Anything that can't be reached is considered dead. In tracing the GC starts from the "roots" - things like stack references, and traces all reachable (live) objects. There are two broad approaches for tracking down dead objects: tracing and reference counting. ![]() On machines with lots of RAM and for processes that use memory only slowly, you might not want to be doing GC at all! Reference counting would be a bit of a drawback there in terms of performance.Īctually reference counting and the strategies used by the Sun JVM are all different types of garbage collection algorithms. Garbage Collection does not need to do this.Īlso, Garbage Collection can be done in a separate thread (though it can be a bit tricky). Each time an object is referenced and dereferenced, the interpreter/VM must check to see if the count has gone down to 0 (and then deallocate if it did). CPython (reference counting is not part of python itself, but part of the C implementation thereof) catches circular references with a separate garbage collection routine that it runs periodically.Īnother drawback: Reference counting can make execution slower. If A were to drop its reference to B, both B and C will still have a reference count of 1 and won't be deleted with traditional reference counting. One of the most mentioned is circular references: Suppose A references B, B references C and C references B. If (MemorySampler.There are drawbacks of using reference counting. For example, CoralReactor, which is a high-performance asynchronous network library, has the following code at the top of its critical selector (i.e. You can plug the MemorySampler in that loop to embody your whole application. Real-time applications usually have a critical loop that is executed non-stop by a high-priority thread. It might sound more complicated than it really is as it will often be straightforward to verify that as you iterate doing the same thing, the same object is allocated over and over again indicating a garbage leak, as it was the case with the. If it is allocating memory linearly as the number of iterations increases, it is most likely creating garbage and you should use the stack trace provided by MemorySampler to investigate it further. The key to make sure your system is not creating any garbage is to warm up your critical path from start to finish a couple of million times and then check for memory allocation another couple of million times. The total memory allocated is incrementing linearly with iterations, in other words, the memory allocated per pass is always 32 and the total memory allocated increases from 144 to 3280:Īt this point it is clear that is not pooling its objects, it is creating garbage and it is just a matter of enough iterations before it triggers the GC. For example the code below:įor(int i = 0 i ignore the first pass (init)Ĭom.test2(Basics.java:104)Ĭom.main(Basics.java:116) Of course the fact that the code is allocating memory does not necessarily mean it is creating garbage as references can be pooled for re-use.
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