5 ms·

The first question I asked myself when I read the topic-line was: "Why would I want a garbage collector for C++" I'm not saying that it's a bad idea. I'm just

by jgaa 4y ago

The first question I asked myself when I read the topic-line was: "Why would I want a garbage collector for C++" I'm not saying that it's a bad idea. I'm just curious about why/when garbage collection is better than conventional use of unique_ptr or shared_ptr.

pebal 4y ago
GC can be faster and allocate less memory. tracked_ptr: 279.95ms (151MB) shared_ptr: 1035.07ms (480MB) source: https://godbolt.org/z/jP7ohPehY https://godbolt.org/z/jP7ohPehY
Kranar 4y ago
Makes writing lock free data structures a heck of a lot easier. Entirely eliminates the ABA problem.
pebal 4y ago
This introduces the tracked smart pointer. This pointer has less overhead than shared_ptr and releases circular data structures.
pjmlp 4y ago
Unreal C++ and C++/CLI have one. When one wants the flexibility of C++ with the productivity of a GC.
- pkolaczk 4y ago
  Whether GC improves productivity is a strong claim that needs a proof. I have yet to see such a proof. So far my experience has been that (nondeterministic) GC is only a productivity improvement over languages with manual memory management like C, but a productivity degradation over languages with deterministic automatic memory management like C++ and Rust. GC manages only memory, while in the latter case, you get automatic management of resources of any type, which can be a huge productivity boost if you use it to your advantage.
  - pjmlp 4y ago
    Do you want an example for C++? Lock free data structures are PhD level skillset unless there is some kind of tracing GC algorithm being used. Also RAII is great and all, yet there lots of ways to get it wrong. Badly implemented destructors, missing virtual destructors in hierachys, throwing exceptions on destructurs, not using handle types on heap allocated data, incorrect pairing of allocators.
    - pkolaczk 4y ago
      Lock-free data structures is a 0.001% niche, which can be implemented as libraries by experts and then used without any trouble by an average Joe programmer. An average Joe wouldn't get the implementation of them right even in a GC language anyway. Also, in those niche cases, GC is available as an option. But you should not optimize the language for the niche use case. > Badly implemented destructors, missing virtual destructors in hierachys, throwing exceptions on destructurs, not using handle types on heap allocated data, incorrect pairing of allocators. Those seem to be more specific problems with C++ rather than RAII. Rust has none of those.
      - pjmlp 4y ago
        The ongoing issues with Drop trait stabilization show otherwise. Having a GC is an option alongside the other ones, an option that both Epic and Microsoft think it is worth enough money to spend development resources on. And Google as well, actually, as Chrome uses a C++ GC, seen as way to fix security issues, Olipan.
        pkolaczk 4y ago
        GC as an opt in, next to to RAII - so you can use it when you need it, that's fine. But it is a productivity issue when the only way to manage memory is non-deterministic GC, e.g. like in Java or Go.
        pjmlp 4y ago
xiphias2 4y ago
One example would be to create higher level languages that compile to / interoperable with C++
plq 4y ago
You use custom memory management where it matters, gc anywhere else.
- pebal 4y ago
  The intention of creating this project was the opposite. You only use the tracked pointer when it is beneficial.
  - plq 4y ago
    Well that'd work too :) The adoption of a particular memory allocation strategy depends on a lot factors. This is C++ after all.
fathyb 4y ago
GCs can be more efficient if you alloc/free memory very often. shared_ptr can be quite slow because of the bookkeeping. It can also help bring more consistent performance, ie. one big malloc instead of millions of small ones.
- aidenn0 4y ago
  I would think destructors would negate some of the performance advantages of GCs, since it's not possible to have zero-cost freeing of objects when destructors exist.
  - pebal 4y ago
    SGCL executes the destructors in a separate thread, so there is no cost to the mutator.
    - aidenn0 4y ago
      > SGCL executes the destructors in a separate thread, so there is no cost to the mutator. Firstly, that's orthogonal to my original point is that, for example, a semi-space nursery can significantly reduce the cost of short-lived allocations (often being faster than malloc/free on such a workload), but only if there are no destructors. Secondly, threads typically share resources so running something on a different thread does not magically cause it to not disturb the mutator. Such a claim makes me start to doubt the "real-time" claim in the title...
      - pebal 4y ago
        Disruption is a lower cost than the cost of executing the destructor by the mutator.
ivzhh 4y ago
In early attempts of removing GIL, atomic reference counting slows down performance by 60%. In comparison, tracing garbage collector can defer/coalesce reference counting. You can read more in [2] section 2.1, and this paper introduced a new referencing counting method. [1] Multithreaded Python without the GIL https://docs.google.com/document/d/18CXhDb1ygxg-YXNBJNzfzZsDFosB5e6BfnXLlejd9l0/edit#heading=h.6nk4ql5o8r2q https://docs.google.com/document/d/18CXhDb1ygxg-YXNBJNzfzZsD... [2] Low-Latency, High-Throughput Garbage Collection https://users.cecs.anu.edu.au/~steveb/pubs/papers/lxr-pldi-2022.pdf https://users.cecs.anu.edu.au/~steveb/pubs/papers/lxr-pldi-2...

by jgaa 4y ago

pebal 4y ago

GC can be faster and allocate less memory. tracked_ptr: 279.95ms (151MB) shared_ptr: 1035.07ms (480MB) source: https://godbolt.org/z/jP7ohPehY https://godbolt.org/z/jP7ohPehY

Kranar 4y ago

Makes writing lock free data structures a heck of a lot easier. Entirely eliminates the ABA problem.

pebal 4y ago

This introduces the tracked smart pointer. This pointer has less overhead than shared_ptr and releases circular data structures.

pjmlp 4y ago

Unreal C++ and C++/CLI have one. When one wants the flexibility of C++ with the productivity of a GC.

pkolaczk 4y ago
Whether GC improves productivity is a strong claim that needs a proof. I have yet to see such a proof. So far my experience has been that (nondeterministic) GC is only a productivity improvement over languages with manual memory management like C, but a productivity degradation over languages with deterministic automatic memory management like C++ and Rust. GC manages only memory, while in the latter case, you get automatic management of resources of any type, which can be a huge productivity boost if you use it to your advantage.
- pjmlp 4y ago
  Do you want an example for C++? Lock free data structures are PhD level skillset unless there is some kind of tracing GC algorithm being used. Also RAII is great and all, yet there lots of ways to get it wrong. Badly implemented destructors, missing virtual destructors in hierachys, throwing exceptions on destructurs, not using handle types on heap allocated data, incorrect pairing of allocators.
  - pkolaczk 4y ago
    Lock-free data structures is a 0.001% niche, which can be implemented as libraries by experts and then used without any trouble by an average Joe programmer. An average Joe wouldn't get the implementation of them right even in a GC language anyway. Also, in those niche cases, GC is available as an option. But you should not optimize the language for the niche use case. > Badly implemented destructors, missing virtual destructors in hierachys, throwing exceptions on destructurs, not using handle types on heap allocated data, incorrect pairing of allocators. Those seem to be more specific problems with C++ rather than RAII. Rust has none of those.
    - pjmlp 4y ago
      The ongoing issues with Drop trait stabilization show otherwise. Having a GC is an option alongside the other ones, an option that both Epic and Microsoft think it is worth enough money to spend development resources on. And Google as well, actually, as Chrome uses a C++ GC, seen as way to fix security issues, Olipan.
      - pkolaczk 4y ago
        GC as an opt in, next to to RAII - so you can use it when you need it, that's fine. But it is a productivity issue when the only way to manage memory is non-deterministic GC, e.g. like in Java or Go.
        pjmlp 4y ago

xiphias2 4y ago

One example would be to create higher level languages that compile to / interoperable with C++

plq 4y ago

You use custom memory management where it matters, gc anywhere else.

pebal 4y ago
The intention of creating this project was the opposite. You only use the tracked pointer when it is beneficial.
- plq 4y ago
  Well that'd work too :) The adoption of a particular memory allocation strategy depends on a lot factors. This is C++ after all.

fathyb 4y ago

GCs can be more efficient if you alloc/free memory very often. shared_ptr can be quite slow because of the bookkeeping. It can also help bring more consistent performance, ie. one big malloc instead of millions of small ones.

aidenn0 4y ago
I would think destructors would negate some of the performance advantages of GCs, since it's not possible to have zero-cost freeing of objects when destructors exist.
- pebal 4y ago
  SGCL executes the destructors in a separate thread, so there is no cost to the mutator.
  - aidenn0 4y ago
    > SGCL executes the destructors in a separate thread, so there is no cost to the mutator. Firstly, that's orthogonal to my original point is that, for example, a semi-space nursery can significantly reduce the cost of short-lived allocations (often being faster than malloc/free on such a workload), but only if there are no destructors. Secondly, threads typically share resources so running something on a different thread does not magically cause it to not disturb the mutator. Such a claim makes me start to doubt the "real-time" claim in the title...
    - pebal 4y ago
      Disruption is a lower cost than the cost of executing the destructor by the mutator.

ivzhh 4y ago

In early attempts of removing GIL, atomic reference counting slows down performance by 60%. In comparison, tracing garbage collector can defer/coalesce reference counting. You can read more in [2] section 2.1, and this paper introduced a new referencing counting method. [1] Multithreaded Python without the GIL https://docs.google.com/document/d/18CXhDb1ygxg-YXNBJNzfzZsDFosB5e6BfnXLlejd9l0/edit#heading=h.6nk4ql5o8r2q https://docs.google.com/document/d/18CXhDb1ygxg-YXNBJNzfzZsD... [2] Low-Latency, High-Throughput Garbage Collection https://users.cecs.anu.edu.au/~steveb/pubs/papers/lxr-pldi-2022.pdf https://users.cecs.anu.edu.au/~steveb/pubs/papers/lxr-pldi-2...