How to Manage Memory

Memory is a precious resource that you can't afford to run out of. You can ignore it for a while but eventually you will have to decide how to manage memory.

Space that needs to persist beyond the scope of a single subroutine is often called heap allocated. A chunk of memory is useless, hence garbage, when nothing refers to it. Depending on the system you use, you may have to explicitly deallocate memory yourself when it is about to become garbage. More often you may be able to use a system that provides a garbage collector. A garbage collector notices garbage and frees its space without any action required by the programmer. Garbage collection is wonderful: it lessens errors and increases code brevity and concision cheaply. Use it when you can.

But even with garbage collection, you can fill up all memory with garbage. A classic mistake is to use a hash table as a cache and forget to remove the references in the hash table. Since the reference remains, the referent is non-collectable but useless. This is called a memory leak. You should look for and fix memory leaks early. If you have long running systems memory may never be exhausted in testing but will be exhausted by the user.

The creation of new objects is moderately expensive on any system. Memory allocated directly in the local variables of a subroutine, however, is usually cheap because the policy for freeing it can be very simple. You should avoid unnecessary object creation.

An important case occurs when you can define an upper bound on the number of objects you will need at one time. If these objects all take up the same amount of memory, you may be able to allocate a single block of memory, or a buffer, to hold them all. The objects you need can be allocated and released inside this buffer in a set rotation pattern, so it is sometimes called a ring buffer. This is usually faster than heap allocation.

Sometimes you have to explicitly free allocated space so it can be reallocated rather than rely on garbage collection. Then you must apply careful intelligence to each chunk of allocated memory and design a way for it to be deallocated at the appropriate time. The method may differ for each kind of object you create. You must make sure that every execution of a memory allocating operation is matched by a memory deallocating operation eventually. This is so difficult that programmers often simply implement a rudimentary form of garbage collection, such as reference counting, to do this for them.

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