Originally published at deepu.tech.
In this multi-part series, I aim to demystify the concepts behind memory management and take a deeper look at memory management in some of the modern programming languages. I hope the series would give you some insights into what is happening under the hood of these languages in terms of memory management. Learning about memory management will also help us to write more performant code as the way we write code also has an impact on memory management regardless of the automatic memory management technique used by the language.
Memory management is the process of controlling and coordinating the way a software application access computer memory. It is a serious topic in software engineering and its a topic that confuses some people and is a black box for some.
When a software runs on a target Operating system on a computer it needs access to the computers RAM(Random-access memory) to:
- load its own bytecode that needs to be executed
- store the data values and data structures used by the program that is executed
- load any run-time systems that are required for the program to execute
When a software program uses memory there are two regions of memory they use, apart from the space used to load the bytecode, Stack and Heap memory.
The stack is used for static memory allocation and as the name suggests it is a last in first out(LIFO) stack (Think of it as a stack of boxes).
- Due to this nature, the process of storing and retrieving data from the stack is very fast as there is no lookup required, you just store and retrieve data from the topmost block on it.
- But this means any data that is stored on the stack has to be finite and static(The size of the data is known at compile-time).
- This is where the execution data of the functions are stored as stack frames(So, this is the actual execution stack). Each frame is a block of space where the data required for that function is stored. For example, every time a function declares a new variable, it is "pushed" onto the topmost block in the stack. Then every time a function exits, the topmost block is cleared, thus all of the variables pushed onto the stack by that function, are cleared. These can be determined at compile time due to the static nature of the data stored here.
- Multi-threaded applications can have a stack per thread.
- Memory management of the stack is simple and straightforward and is done by the OS.
- Typical data that are stored on stack are local variables(value types or primitives, primitive constants), pointers and function frames.
- This is where you would encounter stack overflow errors as the size of the stack is limited compared to the Heap.
- There is a limit on the size of value that can be stored on the Stack for most languages.
Heap is used for dynamic memory allocation and unlike stack, the program needs to look up the data in heap using pointers (Think of it as a big multi-level library).
- It is slower than stack as the process of looking up data is more involved but it can store more data than the stack.
- This means data with dynamic size can be stored here.
- Heap is shared among threads of an application.
- Due to its dynamic nature heap is trickier to manage and this is where most of the memory management issues arise from and this is where the automatic memory management solutions from the language kick in.
- Typical data that are stored on the heap are global variables, reference types like objects, strings, maps, and other complex data structures.
- This is where you would encounter out of memory errors if your application tries to use more memory than the allocated heap(Though there are many other factors at play here like GC, compacting).
- Generally, there is no limit on the size of the value that can be stored on the heap. Of course, there is the upper limit of how much memory is allocated to the application.
Unlike Hard disk drives, RAM is not infinite. If a program keeps on consuming memory without freeing it, ultimately it will run out of memory and crash itself or even worse crash the operating system. Hence software programs can't just keep using RAM as they like as it will cause other programs and processes to run out of memory. So instead of letting the software developer figure this out, most programming languages provide ways to do automatic memory management. And when we talk about memory management we are mostly talking about managing the Heap memory.
Since modern programming languages don't want to burden(more like trust 👅) the end developer to manage the memory of his/her application most of them have devised a way to do automatic memory management. Some older languages still require manual memory handling but many do provide neat ways to do that. Some languages use multiple approaches to memory management and some even let the developer choose what is best for him/her(C++ is a good example). The approaches can be categorized as below
The language doesn't manage memory for you by default, it's up to you to allocate and free memory for the objects you create. For example, C and C++. They provide the
free methods to manage memory and it's up to the developer to allocate and free heap memory in the program and make use of pointers efficiently to manage memory. Let's just say that it's not for everyone 😉.
- Reference counting GC: In this approach, every object gets a reference count which is incremented or decremented as references to it change and garbage collection is done when the count becomes zero. It's not very preferred as it cannot handle cyclic references. PHP, Perl, and Python, for example, uses this type of GC with workarounds to overcome cyclic references. This type of GC can be enabled for C++ as well.
In this type of memory management, an object's memory allocation is tied to its lifetime, which is from construction until destruction. It was introduced in C++ and is also used by Ada and Rust.
It's similar to Reference counting GC but instead of running a runtime process at a specific interval the
release instructions are inserted to the compiled code at compile-time and when an object reference becomes zero its cleared automatically as part of execution without any program pause. It also cannot handle cyclic references and relies on the developer to handle that by using certain keywords. Its a feature of the Clang compiler and provides ARC for Objective C & Swift.
It combines RAII with an ownership model, any value must have a variable as its owner(and only one owner at a time) when the owner goes out of scope the value will be dropped freeing the memory regardless of it being in stack or heap memory. It is kind of like Compile-time reference counting. It is used by Rust, in my research I couldn't find any other language using this exact mechanism.
We have just scratched the surface of memory management. Each programming language uses its own version of these and employs different algorithms tuned for different goals. In the next parts of the series, we will take a closer look at the exact memory management solution in some of the popular languages.
Stay tuned for upcoming parts of this series:
- Part 2: Memory management in JVM(Java, Kotlin, Scala, Groovy)
- Part 4: Memory management in Go
- Part 5: Memory management in Rust
- Part 6: Memory management in Python
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Stack visualization: Created based on pythontutor.
Ownership illustration: Link Clark, The Rust team under Creative Commons Attribution Share-Alike License v3.0.