4 ms·

That might seem counter-intuitive, but tracing GC is the fastest way to manage memory in general (arbitrary life time, graph shapes and allocation patterns). I

That might seem counter-intuitive, but tracing GC is the fastest way to manage memory in general (arbitrary life time, graph shapes and allocation patterns). It permits really fast allocation, defragmentation and is sometime the only way to manage memory (cyclic datastructures for which reachability cannot be known directly from program text). Reference-counted GC on the other hand is notoriously slow, incomplete, and exhibit lots of bad performance patterns (that can be somehow managed by taking some elements of tracing GCs). The drawback is that tracing GCs are very hard to implement, to tune, and sometimes not all of the benefits are available at the same time. And because they are so counter intuitive, programmers in general have a hard time understanding their runtime behavior... Even GC experts struggle (the skills required range from low-level hardware to control theory, otherwise performance at scale is unpredictable).

Rochus 6y ago
In C/C++ we have the possibility to avoid dynamic allocation altogether and to use system features like memory mapping. If we use C++ the same way as Java (everything dynamically) it's not too surprising the result is not (much) faster than Java.
- aidenn0 6y ago
  Their original implementation was in common lisp. Common Lisp also lets you use mmap (in fact it's not that uncommon to do so if you have a large amount of mostly static data) to manage your memory manually. They clearly wanted automatic memory management, so the C++ implementation is reasonable. A fairer comparison might have used MPS or boehm instead of refcounting, but I suspect the results would have been similar.
  - Rochus 6y ago
    > Common Lisp also lets you use mmap But not without allocating dynamic memory and copying data. > They clearly wanted automatic memory management Most likely because of some misconceptions. > so the C++ implementation is reasonable. How so? > but I suspect the results would have been similar Don't forget the data sets to be filtered, sorted an analyzed are up to 200 GB.
    - deleted 6y ago
      [deleted]
    - aidenn0 6y ago
      > > Common Lisp also lets you use mmap > But not without allocating dynamic memory and copying data. Sure it does. In SBCL you can force a stack allocation (though rarely does it improve performance), and very short-lived values do not leave registers in any case. > > They clearly wanted automatic memory management > Most likely because of some misconceptions. There are both good and bad reasons to want automatic memory management. At least one good reason it it would decrease the porting effort by keeping the code similar. > > so the C++ implementation is reasonable. > How so? Using reference counting is a reasonable way to get automatic memory management in C++ > > but I suspect the results would have been similar > Don't forget the data sets to be filtered, sorted an analyzed are up to 200 GB. Which is going to be rough on any automatic memory management system, which makes using a language with a better ecosystem of automatic memory management more performant.
      - Rochus 6y ago
        Common Lisp: It is permissible for an implementation to simply ignore such declarations. And you still have to copy. Ref counting: only makes sense in a few special cases. Avoiding dynamic memory management: have a look at mmap.
        aidenn0 6y ago
        C++: It is permissible for an implementation to allocate every local variable on the heap. Going back to my original point, you are suggesting a complete rearchitecture of their allocation system. That does not require switching languages to C++. If we are talking about working with 100s of GB of data, that's probably even the correct approach! TFA does not, however, claim that they have a working set of 100s of GB of data. The data is 100s of GB at rest, but can be processed in chunks with a single pass. That, by itself, does not scream "mmap" to me. On top of that, the data is compressed at rest, so copying is inevitable.
  - nemetroid 6y ago
    > the C++ implementation is reasonable Fig. 4 shows that deallocation, presumably of objects allocated during the first phase, takes half as long as the first phase itself. Unless the nature of the problem solved by the program requires you to have a ton of objects with shared mutability (the use case for shared_ptr), and requires you to make a lot of allocations up-front without knowing if you will need them, the memory thrashing occurring does not seem reasonable.

Rochus 6y ago

In C/C++ we have the possibility to avoid dynamic allocation altogether and to use system features like memory mapping. If we use C++ the same way as Java (everything dynamically) it's not too surprising the result is not (much) faster than Java.

aidenn0 6y ago
Their original implementation was in common lisp. Common Lisp also lets you use mmap (in fact it's not that uncommon to do so if you have a large amount of mostly static data) to manage your memory manually. They clearly wanted automatic memory management, so the C++ implementation is reasonable. A fairer comparison might have used MPS or boehm instead of refcounting, but I suspect the results would have been similar.
- Rochus 6y ago
  > Common Lisp also lets you use mmap But not without allocating dynamic memory and copying data. > They clearly wanted automatic memory management Most likely because of some misconceptions. > so the C++ implementation is reasonable. How so? > but I suspect the results would have been similar Don't forget the data sets to be filtered, sorted an analyzed are up to 200 GB.
  - deleted 6y ago
    [deleted]
  - aidenn0 6y ago
    > > Common Lisp also lets you use mmap > But not without allocating dynamic memory and copying data. Sure it does. In SBCL you can force a stack allocation (though rarely does it improve performance), and very short-lived values do not leave registers in any case. > > They clearly wanted automatic memory management > Most likely because of some misconceptions. There are both good and bad reasons to want automatic memory management. At least one good reason it it would decrease the porting effort by keeping the code similar. > > so the C++ implementation is reasonable. > How so? Using reference counting is a reasonable way to get automatic memory management in C++ > > but I suspect the results would have been similar > Don't forget the data sets to be filtered, sorted an analyzed are up to 200 GB. Which is going to be rough on any automatic memory management system, which makes using a language with a better ecosystem of automatic memory management more performant.
    - Rochus 6y ago
      Common Lisp: It is permissible for an implementation to simply ignore such declarations. And you still have to copy. Ref counting: only makes sense in a few special cases. Avoiding dynamic memory management: have a look at mmap.
      - aidenn0 6y ago
        C++: It is permissible for an implementation to allocate every local variable on the heap. Going back to my original point, you are suggesting a complete rearchitecture of their allocation system. That does not require switching languages to C++. If we are talking about working with 100s of GB of data, that's probably even the correct approach! TFA does not, however, claim that they have a working set of 100s of GB of data. The data is 100s of GB at rest, but can be processed in chunks with a single pass. That, by itself, does not scream "mmap" to me. On top of that, the data is compressed at rest, so copying is inevitable.
- nemetroid 6y ago
  > the C++ implementation is reasonable Fig. 4 shows that deallocation, presumably of objects allocated during the first phase, takes half as long as the first phase itself. Unless the nature of the problem solved by the program requires you to have a ton of objects with shared mutability (the use case for shared_ptr), and requires you to make a lot of allocations up-front without knowing if you will need them, the memory thrashing occurring does not seem reasonable.