Understanding Go’s Garbage Collection

20 min readJun 14, 2023

Updated 20 October 24

As we delve into the world of programming languages and their internal mechanisms, one aspect that emerges as crucial, yet often overlooked, is memory management. It forms the foundation upon which applications run and perform. The chosen strategy for memory management can profoundly influence the efficiency, speed, and responsiveness of a software system. Among various memory management techniques, garbage collection has been adopted by several modern programming languages, including Go, due to its automation of memory allocation and deallocation.

Garbage collection is a form of automatic memory management that aims to reclaim memory occupied by objects that are no longer in use by the program. It’s the silver bullet that resolves the infamous problem of manual memory management, where negligence could lead to critical issues like memory leaks or dangling pointers. Yet, garbage collection isn’t without its trade-offs. Depending on the algorithm implemented, it can introduce overhead that might affect application performance.

The primary alternatives to garbage collection are manual and reference counting memory management. Manual memory management gives programmers full control over the allocation and deallocation of memory, which can lead to more efficient use of memory, but also opens the door for potential errors. Reference counting, on the other hand, deallocates an object’s memory as soon as there are no more references to it, but it can be computationally expensive and struggle with circular references.

In Go, memory management is streamlined by its concurrent, tri-color, mark-sweep garbage collector (GC). This garbage collector is a core part of the language runtime, designed to efficiently handle memory deallocation while providing considerable speed and performance.

Understanding Go’s GC is more than an academic exercise — it can have a significant impact on the performance of your programs. Without a solid grasp of its workings, developers might produce code leading to unnecessary allocations, bloated memory usage, and suboptimal performance. However, by mastering the principles of Go’s garbage collection, you can leverage its power to write more efficient, performant Go code.

In this detailed examination, we’ll shed light on the internal mechanics of Go’s garbage collection, elucidate the algorithms it employs, its pacing strategy, and the handling of stack frames. We’ll also dive into the world of finalizers and explore how you can fine-tune the garbage collection to your specific needs.

Whether you’re a seasoned Go developer or a newcomer eager to learn, by the end of this article, you’ll possess a deep and robust understanding of Go’s garbage collection. You’ll grasp its influence on your Go programs and how it assists in producing high-performance applications. So, without further ado, let’s dive in and unearth the mysteries of garbage collection in Go.

Understanding Memory Management in Go

When a program is executed, it needs to store data and instructions to execute. For this purpose, it uses computer memory. To maximize efficiency and prevent potential conflicts, memory management comes into play, systematically allocating and deallocating specific blocks of memory according to the program’s needs.

How Go Manages Memory

Memory management in Go is a two-pronged approach involving both the stack and the heap. Both structures provide different services, depending on the nature and lifecycle of the data that needs to be stored.

The Heap and the Stack

The stack is a LIFO (last in, first out) data structure that stores local variables and function calls. Each time a function is invoked, a new stack frame is allocated with all the function’s local variables. When the function finishes executing, its stack frame is deallocated, freeing up the memory for subsequent use. The stack is fast and provides automatic memory management at the expense of size limitations and local scope.

The heap, on the other hand, is a region of memory used for dynamic memory allocation. Unlike the stack, the heap has no inherent organization or order, and blocks of memory can be allocated and deallocated at any time. This flexibility comes at the cost of manual memory management and slower access times. In Go, memory allocation on the heap is used for data that needs to outlive the scope of the function making the allocation.

Memory Allocation in Go

In Go, the size of the heap is managed by the garbage collector (GC). When memory is allocated, and there’s not enough space in the heap, Go’s runtime will increase the size of the heap. The GC later frees up heap memory by identifying and discarding objects that are no longer accessible by the program.

As for the stack, Go uses dynamically sized contiguous stacks. Unlike some languages where the stack size must be defined at thread creation, Go starts each goroutine with a small stack, usually around 2KB, which can dynamically grow and shrink as needed. When a goroutine requires more stack space, the runtime allocates a larger contiguous stack and copies over the existing data.

Go’s memory allocation, both on the heap and stack, is efficient and gives Go an edge in building high-performance applications. Understanding these basics paves the way for the detailed exploration of Go’s garbage collector, which we will dive into next.

Go’s Concurrent, Tri-color, Mark-Sweep Garbage Collector

Garbage collection in Go adopts a concurrent, tri-color, mark-sweep approach. This design allows Go’s GC to be non-disruptive to the application’s performance while ensuring efficient memory management. To understand it better, let’s break down each term.

1. Concurrent

The term “concurrent” in Go’s GC signifies that the garbage collection process doesn’t stop the execution of the application. Traditional garbage collectors often implement a “stop-the-world” phase, during which all program execution halts to allow the garbage collector to examine and reclaim memory. However, such an approach can lead to noticeable pauses in application performance, which is detrimental for real-time or high-throughput systems.

Go’s GC, on the other hand, is designed to work concurrently with the program. Most of the garbage collector’s work is done in the background, alongside the application’s execution. This results in shorter stop-the-world pauses, improving the overall latency profile of Go applications.

2. Tri-color

The “tri-color” term refers to the marking algorithm used by Go’s GC, which considers objects (or blocks of memory) in three different states — white, grey, and black.

White objects are those that the garbage collector has not processed. They may or may not be reachable from the roots (the set of objects directly accessible by the program, like global variables or currently executing function’s local variables).
Grey objects are those that the garbage collector has discovered to be reachable from the roots, but their descendants (objects they reference) haven’t been processed yet.
Black objects are those that the garbage collector has processed entirely — both the object and its descendants have been discovered and found to be reachable.

Initially, all objects are white. The garbage collector starts at the roots and colors them grey. It then proceeds to process each grey object, scanning it for references to other objects. If a referenced object is white, the garbage collector turns it grey. After processing an object, the garbage collector colors it black.

This tri-color algorithm ensures a clear segregation of objects based on their reachability status, assisting the garbage collector in identifying and reclaiming unreachable memory efficiently.

3. Mark-Sweep

The “mark-sweep” term describes the two-phase approach to memory reclamation used by Go’s GC:

Mark phase — During this phase, the garbage collector traverses the object graph, starting from the roots. As described above, it uses the tri-color marking algorithm to discover all reachable objects. The mark phase operates concurrently with the program, interleaving marking work with the execution of Goroutines.
Sweep phase — Once all reachable objects are marked (black), the sweep phase begins. During this phase, the garbage collector reclaims the memory occupied by white (unreachable) objects. This phase also happens concurrently with the execution of Goroutines, cleaning up a bit of memory at a time.

The mark-sweep approach offers a good balance between performance and memory usage, making it a popular choice for garbage collectors, including the one in Go.

Garbage Collection Algorithm

Go’s garbage collector utilizes a tri-color mark-sweep algorithm, a variant of a broader class of tracing garbage collectors. At the core of this process lies the concept of ‘marking’ and ‘sweeping’.

Tri-color Mark and Sweep Algorithm

In the context of garbage collection, ‘marking’ involves traversing the object graph, starting from the root, and marking every object that can be reached. This action tags all objects in active use. The ‘sweeping’ phase then goes through all the objects and reclaims the memory occupied by unmarked objects, which are unreachable and therefore no longer needed.

The tri-color marking model visualizes the object graph using three colors: white, grey, and black. At the start of a garbage collection cycle, all objects are marked white, indicating they are candidates for memory reclamation. The root objects are then colored grey, denoting that they are active but their descendants (objects they reference) have not been marked yet. The garbage collector then successively processes grey objects, marking their descendants grey and turning themselves black. This process continues until there are no more grey objects, at which point all live (reachable) objects will be black, and all white objects can be considered garbage and swept away.

Write Barriers

An integral part of the tri-color mark-sweep algorithm is the concept of a ‘write barrier’. This is a mechanism that makes sure the properties of the tri-color abstraction are maintained while the algorithm is in progress. When a pointer that references a white (not yet processed) object is written to a black (already processed) object, the garbage collector ensures the white object is marked as grey (preventing it from being prematurely collected).

This mechanism is crucial for allowing the garbage collector to run concurrently with the program. Without it, there might be a race condition where the garbage collector might end up sweeping an object that has just been referenced by the program, leading to a catastrophic failure.

Understanding the basics of Go’s garbage collection algorithm is key to grasping how Go manages memory efficiently. The garbage collector’s primary aim is to minimize the impact on the program’s execution, which leads us to the next topic — Garbage Collection Pacing.

Garbage Collection Pacing

To understand Go’s approach to garbage collection, we need to delve into the specifics of how it decides when to initiate a garbage collection cycle and its strategies for minimizing the dreaded GC pause time. This decision-making process, known as garbage collection pacing, involves a delicate balance of various factors.

Heap Growth Ratio and the Role of GOGC

A fundamental mechanism for triggering a garbage collection cycle in Go involves monitoring the heap’s growth over time. When the heap size grows beyond a certain ratio compared to the size at the end of the previous GC cycle, a new cycle gets initiated. This ratio is adjustable through the GOGC environment variable.

By default, GOGC is set to 100, meaning that when the heap size becomes double the size it was at the end of the previous cycle, a new GC cycle gets triggered. If the GOGC value is 200, the heap is allowed to grow to three times its previous end size before a GC cycle begins. Conversely, a GOGC value of 50 would trigger a new GC cycle when the heap is 1.5 times its previous end size. As such, you can manipulate GOGC to control the frequency of GC cycles, trading off between CPU usage and memory usage.

The Art of Minimizing GC Pause Time

One of Go’s garbage collector’s main goals is minimizing the ‘stop-the-world’ pauses — moments when the execution of goroutines halts to let the garbage collector run. While a complete avoidance of pauses is impossible, the designers of Go’s GC have made significant strides in reducing their impact on execution flow.

To keep pauses to a minimum, most of the garbage collection work is performed concurrently with the program’s execution. The work of the garbage collector is divided into four phases: setup, mark, mark termination, and sweep. Out of these, only the mark termination phase requires stopping the execution of the program.

The setup phase is a very short phase that prepares for the mark phase. The mark phase, which can run concurrently with the program, involves tracing through the heap to identify reachable objects, starting from the root set.

Mark termination, the phase that requires a pause, serves to complete the marking process. It ensures that all goroutines are at a GC-safe point, known as a GC safepoint, stops them, and then drains any remaining grey objects in the worklist and scans the stacks and globals again to ensure no reachable objects were missed.

Finally, the sweep phase, which can also run concurrently, reclaims the memory consumed by unreachable objects, making it available for future allocations.

By splitting the process into these phases and allowing much of the work to occur concurrently with the execution of the program, Go’s garbage collector effectively minimizes pause times, leading to smoother and more predictable performance.

It’s important to note that Go’s GC is both concurrent and parallel. The garbage collector performs its work concurrently with the running program and parallelizes GC tasks across multiple processor cores. This means that within a single GC cycle, multiple worker threads can participate in the garbage collection process simultaneously, enhancing efficiency and reducing pause times.

With this understanding of the GC pacing, it’s clear that Go’s garbage collector follows a well-thought-out strategy to ensure efficient memory management. However, memory management in Go is not just about the heap; stack frames play a crucial role as well.

How Go Handles Stack Frames

To understand the role of stack frames in Go’s memory management, we first need to grasp what stack frames are. In essence, a stack frame is a portion of the call stack, a region of memory where a program stores the state of a function. Each time a function is invoked, a new stack frame is allocated. This frame holds local variables, return addresses, and other function-specific data. Once the function completes execution, the stack frame is deallocated, effectively freeing up that portion of memory.

Go’s handling of stack frames is unique and plays a significant part in the language’s memory efficiency, particularly in terms of garbage collection. Let’s delve deeper into the specifics.

Dynamic Stack Size

Unlike many programming languages that allocate a fixed-size stack for each thread or process, Go uses a dynamic stack size. This means the Go runtime allocates a small stack for each goroutine — initially just a few kilobytes — and the stack can grow or shrink as required. This dynamic approach results in significant memory savings, especially considering Go’s emphasis on lightweight goroutines, which are much more numerous than typical OS threads.

The dynamic nature of the stack size is managed through a mechanism known as stack resizing. When a function call finds the stack too small to fit a new frame, Go’s runtime intervenes. It allocates a larger stack, copies the current stack’s contents to the new one, and then updates the relevant pointers. The old stack is left to be reclaimed by the garbage collector.

Stack Shrinking and its Impact on Garbage Collection

Go’s runtime also periodically shrinks stacks that have grown larger than necessary. This process usually happens at the end of a garbage collection cycle. The runtime scans the stacks for all goroutines, and if a large portion of a stack is found unused, the runtime reduces its size.

The mechanism of stack shrinking has implications for garbage collection. Stack shrinking contributes to the efficient use of memory in Go applications by reducing the overall stack space consumed by goroutines. When a stack shrinks, unused stack space is released back to the heap, reducing the application’s memory footprint. This process does not directly affect the number of reachable objects for the garbage collector but helps in overall memory management efficiency.

However, the process of shrinking and growing stacks incurs overhead, as it involves copying data and updating pointers. The Go runtime strikes a balance by not shrinking a stack immediately when space is freed but waiting until a significant amount of stack space is unused.

With an understanding of how Go’s runtime dynamically manages stack sizes, and the implications for garbage collection, it becomes apparent that Go’s memory management goes beyond just garbage collection. Every aspect of the runtime, including stack frame management, is designed with efficiency and performance in mind. Up next, we’ll examine an essential feature related to garbage collection: finalizers.

Finalizers in Go

Finalizers in Go provide a mechanism to execute cleanup actions or finalizing operations before the garbage collector reclaims an object. Typically, they are used to free non-memory resources like file descriptors, network connections, or database handles that the Go garbage collector cannot reclaim.

In Go, finalizers are associated with a specific object and are invoked when the garbage collector sees that the object is unreachable, meaning there are no more references to this object in the program. However, a significant aspect to note is that Go does not guarantee that a finalizer will run if a program does not terminate cleanly, such as in the event of an unexpected shutdown or when os.Exit is called. Therefore, it's recommended to use finalizers only for cleanup actions where failing to execute isn't critical or can be tolerated.

The runtime package in Go provides a function runtime.SetFinalizer which allows you to set finalizers for objects. The function signature is as follows:

func SetFinalizer(obj, finalizer interface{})

The obj parameter is the object you want to attach the finalizer to, and finalizer is the function you want to be executed when the object obj is about to be garbage collected.

Here’s an example of how to set a finalizer for an object:

type File struct {
    fd      int    // file descriptor
    name    string // file name
}

func NewFile(fd int, name string) *File {
    if fd < 0 {
        return nil
    }
    f := File{fd, name}
    runtime.SetFinalizer(&f, func(f *File) {
        fmt.Printf("File %s successfully finalized, closing file descriptor...\n", f.name)
    })
    return &f
}

In the example above, a finalizer is set for each new File object created by the NewFile function. The finalizer is an anonymous function that prints a message stating that the file has been finalized.

While finalizers can be a useful tool, they add an additional layer of complexity to garbage collection and memory management. Due to the non-deterministic nature of garbage collection and finalizer execution, Go encourages the use of deterministic resource management, such as Close methods, instead of relying heavily on finalizers for resource cleanup.

Tuning Garbage Collection in Go

The Go runtime attempts to manage the garbage collector (GC) for most applications efficiently, but there may be situations where specific needs require tuning GC behavior. These needs may arise from unusual workload patterns, the desire for lower latency, or constraints on memory consumption.

The primary tuning mechanism in Go is the GOGC environment variable. GOGC determines the garbage collector's aggressiveness. The value of GOGC is a percentage that controls the amount of additional heap memory allocated relative to the live heap size. If GOGC=100 (the default), Go runtime will trigger a GC cycle when the heap size is twice the size of the retained heap since the last collection. A lower GOGC value makes the GC run more frequently, thus reducing the program's memory footprint, but potentially at the cost of CPU time. Conversely, a higher GOGC value makes the GC run less frequently, potentially improving CPU performance but increasing memory use.

Let’s look at some examples:

GOGC=off: This completely disables the garbage collector. It's not generally recommended but might be useful in some short-lived utilities or tests.
GOGC=50: This means the garbage collector will trigger a GC cycle when the heap size is 50% more than the live heap size after the last collection. It can be useful in memory-constrained environments but may result in more frequent GC cycles, affecting performance.
GOGC=200: Here, the GC will run when the heap size is triple the live heap size after the last collection. This setting may be beneficial in CPU-bound applications, as it reduces GC frequency, but it also increases memory usage.

To set the GOGC variable, you can use an environment variable, or you can adjust it programmatically using the debug package:

debug.SetGCPercent(200)  // sets GOGC=200

In addition to GOGC, you can use the runtime/debug package for more granular control over the garbage collector. Functions such as debug.SetGCPercent() allow you to adjust the garbage collector's target percentage programmatically, and debug.FreeOSMemory() can prompt the runtime to return unused memory back to the operating system. However, these functions should be used cautiously, as they can disrupt the garbage collector's regular operation and potentially degrade performance.

The ability to tune Go’s garbage collector allows developers to optimize for specific conditions. However, it’s always good to profile and understand your application’s behavior before making any adjustments.

In the next section, we’ll look at the benefits and drawbacks of Go’s garbage collector to help you gain a more comprehensive understanding of its trade-offs.

Object Reachability and GC Termination

An essential principle that governs garbage collection in Go is object reachability. An object in memory is considered reachable if it can be accessed directly or indirectly by the root of the object graph, usually a global variable or a local variable on the current call stack. Objects that are not reachable are deemed as garbage and are candidates for memory deallocation during a GC cycle.

Go’s garbage collector employs the tri-color marking algorithm, as discussed earlier, to determine the reachability of objects. At the start of the marking phase, all objects are initially marked white, signifying they are unmarked. The GC then iteratively scans the root objects, marking them as grey, meaning they are reachable but their children have not been examined. The GC will follow pointers from these grey objects, marking them and their children as grey, until there are no unexamined grey objects left.

The termination of the garbage collection process is closely linked to object reachability. The GC cycle ends when all the reachable objects have been examined and marked as black, indicating they are reachable, and all their children have been marked. The remaining white objects are unreachable and hence, considered garbage.

An interesting detail to note is that Go uses a Write Barrier, a mechanism that enforces specific rules during pointer updates, to maintain the invariant of the tri-color mark process. It ensures that no black objects point to a white object, preventing the GC from prematurely considering an object as unreachable when it still has references.

Understanding this reachability concept and the termination of a GC cycle can be crucial for efficient memory management in Go. It assists in predicting and controlling GC behavior, enabling the writing of more efficient and performant Go code. The reachability concept also holds significant value when troubleshooting memory leaks, where unreachable objects are unexpectedly retained, causing the application’s memory consumption to continuously increase over time.

Dedicated Garbage Collection Threads and Work Stealing

The Go runtime employs dedicated operating system (OS) threads for garbage collection tasks. These threads operate independently of the Goroutines managed by the Go scheduler, meaning they can run garbage collection concurrently with program execution, enhancing the overall efficiency of the Go runtime. This approach capitalizes on multi-core processors, allowing for parallel garbage collection.

Work stealing is a strategy used by the garbage collector to optimize the distribution of work across multiple processors. In work stealing, idle processors can “steal” tasks from busy ones, effectively balancing the load across all available processors. This dynamic load balancing helps Go achieve efficient utilization of computational resources and boosts the performance of garbage collection. The work stealing algorithm has been designed to minimize contention and maximize parallelism, which leads to better CPU cache utilization and overall throughput.

When a GC cycle starts, the Go runtime creates a set of tasks, each representing a section of the heap that needs to be scanned. These tasks are stored in a global queue. When a processor is free, it pulls a task from this queue and starts executing it. If another processor finishes its current task and finds the global queue empty, it attempts to steal a task from another processor’s local queue. This mechanism continues until all tasks have been completed, leading to the termination of the GC cycle.

It’s noteworthy that Go’s garbage collector not only balances the work among processors but also adapts to the workload. The garbage collector tracks the allocation rate and the time taken to scan the heap. It uses this data to adjust the rate of object allocation, allowing it to control the frequency and duration of GC cycles.

This clever use of dedicated garbage collection threads and work stealing contribute to the minimal GC pause times in Go, one of the garbage collector’s key objectives. This efficient utilization of resources enhances the overall performance of Go applications, especially in multi-core, multi-processor environments. The following sections will further explore heap partitioning and other optimizations that contribute to Go’s robust garbage collection mechanism.

Heap Partitioning and Coloring

To optimize the garbage collection process and better manage the memory, Go utilizes a heap partitioning scheme. The heap is divided into several small blocks or spans, and each span is usually of a particular size class. These spans are the smallest units of memory that the garbage collector deals with. All objects of a particular size are allocated from the same span, which reduces fragmentation and increases memory utilization.

This partitioning scheme makes the garbage collector’s job easier and more efficient. Since each span consists of objects of the same size, the garbage collector doesn’t need to traverse the entire heap; it can simply scan the spans containing live objects.

To further improve the garbage collection process, Go uses a tri-color marking scheme, which we’ve discussed earlier. To recap, the heap is conceptually divided into three sets or “colors”:

White: Objects that have not been visited yet.
Grey: Objects that have been visited but whose children have not been visited.
Black: Objects that have been visited and whose children have been visited.

At the start of the garbage collection cycle, all objects are white. The garbage collector visits each object, starting from the root, and colors it grey. Then it visits the children of the grey objects and colors them grey, while the parent objects are turned black. This process continues until there are no more grey objects, at which point all reachable objects will be black, and all unreachable objects will be white. The white objects are then considered garbage and reclaimed by the garbage collector.

This mechanism allows the garbage collector to work concurrently with the program execution, reducing pause times and making the garbage collection process more efficient.

Benefits and Drawbacks of Go’s Garbage Collector

Like any technology, Go’s garbage collector comes with its share of advantages and trade-offs. Understanding these factors is essential in weighing its suitability for different applications.

Benefits:

1. Simplicity — Go’s garbage collector uses a relatively straightforward concurrent tri-color mark-sweep algorithm. Its design promotes simplicity and efficient memory management, eliminating many challenges associated with manual memory management.

2. Concurrency — One of the significant advantages of Go’s GC is its concurrent design. It allows most of the garbage collection work to happen concurrently with the application, which means less interruption and better application performance.

3. Safety — Automatic memory management in Go reduces the risks of common memory bugs such as memory leaks, dangling pointers, and double frees that are often prevalent in languages that use manual memory management.

4. Efficient Memory Utilization — Go’s garbage collector, combined with its memory model and heap partitioning scheme, reduces memory fragmentation and increases overall memory utilization.

Drawbacks:

1. Unpredictable Pause Times — While Go’s GC is designed to minimize pause times, the stop-the-world phase, albeit short, still introduces some level of unpredictability.

2. Memory Usage — Go’s garbage collector can sometimes use more memory than manual memory management, especially if the application generates a lot of short-lived garbage or has a large heap.

3. CPU Overhead — While Go’s concurrent garbage collector reduces pause times, it does come with CPU overhead since it runs concurrently with the application.

Conclusion

Garbage collection in Go is a fascinating and essential aspect of the language. As we’ve seen throughout this article, the design and operation of Go’s garbage collector are integral to the efficiency and performance of Go applications. With a strong understanding of how Go manages memory and how its garbage collector works, developers are equipped to write better, more efficient code and to debug and optimize their applications effectively.

At its heart, Go’s garbage collector employs a concurrent, tri-color, mark-sweep algorithm. This algorithm, while simple in its concept, is highly effective in automatically managing memory in Go applications. By marking objects as white, grey, or black based on their reachability, and subsequently sweeping away the unreachable, white objects, Go can efficiently reuse memory and prevent leaks. This takes away the burden of manual memory management from the developer and eliminates many common bugs associated with it.

What’s remarkable about Go’s garbage collector is its ability to do most of its work concurrently with the running application. This concurrent design reduces the disruptive stop-the-world pauses that can hinder application performance. Even though there’s still a stop-the-world phase in Go, it’s typically very short and often unnoticeable in most applications.

Go also introduces interesting concepts like heap partitioning, write barriers, and stack shrinking, which all play essential roles in how Go manages memory and how its garbage collector operates. Understanding these concepts allows developers to better understand the behavior of their Go applications and how memory is being used and freed.

Of course, no technology is without its trade-offs, and Go’s garbage collector is no exception. While it provides many benefits, there are also challenges like unpredictable pause times, increased memory usage, and CPU overhead. However, Go provides ways to tune its garbage collector using the GOGC environment variable and by adjusting how your code generates garbage.

Despite these trade-offs, the advantages of Go’s garbage collector — its simplicity, concurrent design, and efficient memory utilization — typically outweigh the disadvantages for many applications. Go’s garbage collector is a prime example of the language’s philosophy: to make things simple and efficient.

Understanding garbage collection in Go is more than just an academic exercise. It’s a vital part of mastering the language and writing efficient, performance-oriented Go applications. Whether you’re a novice Go developer or an experienced Gopher looking to deepen your understanding of the language, I hope this deep dive into Go’s garbage collector has been enlightening.