2019년 4월 14일 일요일

Do not use garbage collection to catch memory leak

Garbage collection is a technique that automatically releases unnecessary memory. It's very famous because many programming languages adopted garbage collection after John McCarthy implemented it in Lisp. However, there are a few people who misunderstand what garbage collection does. If you think garbage collection prevents a memory leak, unfortunately, you are one of them.

Garbage collection cannot prevent a memory leak. There is no way to avoid all memory leaks if you are using Turing-complete language. To understand it you should know what a memory leak is. Wikipedia describes a memory leak as the following:

a type of resource leak that occurs when a computer program incorrectly manages memory allocations in such a way that memory which is no longer needed is not released.

Briefly, a memory leak is a bug that doesn't release a memory that you don't use. So it is first to find the memory which will not be used in order to detect memory leaks. Unfortunately, it is impossible. I'll explain the reason with the code below. When should x be freed?


Generally, x should be released after use(x). However, what if some_function does not end? If some_function never returns because there is an infinite loop, use(x) will never be called. In this case, x has no future access. Thus keeping this memory while running some_function is a memory leak. If you want to make this function have no memory leak, you need to determine when to release x before executing some_function. It's impossible. It's the halting problem. There is no way to static analyze whether some_function runs forever or not.

It's the reason that there is no way to find all memory leaks. So all automatic memory management schemes, including garbage collection, don't guarantee to catch all memory leaks. They try to release memory which applications will not use. They release only memory that can not be accessed apparently, rather than freeing all unnecessary memory. Formally speaking, all automatic memory management schemes use a sound algorithm in the question "Is it safe to free this memory?" even though the algorithm is not complete.

Releasing only memory which is safe to free is the goal of using garbage collection. Your program doesn't have a dangling pointer if you use it instead of managing memory manually. It means you are free from use-after-free bugs or double-free bugs. In conclusion, garbage collection is not for memory efficiency, but memory safety.

If you are not familiar with memory management, you can reduce the number of memory leaks by using garbage collection. You should not choose garbage-collected languages to catch memory leaks. You should not think your program doesn't have a memory leak because you used garbage-collected languages. As I said before, there is no method to catch all memory leaks. You should diagnose the source code manually to find them. You should use automatic memory management schemes, including garbage collection, as a tool to enhance safety.


This article is a translation of the article written in Korean. Please see this link to see the original post.

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