C++ Programming/Compiler/Linker/Libraries/Garbage Collection

Garbage collection

Garbage collection is a form of automatic memory management. The garbage collector attempts to reclaim garbage, or memory used by objects that will never be accessed or mutated again by the application.

Tracing garbage collectors require some implicit runtime overhead that may be beyond the control of the programmer, and can sometimes lead to performance problems. For example, commonly used Stop-The-World garbage collectors, which pause program execution at arbitrary times, may make garbage collecting languages inappropriate for some embedded systems, high-performance server software, and applications with real-time needs.

A more fundamental issue is that garbage collectors violate locality of reference, since they deliberately go out of their way to find bits of memory that haven't been accessed recently. The performance of modern computer architectures is increasingly tied to caching, which depends on the assumption of locality of reference for its effectiveness. Some garbage collection methods result in better locality of reference than others. Generational garbage collection is relatively cache-friendly, and copying collectors automatically defragment memory helping to keep related data together. Nonetheless, poorly timed garbage collection cycles could have a severe performance impact on some computations, and for this reason many runtime systems provide mechanisms that allow the program to temporarily suspend, delay or activate garbage collection cycles.

Despite these issues, for many practical purposes, allocation/deallocation-intensive algorithms implemented in modern garbage collected languages can actually be faster than their equivalents using explicit memory management (at least without heroic optimizations by an expert programmer). A major reason for this is that the garbage collector allows the runtime system to amortize allocation and deallocation operations in a potentially advantageous fashion. For example, consider the following program in C++:

#include <iostream>
 
class A {
  int x;
public:
  A() { x = 0; ++x; }
};
 
int main() {
 for (int i = 0; i < 1000000000; ++i) {
   A *a = new A();
   delete a;
 }
 std::cout << "DING!" << std::endl;
}

One of more widely used libraries that provides this function is Hans Boehm's conservative GC. Oilpan is a garbage collector for C++ that was originally designed for Blink. As we have seen earlier C++ also supports a powerful idiom called RAII (resource acquisition is initialization) that can be used to safely and automatically manage resources including memory.