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Garbage collectors have the responsibility of tracking heap memory allocations, freeing up allocations that are no longer needed, and keeping allocations that are still in-use. How a language decides to implement this behavior is complex but it shouldn’t be a requirement for application developers to understand the details in order to build software. Plus, with different releases of a language’s VM or runtime, the implementation of these systems are always changing and evolving. What’s important for application developers is to maintain a good working model of how the garbage collector for their language behaves and how they can be sympathetic with that behavior without being concerned as to the implementation.
As of version 1.12, the Go programming language uses a non-generational concurrent tri-color mark and sweep collector. If you want to visually see how a mark and sweep collector works, Ken Fox wrote this great article and provides an animation. The implementation of Go’s collector has changed and evolved with every release of Go. So any post that talks about the implementation details will no longer be accurate once the next version of the language is released.
<aside> 📘 三色标记法 tri-color gc
三色标记法的形象讲解
With all that said, the modeling I will do in this post will not focus on the actual implementation details. The modeling will focus on the behavior you will experience and the behavior you should expect to see for years to come. In this post, I will share with you the behavior of the collector and explain how to be sympathetic with that behavior, regardless of the current implementation or how it changes in the future. This will make you a better Go developer.
Note: Here is more reading you can do about garbage collectors and Go’s actual collector as well.
I will never refer to the heap as a container that you can store or release values from. It’s important to understand that there is no linear containment of memory that defines the “Heap”. Think that any memory reserved for application use in the process space is available for heap memory allocation. Where any given heap memory allocation is virtually or physically stored is not relevant to our model. This understanding will help you better understand how the garbage collector works.
When a collection starts, the collector runs through three phases of work. Two of these phases create Stop The World (STW) latencies and the other phase creates latencies that slow down the throughput of the application. The three phases are:
<aside> 📘 GC 的三个步骤，其中有两个步骤会导致 STW
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Here is a break-down of each phase.
When a collection starts, the first activity that must be performed is turning on the Write Barrier. The purpose of the Write Barrier is to allow the collector to maintain data integrity on the heap during a collection since both the collector and application goroutines will be running concurrently.