**Article Translation and Refinement** --- **Introduction** Hello everyone. Today's presentation focuses on JavaScript's memory management mechanism. This talk will be divided into three parts: 1. **JavaScript Memory Management and Garbage** 2. **Common GC Algorithms** 3. **Garbage Collection in V8 Engine** --- ### 1. JavaScript Memory Management and Garbage JavaScript's memory management is handled automatically by the engine and does not require manual intervention. It involves three core operations: - **Allocate memory space** - **Use memory space** - **Release memory space** **Garbage** refers to objects that are no longer reachable from the root. Objects that are not referenced are considered garbage. This concept is widely understood, and we will not delve into details here. Let's now discuss GC algorithms. --- ### 2. GC Algorithms Garbage collection (GC) algorithms aim to identify and reclaim memory occupied by garbage objects. The term "garbage" here refers to objects that are no longer needed by the program or cannot be accessed from the root. **GC Algorithms** include: - **Reference Counting** - **Mark-Sweep** - **Mark-Compact** - **Generational Collection** --- ### 2.1 Reference Counting Reference counting was historically used in browsers below IE8. Modern browsers no longer use it. Its core principle is to track the number of references to each value. When a value is referenced, the count increases; when it is no longer used, the count decreases. A count of zero indicates the memory is no longer in use. **Advantages**: - Real-time monitoring of reference counts allows timely garbage collection. - Reduces program pauses. **Disadvantages**: - High resource consumption due to continuous operation. - Cannot reclaim objects with cyclic references. --- ### 2.2 Mark-Sweep Mark-sweep is divided into two phases: **Mark** and **Sweep**. The core idea is to traverse all objects, identify reachable objects (reachable objects), and reclaim memory from non-reachable objects. **Example**: - **Mark** phase: Identify reachable objects. - **Sweep** phase: Reclaim memory from non-reachable objects. **Advantages**: - Solves cyclic reference issues. - Simple to implement. **Disadvantages**: - Causes **fragmentation** (space waste). - Cannot efficiently reclaim garbage objects. --- ### 2.3 Mark-Compact Mark-compact optimizes for fragmentation by reorganizing memory after the **Mark** phase. It merges free spaces into larger blocks, improving memory utilization. **Key Features**: - **Mark** phase: Identify reachable objects. - **Compact** phase: Reorganize memory to eliminate fragmentation. **Advantages**: - Reduces fragmentation. - Efficiently reclaims garbage objects. **Disadvantages**: - Cannot monitor garbage objects in real-time. - Requires additional steps for memory reorganization. --- ### 3. V8 Engine Garbage Collection V8 is a modern JavaScript engine with **generational garbage collection**. It divides memory into **young generation** (new objects) and **old generation** (long-living objects), using different algorithms for each. **Key GC Algorithms**: 1. **Generational Collection** 2. **Copying Algorithm** 3. **Mark-Sweep** 4. **Mark-Compact** 5. **Incremental Marking** --- ### 3.1 Young Generation Garbage Collection V8 uses the **Copying Algorithm** for young generation objects. The memory area is divided into two equal parts: **From** (active objects) and **To** (free space). Objects are copied from From to To during garbage collection. **Process**: 1. **Mark-Compact** reorganizes memory to eliminate fragmentation. 2. Objects are copied from From to To, and From and To are swapped. 3. Freed memory is released. **Note**: Objects may be promoted (moved to old generation) if the To space utilization exceeds 25%. --- ### 3.2 Old Generation Garbage Collection Old generation objects are collected using **Mark-Sweep**, **Mark-Compact**, and **Incremental Marking**. When young objects are promoted to old generation, the **Mark-Compact** strategy is used for space optimization, while **Incremental Marking** improves efficiency. --- ### 3.3 Incremental Marking Incremental marking divides marking operations into distinct time intervals. During garbage collection, the marking system allocates different time slots for marking and execution, reducing interference and improving performance. --- **Conclusion** This talk has covered the fundamentals of JavaScript memory management, GC algorithms, and V8's implementation. Understanding these concepts is essential for optimizing memory usage and improving application performance. --- **Images and Diagrams** - **Figure 1**: V8 Garbage Collection Strategy - **Figure 2**: V8 Memory Allocation - **Figure 3**: Young Generation Garbage Collection - **Figure 4**: Old Generation Garbage Collection - **Figure 5**: Incremental Marking --- **Note**: All image URLs and captions are preserved as original.