Memory Management

Memory management is where the physical memory is allocated and deallocated during program execution. Controlled by the OS where the program is running and the programming language used to code the program.

Activities in memory management:

• Memory allocation
• Memory deallocation
• Use of pointers for memory access
• Ensuring memory safety

Memory allocation

When a program needs to store data, memory is requested from the OS by the program. The memory can be allocated statically at compile time or dynamically at run time. Allocated implicitly or explicitly depending on the programming language.

Examples:

• In C, memory is allocated implicitly (automatically) or explicitly (using malloc() system call)
• In Java, memory is allocated implicitly (when defining primitive typed variables) explicitly (using new operator)
• In Python, memory is allocated implicitly

Memmory deallocation

When memory is no longer required by the program, it needs to be deallocated and freed to the OS. If not, memory leaks are resulted. Memory leaks hog up memory and ultimately lead to crashes. Can happen either implicitly or explicitly depending on the programming language.

Examples:

• In C, memory is deallocated implicitly (when the current scope ends) explicitly (using free() system call)
• In Java, memory is deallocated implicitly

Note

In high level programming languages such as Java, Python, Go and JavaScript, memory management is implicitly handled by the runtime. Memory is allocated implicitly using a operator and deallocation is handled by a process called garbage collection.

Garbage

Garbage, in this context, means unreachable memory or memory that is only reachable from other unreachable memory.

Garbage Collection

A process where free unused memory is detected using various algorithms and freed. Handled by a garbage collector. Garbage collectors are preferred because otherwise, the deallocation must be done manually by programmers which is very error-prone and time-consuming.

Terminal programs

GC is not required. When standard output is piped to another program, the output is written to persistent storage.

A process, run by a normal user, cannot access the memory buffer of another process. It would require root permissions to do so, and it is not recommended.

Because of that, garbage collector is run on the same process, in a separate thread.

Many techniques are used to determine which memory must be garbage collected.

Reference counting

A table of objects and their reference count is kept by the GC. An object is marked as garbage when the reference count decrements to 0. In this technique, the GC only has to keep track of a single column. Good for object-oriented programming.

Tracing

A set of memory roots is used. Recursively follows all the pointers to other objects in memory. An object is marked as garbage when it is not reached in the tracing process.