Memory Blocks

Go is a language which supports automatic memory management, such as automatic memory allocation and automatic garbage collection. So Go programmers can do programming without handling the underlying verbose memory management. This not only brings much convenience and saves Go programmers lots of time, but also helps Go programmers avoid many careless bugs.

Although knowing the underlying memory management implementation details is not necessary for Go programmers to write Go code, understanding some concepts and being aware of some facts in the memory management implementation by the standard Go compiler and runtime is very helpful for Go programmers to write high quality Go code.

This article will explain some concepts and list some facts of the implementation of memory block allocation and garbage collection by the standard Go compiler and runtime. Other aspects, such as memory apply and memory release in memory management, will not be touched in this article.

Memory Blocks

A memory block is a continuous memory segment to host value parts at run time. Different memory blocks may have different sizes, to host different value parts. One memory block may host multiple value parts at the same time, but each value part can only be hosted within one memory block, no matter how large the size of that value part is. In other words, for any value part, it never crosses memory blocks.

A Value References the Memory Blocks Which Host Its Value Parts

We have known that a value part can reference another value part. Here, we extend the reference definition by saying a memory block is referenced by all the value parts it hosts. So if a value part v is referenced by another value part, then the other value will also reference the memory block hosting v, indirectly.

When Will Memory Blocks Be Allocated?

Where Will Memory Blocks Be Allocated On?

For every Go program compiled by the official standard Go compiler, at run time, each goroutine will maintain a stack, which is a memory segment. It acts as a memory pool for some memory blocks to be allocated from/on. Before Go Toolchain 1.19, the initial size of a stack is always 2KiB. Since Go Toolchain 1.19, the initial size is adaptive. The stack of a goroutine will grow and shrink as needed in goroutine running. The minimum stack size is 2KiB.

(Please note, there is a global limit of stack size each goroutine may reach. If a goroutine exceeds the limit while growing its stack, the program crashes. As of Go Toolchain 1.20, the default maximum stack size is 1 GB on 64-bit systems, and 250 MB on 32-bit systems. We can call the SetMaxStack function in the runtime/debug standard package to change the size. And please note that, by the current official standard Go compiler implementation, the actual allowed maximum stack size is the largest power of 2 which is not larger than then MaxStack setting. So for the default setting, the actual allowed maximum stack size is 512 MiB on 64-bit systems, and 128 MiB on 32-bit systems.)

Memory blocks can be allocated on stacks. Memory blocks allocated on the stack of a goroutine can only be used (referenced) in the goroutine internally. They are goroutine localized resources. They are not safe to be referenced crossing goroutines. A goroutine can access or modify the value parts hosted on a memory block allocated on the stack of the goroutine without using any data synchronization techniques.

Heap is a singleton in each program. It is a virtual concept. If a memory block is not allocated on any goroutine stack, then we say the memory block is allocated on heap. Value parts hosted on memory blocks allocated on heap can be used by multiple goroutines. In other words, they can be used concurrently. Their uses should be synchronized when needed.

Heap is a conservative place to allocate memory blocks on. If compilers detect a memory block will be referenced crossing goroutines or can't easily confirm whether or not the memory block is safe to be put on the stack of a goroutine, then the memory block will be allocated on heap at run time. This means some values which can be safely allocated on stacks may also be allocated on heap.

If a memory block is allocated somewhere, we can also say the value parts hosted on the memory block are allocated on the same place.

If some value parts of a local variable declared in a function is allocated on heap, we can say the value parts (and the variable) escape to heap. By using Go Toolchain, we can run go build -gcflags -m to check which local values (value parts) will escape to heap at run time. As mentioned above, the current escape analyzer in the standard Go compiler is still not perfect, many local value parts can be allocated on stacks safely will still escape to heap.

An active value part allocated on heap still in use must be referenced by at least one value part allocated on a stack. If a value escaping to heap is a declared local variable, and assume its type is T, Go runtime will create (a memory block for) an implicit pointer of type *T on the stack of the current goroutine. The value of the pointer stores the address of the memory block allocated for the variable on heap (a.k.a., the address of the local variable of type T). Go compiler will also replace all uses of the variable with dereferences of the pointer value at compile time. The *T pointer value on stack may be marked as dead since a later time, so the reference relation from it to the T value on heap will disappear. The reference relation from the *T value on stack to the T value on heap plays an important role in the garbage collection process which will be described below.

Similarly, we can view each package-level variable is allocated on heap, and the variable is referenced by an implicit pointer which is allocated on a global memory zone. In fact, the implicit pointer references the direct part of the package-level variable, and the direct part of the variable references some other value parts.

A memory block allocated on heap may be referenced by multiple value parts allocated on different stacks at the same time.

A memory block created by calling new function may be allocated on heap or stacks. This is different to C++.

package main

// The following directive is to prevent
// calls to the function f being inlined.
//go:noinline
func f(i int) byte {
	var a [1<<20]byte // make stack grow
	return a[i]
}

func main(){
	var x int
	println(&x)
	f(100)
	println(&x)
}

We will find that the two printed addresses are different (as of the standard Go compiler v1.20).

When Can a Memory Block Be Collected?

Memory blocks allocated for direct parts of package-level variables will never be collected.

The stack of a goroutine will be collected as a whole when the goroutine exits. So there is no need to collect the memory blocks allocated on stacks, individually, one by one. Stacks are not collected by the garbage collector.

For a memory block allocated on heap, it can be safely collected only if it is no longer referenced (either directly or indirectly) by all the value parts allocated on goroutine stacks and the global memory zone. We call such memory blocks as unused memory blocks. Unused memory blocks on heap will be collected by the garbage collector.

package main

var p *int

func main() {
	done := make(chan bool)
	// "done" will be used in main and the following
	// new goroutine, so it will be allocated on heap.

	go func() {
		x, y, z := 123, 456, 789
		_ = z  // z can be allocated on stack safely.
		p = &x // For x and y are both ever referenced
		p = &y // by the global p, so they will be both
		       // allocated on heap.

		// Now, x is not referenced by anyone, so
		// its memory block can be collected now.

		p = nil
		// Now, y is also not referenced by anyone,
		// so its memory block can be collected now.

		done <- true
	}()

	<-done
	// Now the above goroutine exits, the done channel
	// is not used any more, a smart compiler may
	// think it can be collected now.

	// ...
}

package main

import "fmt"

func main() {
	// Assume the length of the slice is so large
	// that its elements must be allocated on heap.
	bs := make([]byte, 1 << 31)

	// A smart compiler can detect that the
	// underlying part of the slice bs will never be
	// used later, so that the underlying part of the
	// slice bs can be garbage collected safely now.

	fmt.Println(len(bs))
}

package main

import "fmt"
import "runtime"

func main() {
	bs := make([]int, 1000000)

	fmt.Println(len(bs))

	// A runtime.KeepAlive(bs) call is also
	// okay for this specified example.
	runtime.KeepAlive(&bs)
}

How Are Unused Memory Blocks Detected?

When Will a New Garbage Collection Process Start?