via injection.

AnimalMuppet 7y ago
Your way of saying it isn't wrong, but I think I would say instead that C++ uses destructors both for memory management and management of other resources.
- brianush1 7y ago
  If we're being pedantic here, destructors don't manage memory at all, they only free resources. Memory is freed by the caller of the destructor, either automatically if the memory was allocated on the stack, or manually by the delete operator if the memory was allocated on the heap.
  - Someone 7y ago
    Memory is a resource, too, and C++-style destructors do manage it, sometimes. Destructors do not free the memory of the object being destroyed, but they can free memory that its fields point to.
ErotemeObelus 7y ago
I want a language where the garbage collector not only frees inaccessible dynamically allocated blocks, but also frees all resources when they're inaccessible. Is it possible to design such a generalized GC?
- jarjarbinks455 7y ago
  In c# you can implement IDisposable. The garbage collector will call a dispose method allowing you piggyback on the garbage collector for just about anything. But it's rarely a good idea to piggyback. Most resources are much more limited or time sensitive than memory. So you should free them immediately. Even when your resource -is- memory it sometimes is a very very very bad idea to free via garbage collection.
  - ErotemeObelus 7y ago
    What is even the point of garbage collection? By the time you manage all the resources of a 100k line application you're going to have to do as much work as if you wrote it in C.
    - jarjarbinks455 7y ago
      Well the point of gc is to allow the algorithm to focus on the problem domain. and not worry about mechanical implementation details of a computer. Everything has a trade off. An obvious win still must pay a cost elsewhere. In the case of GC it's simply not a good trade off for many resources. Sometimes it cannot be used even if you were willing to accept a trade.
      - ErotemeObelus 7y ago
        Managing resources by hand is managing the mechanical implementation details of a computer. So in order to have garbage collection that manages more than just a sliver of memory so small that it counts as a rounding error by the same you get to the size of real world projects, you need a custom operating system and possibly even custom hardware too.
        AnimalMuppet 7y ago
        Your statement is false for many real world projects. (For some, you are correct. Those projects should not be written in a garbage-collected language, or should be written to run on a custom OS and custom hardware.)

brudgers 7y ago

The "file" abstraction covers many different types of storage: spinning and solid state disks, in-memory structures, whatever your cloud actually uses, optical medial like DVD's, and even magnetic tape. Typically, these are managed by the underlying operating system's IO interface not Java itself. Historically, operating systems use buffers for IO and closing the file includes flushing the buffers of in-flight data (in addition to things like deallocating pointers to files at the OS level). Without explicit instructions the operating system doesn't know when a file is no longer in use. A programming language/environment could open and close files on every read and every write automatically. But it would entail overhead for operating system calls to allocate/deallocate nodes in the file table. For some types of programs this might be ok. For others it might be problematic, particularly those with moderate or strong dependence on IO.

ErotemeObelus 7y ago
So in order to have complete automatic resource management you need to build a custom OS and possibly even custom hardware.
- codegladiator 7y ago
  How would your new OS/hardware know when the "use" of the resource is complete ? Maybe the dev wanted to read just 10 bytes from a 1 GB file, maybe 100 bytes... maybe its a telnet connection which is never supposed to be closed and just kept in a pool ? For a very small benefit of "automatic closing of resources" this looks like a huge trade-off.
  - ErotemeObelus 7y ago
    Disagree that it is a small benefit. Resource management becomes so unwieldly that there's little difference between a 100k line application in Java and a 100k line application in C with regards to memory/resource management. Are you saying that moving beyond C is a "small benefit"? And it's easy to determine when the use of the resource is complete: when there are no active pointers to the resource (the circular reference problem I don't think can happen with external resources like files).

frou_dh 7y ago

That's what the concept of "Finalizers" is for. They indicate that something should be done to an object just before the runtime garbage collects it. https://en.wikipedia.org/wiki/Finalizer https://en.wikipedia.org/wiki/Finalizer Files specifically are a bit thorny because the OS generally places a limit on how many simultaneous open files a process can have, and you don't want to risk bumping in to that limit due to unpredictable garbage collection timing.